Middle School Science Quiz: Motion Caused By Gravity
20 questions · exam conditions
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Motion Caused By GravityQuestion 1 of 20

A satellite moves sideways around Earth in a nearly circular path instead of flying off into space in a straight line. What role does gravity play in keeping it in orbit?

Motion: orbiting (curved path around Earth) Gravitational force: toward Earth's center Effect: changes direction of velocity (inward acceleration)​

Gravity pulls the satellite inward toward Earth, continuously changing its direction so it follows a curved orbital path.
Gravity pushes the satellite outward, balancing Earth's pull so it stays at the same distance.
No force is needed for orbit; the satellite naturally curves because it is moving fast.
Air resistance provides the inward force needed for the satellite to orbit Earth.
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Middle School Science Quiz

Middle School Science Quiz: Motion Caused By Gravity

Practice Motion Caused By Gravity in Middle School Science with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Motion Caused By Gravity, giving you a quick way to practice the rules, question types, and explanations that matter most for Middle School Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A satellite moves sideways around Earth in a nearly circular path instead of flying off into space in a straight line. What role does gravity play in keeping it in orbit?

Motion: orbiting (curved path around Earth) Gravitational force: toward Earth's center Effect: changes direction of velocity (inward acceleration)​

  1. Gravity pulls the satellite inward toward Earth, continuously changing its direction so it follows a curved orbital path. (correct answer)
  2. Gravity pushes the satellite outward, balancing Earth's pull so it stays at the same distance.
  3. No force is needed for orbit; the satellite naturally curves because it is moving fast.
  4. Air resistance provides the inward force needed for the satellite to orbit Earth.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). A satellite orbiting Earth is continuously falling toward Earth due to gravitational force, but it's also moving sideways (tangent to orbit) fast enough that as it falls, the curved surface of Earth falls away beneath it at the same rate—the result is a circular path where the satellite keeps falling but never gets closer to Earth (stable orbit). The gravitational force F = mg pulls inward (toward Earth center) providing the centripetal acceleration that bends the straight-line motion into a circle; without gravity, the satellite would fly off into space in a straight line (Newton's First Law), but gravity's continuous inward pull curves the path into orbit. Choice A is correct because it accurately explains that gravitational force pulling inward causes the curved orbital path by continuously changing the satellite's direction. Choice B incorrectly claims gravity pushes outward, when gravity always pulls inward toward Earth's center. Choice C incorrectly states no force is needed for orbit and the satellite naturally curves, violating Newton's First Law. Choice D incorrectly attributes orbit to air resistance, when satellites orbit in the vacuum of space where there is no air. Understanding orbits: satellites are continuously falling inward (gravity) while moving sideways fast enough to maintain orbit—gravity is the force keeping them from flying off in straight lines.

Question 2

A student releases a ball from rest. Which statement correctly connects gravitational force to the ball's acceleration near Earth's surface (ignoring air resistance)?​

  1. The gravitational force is F=mgF=mg downward, so the ball's acceleration is a=ga=g downward (about 10 m/s210\,\text{m/s}^2). (correct answer)
  2. The gravitational force is F=mgF=mg upward, so the ball's acceleration is upward.
  3. The gravitational force is constant, so the ball must fall at constant speed (zero acceleration).
  4. The gravitational force depends only on the ball's speed, so the acceleration is zero at the start and increases later.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you release a ball from rest, Earth's gravitational force pulls it downward with force F = mg (mass times gravitational field strength)—applying Newton's Second Law F = ma, we get ma = mg, which simplifies to a = g, showing that the ball's acceleration equals the gravitational field strength g ≈ 10 m/s² downward, regardless of the ball's mass. Choice A is correct because it accurately connects gravitational force F = mg downward to acceleration a = g downward, properly applying Newton's Second Law. Choice B incorrectly states gravitational force is upward when it always pulls toward Earth's center (downward near surface); Choice C incorrectly claims constant force means constant speed, missing that F = ma shows constant force produces constant acceleration not constant velocity; Choice D incorrectly claims gravitational force depends on speed, when F = mg depends only on mass and gravitational field strength. Recognizing gravity as motion cause: the direct connection F = mg → a = g is fundamental to understanding falling motion—gravitational force causes gravitational acceleration. Understanding that gravitational force near Earth's surface is essentially constant (F = mg with constant g) and produces constant acceleration (not constant speed) is fundamental physics: this explains why all objects fall with the same acceleration and why we can make precise predictions about falling motion—this force-causes-motion thinking using F = ma is the foundation of mechanics.

Question 3

A student drops a tennis ball from a balcony. As it falls, it moves downward faster and faster until it hits the ground. Which statement best explains why the ball speeds up as it falls?​

  1. The ball speeds up because air pushes it downward more and more as it falls.
  2. The ball speeds up because Earth's gravity pulls downward with a net force F=mgF=mg, causing a downward acceleration of about g≈10 m/s2g\approx 10\,\text{m/s}^2. (correct answer)
  3. The ball speeds up because its motion creates extra gravity that increases over time.
  4. The ball speeds up because objects naturally fall faster without any force acting on them.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you drop a ball, it falls to the ground because Earth's gravitational force pulls it downward with force F = mg (mass of ball times gravitational field strength g ≈ 10 m/s²)—this downward force causes downward acceleration a = g, meaning the ball's downward velocity increases as it falls: it starts at rest (v = 0), then after 1 second is moving 10 m/s downward, after 2 seconds 20 m/s downward, continuously speeding up until it hits the ground. Choice B is correct because it accurately explains that gravitational force pulling downward causes the falling motion and properly identifies the force F = mg and resulting acceleration a = g ≈ 10 m/s². Choice A incorrectly attributes the motion to air pressure pushing downward, when gravity is the actual cause of falling; Choice C reverses cause and effect, suggesting motion creates gravity instead of gravity creating the motion; Choice D claims no force is needed for falling, missing that acceleration requires force and gravity provides that force (F = ma, falling has a so must have F). Recognizing gravity as motion cause: whenever you see objects falling or dropping (cause: gravity pulling down), you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses)—this force-causes-motion thinking explains all motion around us and allows predicting future motion.

Question 4

A ball is thrown straight upward. It slows down as it rises, stops for an instant at the top, and then speeds up downward. What role does gravity play in this motion?​

  1. Gravity pulls downward the whole time, so it slows the ball while it moves up and then accelerates it downward after it turns around. (correct answer)
  2. Gravity pushes upward while the ball rises, then turns off at the top, then turns back on to pull it down.
  3. Gravity pulls sideways, which is why the ball changes direction at the top.
  4. Gravity is not involved; the ball slows down because upward motion naturally fades away.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you jump, your legs push you upward giving you initial upward velocity, but gravity immediately begins pulling you downward with force F = mg—this downward force causes downward acceleration a = g ≈ 10 m/s² throughout your jump, which slows your upward motion (deceleration: gravity opposes upward velocity), brings you to a stop at the peak (velocity becomes zero), then accelerates you downward bringing you back to ground. Choice A is correct because it accurately explains that gravitational force pulling downward causes both the slowing during upward motion and the speeding up during downward motion. Choice B incorrectly describes gravity as pushing upward and turning on/off, when gravity constantly pulls downward; Choice C claims gravity pulls sideways, missing that gravity always pulls toward Earth's center (downward); Choice D denies that gravity affects the motion, claiming upward motion naturally fades away, missing that gravity is the force causing the deceleration and reversal. Recognizing gravity as motion cause: whenever you see jumps being temporary (cause: gravity slowing upward motion and pulling back down), you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and that gravity acts continuously (not turning on/off) is fundamental physics: the entire up-peak-down trajectory is shaped by constant gravitational force pulling downward the whole time—this force-causes-motion thinking explains projectile motion and allows predicting how high objects will go and when they'll return.

Question 5

Two students drop two different balls at the same time from the same height: a heavy steel ball and a lighter rubber ball. Ignoring air resistance, which statement best describes their motion due to gravity?​

  1. The heavier ball falls faster because it has a larger gravitational force, so it must have a larger downward acceleration.
  2. The lighter ball falls faster because it is easier for gravity to pull down.
  3. They fall with the same downward acceleration a=ga=g because gravity gives objects the same acceleration near Earth's surface, even though the forces F=mgF=mg are different. (correct answer)
  4. Neither ball accelerates; both fall at a constant speed because gravity is constant.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you drop a ball, it falls to the ground because Earth's gravitational force pulls it downward—the key insight is that while heavier objects experience larger gravitational forces (F = mg, larger m means larger F), they also have more mass to accelerate (F = ma, larger m means same a requires larger F), and these effects exactly cancel: a = F/m = mg/m = g, so all objects fall with the same acceleration g ≈ 10 m/s² near Earth's surface regardless of their mass. Choice C is correct because it properly identifies that both balls have the same downward acceleration a = g even though the forces are different, capturing the fundamental principle that gravitational acceleration is independent of mass. Choice A incorrectly claims the heavier ball falls faster, missing that larger force on larger mass gives same acceleration; Choice B incorrectly claims the lighter ball falls faster; Choice D incorrectly claims neither accelerates and both fall at constant speed, missing that gravity causes acceleration not constant velocity. Recognizing gravity as motion cause: the fact that all objects fall with the same acceleration (ignoring air resistance) is one of gravity's most important properties—a feather and hammer dropped on the Moon fall together because gravity gives all objects the same acceleration. Understanding that a = g for all masses near Earth's surface (Galileo's discovery) is fundamental physics: this universality of gravitational acceleration allows us to predict motion without knowing object mass—this force-causes-motion thinking combined with mass-independence makes projectile motion analysis possible.

Question 6

A rock is dropped from rest. One second after being dropped (ignore air resistance), its speed is greater than it was right after release. Which statement best connects the force to this change in speed?

  1. Gravity provides a net downward force F=mgF = mg, so the rock accelerates downward and its speed increases. (correct answer)
  2. Gravity keeps the rock's speed constant, so the rock falls at the same speed the whole time.
  3. The rock speeds up because it is moving toward the ground, and the ground pulls it down.
  4. The rock speeds up because air pressure increases as it gets closer to the ground.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you drop a ball, it falls to the ground because Earth's gravitational force pulls it downward with force F = mg (mass of ball times gravitational field strength g ≈ 10 m/s²)—this downward force causes downward acceleration a = g, meaning the ball's downward velocity increases as it falls: it starts at rest (v = 0), then after 1 second is moving 10 m/s downward, after 2 seconds 20 m/s downward, continuously speeding up until it hits the ground. Choice A is correct because it accurately explains that gravitational force pulling downward causes the falling motion. Choice B incorrectly states speed is constant, but gravity causes acceleration; Choice C reverses cause and effect, suggesting motion creates gravity; Choice D attributes to air pressure, not gravity. Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).

Question 7

A ball is thrown horizontally. If gravity could be "turned off" right after the throw (ignore air resistance), what would the ball do after leaving the thrower's hand?

  1. It would continue moving in a straight line at constant speed in the direction it was thrown. (correct answer)
  2. It would immediately stop moving because gravity is required to keep it going.
  3. It would curve upward because the forward motion would create an upward force.
  4. It would still curve downward because curved motion happens automatically once an object is thrown.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you throw a ball horizontally, it would travel in a straight line if no forces acted (Newton's First Law), but gravity continuously pulls downward creating a downward acceleration while the ball maintains its forward velocity—the combination produces a curved path (parabola): the ball moves forward at constant speed (no horizontal force) while simultaneously accelerating downward (gravity pulls down), so it travels forward and down simultaneously, creating the characteristic curved trajectory we see in thrown objects. Choice A is correct because it correctly describes how without gravity, the path would be straight, highlighting gravity's role in curving it. Choice B incorrectly says it stops, but motion continues without force; Choice C suggests upward curve, wrong direction; Choice D claims curve automatic, missing force requirement. Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).

Question 8

A student throws a ball horizontally from the top of a small hill. The ball moves forward but also curves downward and hits the ground. Why doesn't the ball travel in a straight horizontal line?

  1. The ball curves because air always pushes downward harder than gravity does.
  2. The ball curves because its forward motion creates a downward force.
  3. The ball curves because gravity continuously pulls it downward while it keeps moving forward, creating a curved (projectile) path. (correct answer)
  4. The ball curves because gravity only acts after the ball has traveled a certain distance forward.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you throw a ball horizontally, it would travel in a straight line if no forces acted (Newton's First Law), but gravity continuously pulls downward creating a downward acceleration while the ball maintains its forward velocity—the combination produces a curved path (parabola): the ball moves forward at constant speed (no horizontal force) while simultaneously accelerating downward (gravity pulls down), so it travels forward and down simultaneously, creating the characteristic curved trajectory we see in thrown objects. Choice C is correct because it accurately explains that gravitational force pulling downward causes the curved projectile path by combining constant forward motion with accelerating downward motion. Choice A incorrectly attributes the curve to air pushing harder than gravity; Choice B reverses cause and effect, suggesting forward motion creates downward force when gravity is the actual cause; Choice D incorrectly claims gravity only acts after a certain distance, when gravity acts continuously from the moment of release. Recognizing gravity as motion cause: whenever you see thrown objects curving down (cause: gravity bending straight-line motion), you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that projectile motion results from the combination of horizontal motion (maintained by inertia) and vertical acceleration (caused by gravity) is fundamental physics: the curve is evidence of gravitational force acting throughout the flight, not air or delayed effects—this force-causes-motion thinking explains why all projectiles follow parabolic paths near Earth's surface.

Question 9

Two students drop two different balls at the same time from the same height: a heavy steel ball and a lighter rubber ball. Ignoring air resistance, which statement best describes their motion due to gravity?

  1. The heavier ball falls faster because it has a larger gravitational force, so it must have a larger downward acceleration.
  2. The lighter ball falls faster because it is easier for gravity to pull down.
  3. They fall with the same downward acceleration a=ga=g because gravity gives objects the same acceleration near Earth's surface, even though the forces F=mgF=mg are different. (correct answer)
  4. Neither ball accelerates; both fall at a constant speed because gravity is constant.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you drop a ball, it falls to the ground because Earth's gravitational force pulls it downward—the key insight is that while heavier objects experience larger gravitational forces (F = mg, larger m means larger F), they also have more mass to accelerate (F = ma, larger m means same a requires larger F), and these effects exactly cancel: a = F/m = mg/m = g, so all objects fall with the same acceleration g ≈ 10 m/s² near Earth's surface regardless of their mass. Choice C is correct because it properly identifies that both balls have the same downward acceleration a = g even though the forces are different, capturing the fundamental principle that gravitational acceleration is independent of mass. Choice A incorrectly claims the heavier ball falls faster, missing that larger force on larger mass gives same acceleration; Choice B incorrectly claims the lighter ball falls faster; Choice D incorrectly claims neither accelerates and both fall at constant speed, missing that gravity causes acceleration not constant velocity. Recognizing gravity as motion cause: the fact that all objects fall with the same acceleration (ignoring air resistance) is one of gravity's most important properties—a feather and hammer dropped on the Moon fall together because gravity gives all objects the same acceleration. Understanding that a = g for all masses near Earth's surface (Galileo's discovery) is fundamental physics: this universality of gravitational acceleration allows us to predict motion without knowing object mass—this force-causes-motion thinking combined with mass-independence makes projectile motion analysis possible.

Question 10

Imagine a ball is thrown forward in a world with no gravity (and no air). What would happen to its motion after it leaves the thrower's hand?

  1. It would curve downward because forward motion naturally turns into downward motion.
  2. It would travel in a straight line at constant speed because there would be no net force to change its velocity. (correct answer)
  3. It would immediately stop because gravity is required to keep objects moving.
  4. It would move upward because gravity is the only force that prevents things from rising.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). In a world with no gravity (and no air resistance), a thrown ball would experience no net force after leaving the hand—according to Newton's First Law, an object in motion stays in motion at constant velocity when no net force acts on it, so the ball would continue forward in a straight line at whatever speed it was thrown, never curving down, never slowing, just constant straight-line motion forever. Choice B is correct because it properly applies Newton's First Law: with no forces acting (no gravity, no air), the ball maintains its velocity (both speed and direction) indefinitely. Choice A incorrectly suggests forward motion naturally becomes downward motion without any force to cause the change; Choice C incorrectly claims gravity is needed to keep objects moving, when actually objects maintain motion without force (inertia); Choice D incorrectly suggests objects would rise without gravity, missing that no force means no acceleration in any direction. Recognizing gravity as motion cause: this thought experiment highlights gravity's role—without it, there would be no falling, no curved projectile paths, no orbits, just straight-line motion at constant speed. Understanding that motion continues without force (inertia) but changes in motion require force is fundamental physics: gravity is what makes our everyday world different from this no-gravity scenario—this force-causes-motion thinking helps us recognize that the curved paths and falling we observe are evidence of gravitational force, not natural tendencies of objects.

Question 11

Imagine a ball is thrown forward on Earth. If gravity could be turned off right after the ball leaves the thrower's hand (and air resistance is ignored), how would the ball move?

Compare with/without gravity: gravity normally provides downward acceleration

  1. It would continue in a straight line at constant velocity because there would be no downward gravitational force to curve its path. (correct answer)
  2. It would curve downward the same way as usual because gravity is not needed once the ball is moving.
  3. It would immediately stop moving because gravity is required to keep objects in motion.
  4. It would accelerate upward because removing gravity makes objects float upward on their own.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you throw a ball forward, it would normally follow a curved path because gravity continuously pulls downward while the ball moves forward, but if gravity could be turned off after release, there would be no downward force and thus no downward acceleration—according to Newton's First Law, an object in motion stays in motion at constant velocity unless acted upon by a force, so without gravity the ball would continue forward in a straight line at whatever velocity it had when gravity was turned off. Choice A is correct because it accurately explains that without downward gravitational force there would be no downward acceleration to curve the path, so the ball would move in a straight line at constant velocity. Choice B incorrectly claims the ball would curve downward without gravity, missing that the curve is caused by gravitational force. Choice C incorrectly states gravity is required to keep objects in motion, when Newton's First Law shows objects maintain motion without force. Choice D incorrectly suggests removing gravity makes objects accelerate upward spontaneously. This thought experiment demonstrates that projectile curves are caused by gravity—remove gravity and the curve disappears, proving gravity is the cause of the curved motion we observe.

Question 12

A student drops a heavy metal ball and a light plastic ball from the same height at the same time. Ignoring air resistance, which prediction is best?

Gravitational force on each object: F=mgF = mg downward Acceleration near Earth: a=g≈10 m/s2a = g \approx 10\ \text{m/s}^2 downward

  1. The heavier ball hits first because it has a larger gravitational force, so it must have a larger downward acceleration.
  2. The lighter ball hits first because it has less mass to pull down, so it accelerates faster.
  3. They hit at the same time because both experience the same downward acceleration gg at the same location. (correct answer)
  4. Neither hits the ground because objects in free-fall are weightless, meaning gravity is not acting on them.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you drop objects of different masses, each experiences gravitational force F = mg proportional to its mass, but since acceleration a = F/m = mg/m = g, the mass cancels out and all objects experience the same downward acceleration g ≈ 10 m/s² regardless of their mass—this means a heavy ball and light ball dropped together will fall at the same rate and hit the ground simultaneously (ignoring air resistance). Choice C is correct because it accurately explains that both balls experience the same downward acceleration g at the same location, regardless of their different masses. Choice A incorrectly assumes larger force means larger acceleration, missing that a = F/m = mg/m = g (mass cancels). Choice B incorrectly claims lighter objects accelerate faster, contradicting the fact that all objects fall with acceleration g. Choice D incorrectly states objects in free-fall are weightless so gravity isn't acting, confusing the sensation of weightlessness with absence of gravitational force. Recognizing gravity as motion cause: all objects near Earth's surface experience the same gravitational acceleration g, making dropped objects fall together regardless of mass—this counterintuitive result shows gravity's universal nature.

Question 13

A student throws a ball straight up. The ball slows down as it rises, stops for an instant at the top, and then speeds up downward as it falls back. How does gravity explain this up-then-down motion?

Gravitational force: F=mgF = mg downward (toward Earth) Acceleration: a=g≈10 m/s2a = g \approx 10\ \text{m/s}^2 downward

  1. Gravity pulls downward the whole time, so it slows the ball while it's moving up and then accelerates it downward after it reaches the top. (correct answer)
  2. Gravity pushes upward on the ball while it rises and then turns off at the top, letting the ball fall.
  3. The ball stops at the top because no forces act on it there, so it stays still until air starts pushing it down.
  4. The ball falls because its upward force runs out; gravity is not involved after the throw.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you jump or throw a ball upward, your legs or hand push you/it upward giving initial upward velocity, but gravity immediately begins pulling downward with force F = mg—this downward force causes downward acceleration a = g ≈ 10 m/s² throughout the motion, which slows the upward motion (deceleration: gravity opposes upward velocity), brings the object to a stop at the peak (velocity becomes zero), then accelerates it downward bringing it back to ground. Choice A is correct because it accurately explains that gravitational force pulling downward causes both the slowing while rising and the speeding up while falling. Choice B incorrectly claims gravity pushes upward while the ball rises and then turns off, when gravity always pulls downward. Choice C incorrectly states no forces act at the top and air pushes it down, missing that gravity acts continuously throughout. Choice D denies gravity's involvement after the throw, missing that gravity is what brings the ball back down. Recognizing gravity as motion cause: the entire up-peak-down trajectory is shaped by constant gravitational force pulling downward the whole time—understanding that motion results from forces and identifying which force causes which motion is fundamental physics.

Question 14

A student rolls a ball off a table so it leaves the edge moving forward. Instead of traveling in a straight line, it follows a curved path downward while still moving forward. Why does the ball curve downward?

Motion: projectile motion (forward + downward curve) Gravitational force: F=mgF = mg downward Acceleration: a=ga = g downward​

  1. The ball curves downward because gravity continuously pulls it downward while it keeps moving forward, bending its path. (correct answer)
  2. The ball curves downward because its forward motion turns into downward motion even without any force.
  3. The ball curves downward because air resistance pulls it straight down with a constant force larger than gravity.
  4. The ball curves downward because gravity pulls it forward, making it speed up horizontally.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you throw a ball horizontally or roll it off a table, it would travel in a straight line if no forces acted (Newton's First Law), but gravity continuously pulls downward creating a downward acceleration while the ball maintains its forward velocity—the combination produces a curved path (parabola): the ball moves forward at constant speed (no horizontal force) while simultaneously accelerating downward (gravity pulls down), so it travels forward and down simultaneously, creating the characteristic curved trajectory we see in thrown objects. Choice A is correct because it accurately explains that gravitational force pulling downward causes the curved path by continuously bending the straight-line motion. Choice B incorrectly suggests forward motion turns into downward motion without any force, violating Newton's First Law. Choice C incorrectly attributes the curve to air resistance pulling straight down with force larger than gravity. Choice D incorrectly claims gravity pulls forward, when gravity only pulls downward toward Earth's center. The curve is evidence of gravitational force acting throughout the flight—projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion).

Question 15

A satellite moves sideways around Earth in a nearly circular path instead of flying off into space in a straight line. What causes the satellite's path to curve around Earth?

  1. Earth's gravity pulls the satellite inward toward Earth's center, continuously changing the satellite's direction and bending its path into an orbit. (correct answer)
  2. The satellite's engines constantly push it outward, which keeps it circling Earth.
  3. There is no force on the satellite; objects naturally move in circles in space.
  4. A "centrifugal force" pulls the satellite outward and is the only force needed to make it orbit.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). In all cases, the observed motion results from gravitational force acting continuously on the object. A satellite orbiting Earth is continuously falling toward Earth due to gravitational force, but it's also moving sideways (tangent to orbit) fast enough that as it falls, the curved surface of Earth falls away beneath it at the same rate—the result is a circular path where the satellite keeps falling but never gets closer to Earth (stable orbit). The gravitational force F = mg pulls inward (toward Earth center) providing the centripetal acceleration that bends the straight-line motion into a circle; without gravity, the satellite would fly off into space in a straight line (Newton's First Law), but gravity's continuous inward pull curves the path into orbit. Choice A is correct because it properly identifies gravity as the inward force keeping satellite in orbit. Choice B incorrectly attributes the motion to a different force like air pressure or magnetism, when gravity is the actual cause of falling and curved paths; Choice C claims no force is needed for falling, missing that acceleration requires force and gravity provides that force (F = ma, falling has a so must have F); Choice D misunderstands orbits, suggesting satellite needs upward force to stay up, when actually it's continuously falling inward (gravity) while moving sideways fast enough to maintain orbit. Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).

Question 16

A ball is dropped from a balcony 5 m above the ground. As it falls, it moves downward faster and faster until it hits the ground. Which statement best explains why the ball speeds up as it falls (ignore air resistance)?

  1. The ball speeds up because air pushes it downward more and more as it falls.
  2. The ball speeds up because Earth's gravity pulls downward with force F=mgF = mg, causing a downward acceleration a=g≈10 m/s2a = g \approx 10\,\text{m/s}^2. (correct answer)
  3. The ball speeds up because falling motion creates gravity, which then pulls it down faster.
  4. The ball speeds up because no force is needed for objects to start moving faster on their own.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). In all cases, the observed motion results from gravitational force acting continuously on the object. When you drop a ball, it falls to the ground because Earth's gravitational force pulls it downward with force F = mg (mass of ball times gravitational field strength g ≈ 10 m/s²)—this downward force causes downward acceleration a = g, meaning the ball's downward velocity increases as it falls: it starts at rest (v = 0), then after 1 second is moving 10 m/s downward, after 2 seconds 20 m/s downward, continuously speeding up until it hits the ground. The motion (accelerating downward) is the direct result of the gravitational force (pulling downward), demonstrating cause and effect: gravity is the cause, falling is the effect. Choice B is correct because it accurately explains that gravitational force pulling downward causes the falling motion. Choice A incorrectly attributes the motion to a different force like air pressure or magnetism, when gravity is the actual cause of falling and curved paths; Choice C reverses cause and effect, suggesting motion creates gravity instead of gravity creating the motion; Choice D claims no force is needed for falling, missing that acceleration requires force and gravity provides that force (F = ma, falling has a so must have F). Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).

Question 17

A student throws a tennis ball straight up. The ball rises, slows down, stops for an instant at the top, and then falls back down. What role does gravity play in this motion?

  1. Gravity pulls downward the whole time, slowing the ball as it goes up and speeding it up as it comes down. (correct answer)
  2. Gravity pushes upward while the ball rises, then switches direction and pulls downward after the ball stops.
  3. Gravity only acts at the top of the path, which is why the ball stops there.
  4. The ball falls back down mainly because the air above it is heavier than the air below it.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). In all cases, the observed motion results from gravitational force acting continuously on the object. When you jump, your legs push you upward giving you initial upward velocity, but gravity immediately begins pulling you downward with force F = mg—this downward force causes downward acceleration a = g ≈ 10 m/s² throughout your jump, which slows your upward motion (deceleration: gravity opposes upward velocity), brings you to a stop at the peak (velocity becomes zero), then accelerates you downward bringing you back to ground. The entire up-peak-down trajectory is shaped by constant gravitational force pulling downward the whole time. Choice A is correct because it appropriately connects gravitational force to observed trajectory or acceleration. Choice B incorrectly attributes the motion to a different force like air pressure or magnetism, when gravity is the actual cause of falling and curved paths; Choice C claims no force is needed for falling, missing that acceleration requires force and gravity provides that force (F = ma, falling has a so must have F); Choice D denies that gravity affects the motion, claiming ball falls due to its own nature, all missing that gravity is the force causing these motions. Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).

Question 18

A ball is thrown horizontally off a table. It keeps moving forward but also curves downward and hits the floor. Which explanation best connects gravity to the curved path?

  1. The ball curves downward because its forward speed runs out and turns into downward motion.
  2. The ball curves downward because gravity provides a continuous downward force (F=mgF=mg), giving it downward acceleration while it continues moving forward. (correct answer)
  3. The ball curves downward because the air pushes it downward more than it pushes it forward.
  4. The ball curves downward because gravity pulls it sideways toward the edge of the table.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). In all cases, the observed motion results from gravitational force acting continuously on the object. When you throw a ball horizontally, it would travel in a straight line if no forces acted (Newton's First Law), but gravity continuously pulls downward creating a downward acceleration while the ball maintains its forward velocity—the combination produces a curved path (parabola): the ball moves forward at constant speed (no horizontal force) while simultaneously accelerating downward (gravity pulls down), so it travels forward and down simultaneously, creating the characteristic curved trajectory we see in thrown objects. The curve is evidence of gravitational force acting throughout the flight. Choice B is correct because it correctly describes how gravity creates curved path by continuously pulling object down while it moves forward. Choice A reverses cause and effect, suggesting motion creates gravity instead of gravity creating the motion; Choice C incorrectly attributes the motion to a different force like air pressure or magnetism, when gravity is the actual cause of falling and curved paths; Choice D describes gravity effect incorrectly: pulling sideways or upward instead of downward toward Earth center. Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).

Question 19

A student throws a ball horizontally. Ignoring air resistance, which statement best describes how gravity affects the ball's motion while it is in the air?

Gravitational force: F=mgF = mg downward Acceleration: a=ga = g downward (constant near Earth's surface)​

  1. Gravity makes the ball speed up forward, so its horizontal velocity increases.
  2. Gravity provides a constant downward acceleration, changing the ball's vertical velocity and creating a curved path. (correct answer)
  3. Gravity only acts at the moment the ball leaves the hand; after that, no force acts on it.
  4. Gravity pushes upward on the ball, slowing its fall and keeping it in the air longer.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). When you throw a ball horizontally, it would travel in a straight line if no forces acted (Newton's First Law), but gravity continuously pulls downward creating a downward acceleration while the ball maintains its forward velocity—the combination produces a curved path (parabola): the ball moves forward at constant speed (no horizontal force) while simultaneously accelerating downward (gravity pulls down), so it travels forward and down simultaneously, creating the characteristic curved trajectory we see in thrown objects. Choice B is correct because it properly identifies gravity as providing constant downward acceleration that changes vertical velocity and creates the curved path. Choice A incorrectly states gravity makes the ball speed up forward/horizontally, when gravity only acts downward. Choice C incorrectly claims gravity only acts at the moment of release then stops, when gravity acts continuously. Choice D incorrectly states gravity pushes upward, when gravity always pulls downward toward Earth. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: projectile curves are caused by gravity (downward gravitational force combined with horizontal motion).

Question 20

A satellite moves sideways around Earth in a curved path and stays in orbit instead of flying off into space. What causes the satellite's path to curve around Earth?

  1. Earth's gravity pulls inward toward Earth's center, continuously changing the satellite's direction. (correct answer)
  2. The satellite's engines constantly push it sideways to make it curve.
  3. Gravity pushes outward, balancing space, so the satellite is held up.
  4. There is no force in space; the satellite curves because it is already moving in a circle.
Explanation: This question tests understanding that gravitational force causes the motion we observe in falling objects, projectiles, and orbits. Gravity is the force that: (1) makes dropped objects fall to the ground (gravitational force F = mg pulls downward → acceleration a = g downward → object speeds up as it falls), (2) makes thrown objects follow curved paths instead of straight lines (gravity continuously pulls downward even as object moves forward, bending trajectory into parabola), (3) keeps satellites and the Moon in orbit around Earth (gravity pulls inward toward Earth preventing straight-line motion, curving path into circle or ellipse), and (4) makes jumping temporary (gravity pulls you down as you go up, slowing your upward motion, stopping it at peak, and accelerating you downward on the way down). A satellite orbiting Earth is continuously falling toward Earth due to gravitational force, but it's also moving sideways (tangent to orbit) fast enough that as it falls, the curved surface of Earth falls away beneath it at the same rate—the result is a circular path where the satellite keeps falling but never gets closer to Earth (stable orbit). Choice A is correct because it properly identifies gravity as the inward force keeping satellite in orbit. Choice B attributes to engines, but orbits don't require thrust; Choice C describes gravity incorrectly as pushing outward; Choice D claims no force, but centripetal force is needed for curved motion, provided by gravity. Recognizing gravity as motion cause: whenever you see (1) objects falling or dropping (cause: gravity pulling down), (2) thrown objects curving down (cause: gravity bending straight-line motion), (3) satellites or Moon in orbit (cause: gravity pulling inward), (4) jumps being temporary (cause: gravity slowing upward motion and pulling back down), or (5) anything accelerating toward Earth/planet, you're seeing gravitational force in action—gravity is invisible but its effects on motion are very visible and predictable. Understanding that motion results from forces (not spontaneous) and identifying which force causes which motion is fundamental physics: falling is caused by gravity (not air, not object nature, but gravitational attraction between masses), projectile curves are caused by gravity (not air bending path, but downward gravitational force combined with horizontal motion), and orbits are caused by gravity (not mysterious orbit-maintaining force, but continuous gravitational attraction pulling object into curved path)—this force-causes-motion thinking explains all motion around us and allows predicting future motion (if you know forces, you can predict how objects will move using Newton's Laws).