- When a person jumps off the ground, their legs apply a force to the Earth. According to Newton's Third Law of Motion, what is the corresponding reaction force?
Answer: The Earth exerting an equal and opposite upward force on the person.
Newton's Third Law states that for every action, there is an equal and opposite reaction. When analyzing force pairs, you need to identify two objects exerting forces directly on each other.
When you jump, your legs push down on the Earth with a certain force. According to Newton's Third Law, the Earth must simultaneously push back up on you with an equal force in the opposite direction. This upward force from the Earth is what actually propels you into the air. The action-reaction pair consists of: (1) your legs pushing down on Earth, and (2) Earth pushing up on your legs.
Answer D correctly identifies this reaction force - the Earth exerting an equal and opposite upward force on the person.
Answer A describes gravity, which is a separate force that acts on you throughout the jump. Gravity isn't the reaction to your legs pushing on Earth; it's an independent attractive force between you and Earth that exists whether you're jumping or not.
Answer B refers to air resistance, which only occurs as you move through the air and isn't directly related to your legs pushing off the ground. Air resistance is a friction-like force, not part of the action-reaction pair from your jump.
Answer C describes the internal force of your muscles, but Newton's Third Law deals with forces between different objects, not internal forces within your body. Your muscle contraction is what enables the push, but isn't the reaction force.
Remember: Newton's Third Law always involves two different objects pushing or pulling on each other with equal and opposite forces simultaneously.
- A 2,500 kg truck and a 1,000 kg car are both traveling at the same velocity down a highway. Which statement correctly compares their momentum?
Answer: The truck has a greater momentum because it has a greater mass.
When you encounter momentum questions, remember that momentum depends on both mass and velocity. Momentum is calculated using the formula p=mv, where p is momentum, m is mass, and v is velocity.
Since both vehicles travel at the same velocity, you can compare their momentum by looking at their masses. The truck has 2,500 kg of mass while the car has only 1,000 kg. Using the momentum formula, the truck's momentum will be 2.5 times greater than the car's momentum because it has 2.5 times more mass.
Answer A correctly identifies that the truck has greater momentum due to its greater mass. This is the fundamental relationship in the momentum equation - when velocity is constant, momentum is directly proportional to mass.
Answer B incorrectly suggests the car has greater momentum and introduces irrelevant factors. While cars may be more agile due to less mass, agility doesn't determine momentum. Less mass actually means less momentum when velocity is equal.
Answer C falls into a common trap by assuming equal velocities mean equal momentum. This ignores the mass component entirely. Momentum depends on both factors - you can't ignore mass just because velocities are the same.
Answer D misunderstands the basic concept of momentum. Moving objects always have momentum regardless of external forces. External forces change momentum, but they're not required for momentum to exist.
Remember this pattern: when comparing momentum at equal velocities, the object with greater mass always has greater momentum. Focus on the p=mv relationship and consider both variables. - What is the momentum of a 1,200 kg car that is at rest in a parking lot?
Answer: 0 kg·m/s
When you encounter momentum problems, remember that momentum measures how much motion an object has. Momentum is calculated using the formula p=mv, where p is momentum, m is mass, and v is velocity.
In this problem, you have a 1,200 kg car that is "at rest" in a parking lot. The key phrase here is "at rest," which means the car's velocity is zero. When you substitute into the momentum formula: p=(1,200 kg)(0 m/s)=0 kg⋅m/s. Any number multiplied by zero equals zero, so the momentum is zero.
Let's examine why the other answers are incorrect. Answer B (1,200 kg·m/s) incorrectly assumes the car has some velocity, perhaps by mistaking mass for momentum or forgetting that velocity is zero. Answer C (approximately 11,760 kg·m/s due to gravity) represents a fundamental misunderstanding—while gravity acts on the car, it doesn't contribute to momentum when the car isn't moving. The number 11,760 appears to come from multiplying mass by gravitational acceleration (9.8 m/s²), but that would give you weight or force, not momentum. Answer D (cannot be determined without potential energy) confuses momentum with energy concepts—potential energy is completely irrelevant to calculating momentum.
For momentum problems on the GED, always identify the object's velocity first. If something is at rest, stopped, or stationary, its momentum is automatically zero regardless of its mass. Don't let large masses fool you into thinking there must be momentum without motion. - A person pushes two boxes across a frictionless surface. Box A has a mass of 10 kg and Box B has a mass of 20 kg. If the person applies the same constant force to each box, what will be the result?
Answer: Box A will accelerate at a greater rate than Box B.
When you encounter physics problems involving force and motion, think about Newton's Second Law: Force equals mass times acceleration, or F=ma. This fundamental relationship tells us how force, mass, and acceleration are connected.
Since the same constant force is applied to both boxes, we can rearrange the equation to solve for acceleration: a=mF. This shows that acceleration is inversely proportional to mass when force remains constant. Box A (10 kg) will experience acceleration of aA=10F, while Box B (20 kg) will experience acceleration of aB=20F. Since Box A has half the mass of Box B, it will accelerate twice as fast. Therefore, Box A will accelerate at a greater rate than Box B, making answer B correct.
Answer A is wrong because it suggests the heavier object accelerates faster, which contradicts Newton's Second Law. Answer C represents a common misconception—while the applied force is identical, the resulting acceleration depends on both force AND mass. Equal forces don't produce equal accelerations when masses differ. Answer D is incorrect because objects can definitely accelerate on frictionless surfaces; friction isn't required for acceleration to occur when a force is applied.
Remember this key relationship: when the same force acts on objects of different masses, the lighter object always accelerates more. On the GED, physics problems often test whether you can apply Newton's Second Law correctly, so practice rearranging F=ma to solve for the unknown variable. - A swimmer pushes backward on the water to move forward. Which statement best describes this situation?
Answer: The force of the swimmer on the water and the force of the water on the swimmer are an action-reaction pair.
This question tests your understanding of Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. When you see scenarios involving pushes, pulls, or forces between two objects, think about how forces always come in pairs.
When a swimmer pushes backward against the water, they exert a force on the water. According to Newton's Third Law, the water simultaneously exerts an equal and opposite force on the swimmer, pushing them forward. These two forces—the swimmer's push on the water and the water's push on the swimmer—form an action-reaction pair. This is exactly what choice C describes, making it correct.
Let's examine why the other answers miss the mark. Choice A incorrectly focuses on inertia, which is an object's tendency to resist changes in motion, but doesn't explain the mechanism that creates forward motion in the first place. Choice B suggests mass difference is the key factor, but Newton's Third Law applies regardless of the masses involved—the forces are always equal and opposite. Choice D brings up gravity and buoyancy, but while buoyancy does make swimming easier by reducing the swimmer's effective weight, it doesn't explain how the swimmer generates forward motion.
For GED science questions involving motion and forces, always look for Newton's laws in action. When you see pushing, pulling, or objects interacting, ask yourself: "What are the force pairs involved?" This approach will help you identify action-reaction relationships and avoid getting distracted by other physics concepts that might be present but aren't the main focus.
- An astronaut has a mass of 80 kg on Earth. The Moon's gravitational acceleration is approximately one-sixth of Earth's. What can be concluded about the astronaut's mass and weight on the Moon?
Answer: Her mass will be 80 kg, and her weight will be significantly less than on Earth.
When you encounter questions about mass and weight in different gravitational environments, remember that mass and weight are fundamentally different properties. Mass is the amount of matter in an object and remains constant everywhere in the universe. Weight is the gravitational force acting on that mass and depends on the local gravitational field strength.
The astronaut's mass stays at 80 kg regardless of location because the amount of matter in her body doesn't change. However, her weight changes dramatically. Weight is calculated as W=mg, where m is mass and g is gravitational acceleration. On Earth, if her weight is WEarth=80 kg×9.8 m/s2, then on the Moon it becomes WMoon=80 kg×(9.8/6) m/s2, which is one-sixth her Earth weight.
Answer A incorrectly assumes weight remains constant across different gravitational fields. Answer B makes the classic error of confusing mass with weight, suggesting her mass decreases to 13.3 kg (which would be her weight in kg-force units). Answer C compounds this error by claiming both her mass changes AND her weight stays the same, violating basic physics principles.
Answer D correctly identifies that mass remains constant at 80 kg while weight decreases significantly due to the Moon's weaker gravity.
Study tip: Remember the phrase "mass is intrinsic, weight is extrinsic." Mass never changes based on location, but weight always depends on local gravity. This distinction appears frequently on science exams. - Compared with the 100 kg skater's, the 50 kg skater's acceleration during a push-off on ice is
Answer: twice as large, opposite
During a push-off, the forces on the two skaters are equal in size and opposite in direction. Acceleration equals force divided by mass, so the 50 kg skater, having half the mass, accelerates twice as much as the 100 kg skater, in the opposite direction. The tempting mistake is thinking equal forces give equal accelerations, but the smaller mass changes the result.
- A toy car is moving in a circle at a constant speed. What is the direction of the net force acting on the car?
Answer: Toward the center of the circle.
When you see questions about objects moving in circular paths, you're dealing with circular motion and centripetal force. The key insight is that even when speed is constant, the velocity is constantly changing because direction is changing.
For any object moving in a circle, there must be a net force pointing toward the center of the circle. This centripetal force is what keeps the object moving in its circular path rather than flying off in a straight line. In the case of the toy car, this inward force could come from friction between the wheels and the surface, tension in a string, or the banking of a track.
Choice A is incorrect because a force in the direction of motion (tangent to the circle) would change the car's speed, not its direction. Since the speed is constant, the net force cannot be tangential.
Choice C is wrong because a force pointing away from the center would cause the object to spiral outward, not maintain a circular path.
Choice D represents a common misconception. Many students think that constant speed means no acceleration and therefore no net force. However, acceleration includes changes in direction, not just changes in speed. Since the car is constantly changing direction as it moves in a circle, it is accelerating and must have a net force acting on it.
Remember: In circular motion problems, always look for the force that points toward the center of the circle. Constant speed doesn't mean no force—it means the force is perpendicular to the motion.
- Which of the following objects has the greatest inertia?
Answer: A large, stationary cargo ship.
Inertia questions test your understanding that inertia depends solely on an object's mass, not its motion or position. Inertia is an object's resistance to changes in its state of motion—the more massive an object, the greater its inertia.
A large, stationary cargo ship (B) has the greatest inertia because it has by far the most mass among these options. Cargo ships typically weigh thousands of tons, making them extremely resistant to changes in motion. This is why massive ships need powerful tugboats to start moving and require long distances to stop.
Let's examine why the other choices have less inertia: A feather (A) has very little mass, so it has minimal inertia despite appearing to "resist" motion due to air resistance—that's a force effect, not inertia. A student sitting at a desk (C) has moderate mass compared to the other objects, giving them moderate inertia, but nowhere near that of a massive ship. A baseball traveling at 90 mph (D) is the trickiest distractor because its high speed might seem important, but inertia depends only on mass, not velocity. The baseball's small mass means it has relatively little inertia regardless of how fast it's moving.
Remember this key principle: when comparing inertia, ignore motion and focus entirely on mass. The most massive object always has the greatest inertia. On the GED, watch for questions that try to confuse you by mentioning speed or describing objects in motion—inertia is purely about mass.
- A force is applied to an object, but the object does not move. Which statement about the work done on the object is correct?
Answer: No work is done because the object did not move a distance.
When you encounter physics problems involving forces and work, remember that work has a very specific scientific definition that differs from everyday usage. Work is defined as force applied over a distance: W=F×d×cos(θ), where F is force, d is displacement, and θ is the angle between them.
The correct answer is D because work requires both force AND displacement. Since the object doesn't move (displacement = 0), no work is done regardless of how much force is applied. Even if you push against a brick wall with tremendous force, you do zero work on the wall if it doesn't budge.
Choice A incorrectly assumes that applying force automatically means work is done. This reflects the common misconception that effort equals work in physics terms. Choice B suggests negative work occurs due to resistance, but negative work specifically refers to force opposing motion (like friction slowing a sliding object), not simply preventing motion. Choice C focuses on the object's mass, but mass doesn't directly determine work done—only force and displacement matter in the work equation.
For GED Science questions about work and energy, always check whether the object actually moves. If there's no displacement, there's no work, period. This is a frequent test trap because it conflicts with our intuitive sense that exerting force should count as "doing work." Remember: in physics, work = force × distance, and zero distance always means zero work. - A student walks 10 meters north and then 10 meters south, returning to her starting point. The entire trip takes 20 seconds. What is the student's average velocity for the entire trip?
Answer: 0 m/s
When you encounter physics problems involving motion, it's crucial to distinguish between speed and velocity. Speed measures how fast something moves regardless of direction, while velocity includes both speed and direction—making it a vector quantity.
To find average velocity, you use the formula: Average velocity=total timetotal displacement. Notice this uses displacement, not distance. Displacement measures the straight-line distance between starting and ending positions, considering direction.
In this problem, the student walks 10 meters north, then 10 meters south, returning to her starting point. Her total displacement is zero—she ends up exactly where she began. With zero displacement and 20 seconds of travel time: Average velocity=20 seconds0 meters=0 m/s
Choice A (0 m/s) is correct because the student's displacement is zero.
Choice B (0.5 m/s) incorrectly uses only half the total distance (10 meters) divided by time.
Choice C (1 m/s) represents a common error—calculating average speed instead of velocity by using total distance (20 meters) divided by time (20 seconds).
Choice D (200 m/s) appears to reverse the calculation, putting time in the numerator instead of displacement.
Remember this key distinction: if an object returns to its starting point, its displacement is always zero, making its average velocity zero regardless of how far it actually traveled. Focus on the endpoint, not the journey. - A car moves at a steady 25 m/s on a level road; engine force is 4,000 N forward. What is the total resistive force?
Answer: 4,000 N backward
At steady speed there is zero acceleration, so the forward and resistive forces must balance. The engine supplies 4,000 N forward, so total resistance is exactly 4,000 N backward. If you chose less than 4,000 N, that would leave a net forward force and the car would speed up.
- A skydiver jumps from a plane. After some time, she reaches a constant downward speed, known as terminal velocity. This occurs when:
Answer: the upward force of air resistance equals the downward force of gravity.
When objects fall through air, they experience two competing forces that determine their motion. Understanding how these forces interact is key to solving terminal velocity problems.
As a skydiver falls, gravity pulls her downward with a constant force equal to her weight. However, air resistance pushes upward against her motion, and this resistance force increases as her speed increases. Initially, gravity is stronger than air resistance, so she accelerates downward. But as she falls faster, air resistance grows until it eventually equals the gravitational force. At this point, the net force becomes zero, acceleration stops, and she continues falling at a constant speed—terminal velocity.
The correct answer is C because terminal velocity occurs precisely when the upward air resistance force equals the downward gravitational force, creating equilibrium.
Option A is wrong because gravity never becomes zero—it remains constant throughout the fall. Option B incorrectly relates mass and volume, which have nothing to do with force balance during terminal velocity. The relationship between mass and volume determines density, not terminal velocity. Option D is incorrect because terminal velocity depends on force balance, not a predetermined time period. Different objects with different shapes, masses, and surface areas reach terminal velocity at different times.
Remember this key principle: terminal velocity always results from balanced forces, not from gravity disappearing or time limits. Look for force equilibrium in any terminal velocity question on the GED.
- A satellite orbits the Earth at a high altitude. What is the main force that keeps the satellite in its orbit?
Answer: The gravitational force between the satellite and the Earth.
When you encounter questions about orbital mechanics, focus on identifying the fundamental forces at work. Satellites stay in orbit because they're constantly falling toward Earth while moving fast enough sideways that they keep missing it.
The gravitational force between the satellite and Earth provides the centripetal force needed to keep the satellite moving in a circular path. This attractive force continuously pulls the satellite toward Earth's center, causing its straight-line motion to curve into an orbit. Without gravity, the satellite would fly off in a straight line into space.
Let's examine why the other options are incorrect:
Option B suggests engine thrust keeps the satellite moving. Once in orbit, satellites don't need continuous thrust - they maintain their speed due to Newton's first law of motion (objects in motion stay in motion). Engines are only used for occasional adjustments.
Option C mentions centrifugal force balancing weight. Centrifugal force isn't a real force - it's what you feel when you're in a rotating reference frame. The actual physics involves centripetal force (provided by gravity) pulling inward.
Option D claims the initial launch force maintains the orbit. The launch only gets the satellite to the right speed and altitude. Once there, the launch force is gone, and gravity alone maintains the orbital motion.
Remember this key principle: in orbital mechanics, gravity isn't the enemy of staying in space - it's what makes orbits possible. When you see questions about satellites, planets, or moons, always consider gravity as the force providing the centripetal acceleration needed for circular motion.
- A passenger in a car is not wearing a seatbelt. If the car suddenly brakes, the passenger will lurch forward. Which principle best explains this phenomenon?
Answer: Newton's First Law of Motion, which describes an object's inertia.
When you encounter questions about objects in motion and what happens when forces change, you're dealing with Newton's Laws of Motion. The key is identifying which specific law explains the observed behavior.
In this scenario, the passenger continues moving forward at the car's original speed even after the car brakes. This perfectly demonstrates Newton's First Law of Motion, also known as the law of inertia. This law states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an external force. The passenger was moving with the car, and when the car suddenly stops, the passenger's body has no force acting on it to make it stop, so it continues moving forward.
Looking at the wrong answers: Choice A, Newton's Third Law, deals with action-reaction force pairs (like when you push a wall, it pushes back with equal force), which isn't what's happening here. Choice B, conservation of energy, involves energy changing from one form to another, but this question is about motion continuing due to inertia, not energy transformation. Choice D, universal gravitation, explains the attractive force between masses like Earth and objects, but gravity isn't the primary factor causing the forward motion.
For GED Science questions about motion, remember that inertia problems typically involve objects continuing their motion when external conditions change suddenly. Look for scenarios where something keeps moving in the same direction when forces are removed or changed - that's usually Newton's First Law.
- Ignoring air resistance, a 1 kg stone and a 10 kg boulder are dropped from the top of a tall building at the same time. Which of the following will happen?
Answer: Both will hit the ground at the same time because they have the same acceleration.
When you encounter questions about falling objects, you're dealing with one of physics' most fundamental principles: gravity affects all objects equally, regardless of their mass.
The key insight here is that gravitational acceleration is constant at approximately 9.8 m/s² near Earth's surface for all objects. This means both the stone and boulder experience the same downward acceleration, causing them to fall at identical rates and hit the ground simultaneously. This counterintuitive result was famously demonstrated by Galileo and explains why astronauts on the moon could drop a feather and hammer that landed together in the airless environment.
Looking at the incorrect choices: Choice B reflects the common misconception that lighter objects should fall faster because they have "less to carry." In reality, while lighter objects have less gravitational force acting on them, they also have less mass to accelerate, so these effects cancel out perfectly. Choice C represents the opposite misconception—that heavier objects fall faster because gravity pulls them harder. Again, the increased gravitational force is exactly balanced by the increased mass. Choice D is wrong because falling objects accelerate continuously rather than falling at constant velocity; they start from rest and speed up at 9.8 m/s² every second.
For GED science questions about motion, remember that mass doesn't affect how fast objects fall in a vacuum. When you see "ignoring air resistance," expect both objects to behave identically regardless of their weight—this phrase is your clue that the answer involves equal acceleration due to gravity.
- A person applies a constant horizontal force to a heavy crate, but the crate does not move. What is the primary reason the crate remains stationary?
Answer: The force of static friction between the crate and the floor is equal and opposite to the applied force.
When you encounter a question about forces and motion, think about Newton's laws and how forces interact to create equilibrium or movement.
In this scenario, the crate remains stationary because all forces acting on it are balanced. Since the person applies a horizontal force but the crate doesn't move, there must be an equal and opposite force preventing motion. This opposing force is static friction between the crate and floor, which automatically adjusts to match the applied force up to its maximum limit. The correct answer is C because static friction provides exactly the right amount of resistance to keep the crate in equilibrium.
Let's examine why the other options are incorrect. Option A confuses vertical and horizontal forces – gravity acts downward while the applied force acts horizontally, so they don't directly oppose each other. Option B misapplies the concept of inertia. While inertia explains why objects at rest tend to stay at rest, it's not a force that actively resists motion; rather, it's friction that provides the actual opposing force. Option D incorrectly relates horizontal force to vertical weight. The person doesn't need to overcome the crate's weight to move it horizontally – they need to overcome friction.
For GED Science questions involving forces, remember that when an object remains stationary despite an applied force, look for the specific force that's providing resistance. Static friction is often the answer when dealing with objects on surfaces, as it's the force that prevents sliding motion.
- Which of the following scenarios is a clear example of acceleration?
Answer: A satellite orbiting Earth at a constant speed of 17,000 mph.
When you encounter questions about acceleration, remember that acceleration isn't just about speeding up—it's any change in velocity, which includes changes in speed OR direction.
The satellite in option D is the correct answer because even though it maintains a constant speed of 17,000 mph, it's constantly changing direction as it follows its circular orbital path around Earth. Since velocity is a vector quantity (having both magnitude and direction), any change in direction constitutes acceleration. This type is called centripetal acceleration, where the object accelerates toward the center of the circular path.
Let's examine why the other options don't show acceleration. Option A describes a motionless helicopter—with zero velocity and no change in motion, there's no acceleration. Option B shows a train moving in a straight line at constant speed, meaning both the magnitude and direction of velocity remain unchanged, so acceleration is zero. Option C presents a stationary book, which like the helicopter, has no motion and therefore no acceleration.
The key trap here is assuming acceleration only occurs when objects speed up or slow down. Many students incorrectly eliminate option D because they focus on the "constant speed" and miss that the direction is continuously changing.
Remember this pattern: any object moving in a curved path—whether it's a car turning a corner, a planet orbiting the sun, or a ball swinging on a string—is accelerating due to its changing direction, even if its speed stays the same.
- If a net force of 30 Newtons is applied to a 5 kg object that is free to move, what will be the object's acceleration?
Answer: 6 m/s²
When you encounter physics problems involving force, mass, and acceleration, you're working with Newton's Second Law of Motion: F=ma, where F is force in Newtons, m is mass in kilograms, and a is acceleration in meters per second squared.
To find acceleration, you rearrange the formula to solve for a: a=mF. With a net force of 30 N applied to a 5 kg object, you calculate: a=5 kg30 N=6 m/s2. This confirms that A) 6 m/s² is correct.
Looking at the wrong answers: B) 25 m/s² likely comes from subtracting mass from force (30 - 5 = 25), which has no physical meaning in Newton's Second Law. C) 35 m/s² results from adding force and mass (30 + 5 = 35), another mathematically meaningless operation for this relationship. D) 150 m/s² comes from multiplying force by mass (30 × 5 = 150), which actually gives you the wrong formula—this would be solving for F given F and m, resulting in F2/m.
For GED Science success, always identify the correct physics formula first, then carefully rearrange it to solve for the unknown variable. Newton's Second Law problems are common, so memorize F=ma and practice rearranging it. Double-check that your final answer has the correct units—acceleration should always be in m/s², not just a number. - A rocket propels itself upward by expelling hot gases from its engine. Which statement correctly describes the forces involved according to Newton's Third Law?
Answer: The force of the gases pushing on the rocket is equal in magnitude and opposite in direction to the force of the rocket pushing on the gases.
When you encounter rocket propulsion questions, you're dealing with Newton's Third Law of Motion: for every action, there's an equal and opposite reaction. The key insight is that rockets don't need air to push against—they create their own reaction force.
In rocket propulsion, the rocket engine pushes hot gases downward and backward with tremendous force. According to Newton's Third Law, those gases simultaneously push back on the rocket with exactly the same magnitude of force, but in the opposite direction (upward). This reaction force is what propels the rocket forward. The forces form what physicists call an "action-reaction pair"—they're always equal in magnitude and opposite in direction.
Choice A correctly describes this relationship. The rocket pushes on the gases with force X downward, and the gases push on the rocket with force X upward.
Choice B violates Newton's Third Law by suggesting unequal forces. Action-reaction pairs are always equal in magnitude—this is a fundamental principle.
Choice C reflects a common misconception that rockets need air to push against. This is wrong because rockets work perfectly in the vacuum of space where there's no air. The rocket creates its own reaction mass through expelled gases.
Choice D confuses two completely different forces. Gravity pulls the rocket downward, while engine thrust pushes it upward. These aren't action-reaction pairs—they're separate forces acting on the same object.
Remember: Newton's Third Law pairs always involve two different objects acting on each other with equal and opposite forces. Look for this reciprocal relationship in physics problems.
