AP Physics 1 Quiz: Circular Motion

What this quiz covers

This quiz focuses on Circular Motion, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Physics 1.

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.

Question 1

1. It doubles
2. It stays the same
3. It becomes four times as large (correct answer)
4. It becomes half as large

Explanation: This question tests understanding of how centripetal force depends on speed. The centripetal force formula is F = mv²/r, showing that force is proportional to the square of the speed. When speed doubles while mass and radius remain constant, the required inward force becomes four times as large (C). This quadratic relationship means that small increases in speed require much larger increases in centripetal force. The force doesn't just double (A) because of the v² term, and it certainly doesn't stay the same (B) or decrease (D). Remember that centripetal force increases with the square of speed - doubling speed quadruples the required force.

Question 2

1. Upward, because the mass is about to move downward
2. Downward, toward the center of the circle (correct answer)
3. Horizontally, in the direction of motion
4. Zero, because the speed is constant

Explanation: This question tests understanding of acceleration in vertical circular motion. At the top of a vertical circle where the mass moves horizontally, the acceleration must point toward the center of the circle, which is downward (B). This centripetal acceleration is responsible for changing the direction of the velocity vector from horizontal to downward as the mass continues its circular path. The acceleration is not upward (A) - that would cause the mass to slow down and reverse direction, and it cannot be horizontal (C) as that would not curve the path downward. Remember that in circular motion, acceleration always points toward the center regardless of whether the circle is horizontal or vertical.

Question 3

1. North, because the speed is constant
2. West, toward the center of the circle (correct answer)
3. East, away from the center due to centrifugal force
4. South, opposite the velocity

Explanation: This question tests understanding of centripetal acceleration direction in circular motion. For an object moving in a circle at constant speed, the acceleration is always centripetal, meaning it points toward the center of the circle. When the car's velocity is north at a particular instant, and the car is turning in a circular path, the acceleration must be perpendicular to the velocity and point toward the center. Since the velocity is north, the acceleration must point west (toward the center). Choice C incorrectly suggests acceleration away from the center due to "centrifugal force," which is not a real force in an inertial reference frame. The strategy is to identify the velocity direction, then determine which way is toward the center of the circle at that instant.

Question 4

1. The skater’s inertia pulling outward
2. The rope’s tension pulling toward the post (correct answer)
3. A separate centripetal force in addition to tension
4. The skater’s velocity pushing the skater inward

Explanation: This question assesses understanding of the source of centripetal force in uniform circular motion. Centripetal acceleration toward the center is provided by a net force in that direction, such as tension in this case. The tension in the rope pulls inward, serving as the centripetal force to keep the skater circling. No outward forces act; the motion is maintained by this inward net force. Choice A is a distractor attributing the force to inertia pulling outward, which confuses the tendency to move tangentially with an actual force. For identifying centripetal forces, examine real forces acting on the object and determine which provides the inward component.

Question 5

1. Tangent to the circle, in the direction of motion
2. Radially outward, away from the center
3. Toward the center of the circle (northward) (correct answer)
4. Zero, because the speed is constant

Explanation: This question assesses understanding of net force direction in horizontal uniform circular motion. The net force provides centripetal acceleration toward the center, maintaining the circular path. This force is inward, countering the tendency to move in a straight line. At the southmost point, the center is northward, so net force points north. Choice B is a distractor implying an outward force, which might confuse centripetal with centrifugal concepts. Always remember that in circular motion problems, the net force direction is toward the center, helping to identify it regardless of the setup.

Question 6

1. Tangent to the orbit in the direction of motion
2. Radially outward due to the satellite’s inertia
3. Toward Earth’s center (radially inward) (correct answer)
4. Zero, because the satellite’s speed is constant

Explanation: This question tests understanding of orbital motion and centripetal force. A satellite in circular orbit experiences only Earth's gravitational force, which points toward Earth's center (C), providing the centripetal acceleration needed for circular motion. This inward force continuously changes the satellite's velocity direction without changing its speed, maintaining the circular orbit. The force cannot be tangential (A) as this would change the orbital speed, and there's no outward force due to inertia (B) - inertia simply means the satellite would move in a straight line if no force acted on it. The key insight is that gravity serves as the centripetal force in orbital motion, always pointing toward the center of the orbit.

Question 7

1. North, because the velocity is north
2. South, opposite the motion
3. West, away from the center
4. East, toward the center of the circle (correct answer)

Explanation: This question assesses understanding of acceleration direction in circular motion on a path. Centripetal acceleration points toward the center, perpendicular to the tangential velocity. The net force causing this is inward, maintaining the curve. At the westernmost point moving north, the center is eastward, so acceleration is east. Choice C is a distractor implying westward away from the center, possibly confusing with centrifugal ideas. To determine directions in such scenarios, sketch the position and velocity, then point acceleration inward toward the circle's center.

Question 8

1. Toward the west (toward the center of the circle) (correct answer)
2. Toward the east (away from the center)
3. Toward the north (tangent to the circle at that point)
4. Zero, because the velocity is not changing in magnitude

Explanation: This question tests understanding of net force direction in horizontal circular motion. When the stopper is at the easternmost point of its circular path, the net force must point toward the center of the circle, which is west (A). This centripetal force is what causes the continuous change in the velocity's direction, keeping the stopper moving in a circle rather than a straight line. The force cannot point away from the center (B) as this would cause the stopper to spiral outward, and it cannot be tangential (C) as this would change the speed rather than just the direction. The strategy is to always identify the center of the circular path and remember that net force points from the object toward that center.

Question 9

1. Directly outward from the center
2. Along the tangent to the circle at the release point (correct answer)
3. Directly toward the center
4. It remains at rest momentarily due to inertia

Explanation: This question tests understanding of motion after centripetal force removal. While the ball moves in a circle, the string provides centripetal force toward the center, continuously changing the ball's direction. When the string breaks, this inward force disappears, and by Newton's first law, the ball continues with the velocity it had at the instant of release. Since velocity in circular motion is always tangent to the circle, the ball moves along the tangent at the release point. Choice A incorrectly suggests outward motion, confusing the absence of inward force with the presence of an outward force. The strategy is to recognize that objects continue with their instantaneous velocity when forces are removed, and velocity in circular motion is always tangential.

Question 10

1. Upward, toward the center of the circle (correct answer)
2. Downward, because weight is largest there
3. Horizontal, in the direction of motion
4. Radially outward, due to centrifugal force

Explanation: This question tests understanding of net force direction in vertical circular motion. At the bottom of a vertical circle where the stone moves horizontally, the center is directly above, so centripetal acceleration and net force must point upward. The net force is the vector sum of all forces: weight (downward) and tension (upward), with tension being larger to provide the net upward force needed for circular motion. This net upward force provides the centripetal acceleration that curves the stone's path upward. Choice D incorrectly invokes "centrifugal force," which is not a real force in an inertial reference frame. The key is to identify the center's location relative to the object and remember that net force always points toward the center in circular motion.

Question 11

1. East, because acceleration must be in the direction of velocity
2. West, opposite the velocity
3. Toward the center of the circle (perpendicular to the velocity) (correct answer)
4. Away from the center due to centrifugal force

Explanation: This question tests understanding of centripetal acceleration direction relative to velocity. In circular motion at constant speed, centripetal acceleration always points toward the center of the circle, perpendicular to the instantaneous velocity. When the velocity is east, the acceleration must point in a direction perpendicular to east and toward the center—this could be north, south, or any direction perpendicular to east depending on the object's position in the circle. The acceleration is never in the direction of velocity (which would change speed) or away from center. Choice D incorrectly invokes "centrifugal force," which doesn't exist in inertial frames. The strategy is to remember that centripetal acceleration is always perpendicular to velocity and points toward the center.

Question 12

1. It is zero because the speed is constant
2. It points tangent to the orbit, in the direction of motion
3. It points toward Earth’s center due to gravity (correct answer)
4. It points away from Earth due to centrifugal force

Explanation: This question tests understanding of orbital motion as circular motion. A satellite in circular orbit experiences centripetal acceleration toward Earth's center, requiring a net force in the same direction. This force is provided by Earth's gravity, which always points toward Earth's center. Even though the satellite's speed is constant, its velocity direction continuously changes, requiring this inward force. Choice D incorrectly mentions "centrifugal force," which is not a real force but a fictitious effect in rotating reference frames. The strategy is to recognize that orbital motion is circular motion where gravity provides the centripetal force, always pointing toward the central body.

Question 13

1. Upward, toward the center of the loop (correct answer)
2. Downward, because gravity points down
3. Forward along the track, because speed is constant
4. Outward from the loop due to centrifugal force

Explanation: This question assesses understanding of centripetal acceleration in uniform circular motion. In uniform circular motion, the centripetal acceleration always points toward the center of the circle, changing the direction of the velocity while keeping the speed constant. The net force, according to Newton's second law, must also point toward the center to provide this acceleration. At the bottom of the loop, the center is upward, so acceleration is upward. Choice D is incorrect because centrifugal force is not real and does not determine acceleration direction. To determine acceleration in loops, locate the center relative to the position and direct acceleration inward.

Question 14

1. East, in the direction of motion
2. Toward the center of the circle (west) (correct answer)
3. Upward, because the string pulls up
4. Away from the center due to a centrifugal force

Explanation: This question assesses understanding of centripetal acceleration in uniform circular motion. In uniform circular motion, the centripetal acceleration always points toward the center of the circle, changing the direction of the velocity while keeping the speed constant. The net force, according to Newton's second law, must also point toward the center to provide this acceleration. For the ball at the east point, the center is to the west, so both acceleration and net force are westward. Choice D is incorrect because centrifugal force is a fictitious force in non-inertial frames and does not explain acceleration in the inertial frame. To analyze direction in circular motion, identify the center and remember acceleration is radial inward.

Question 15

1. Toward the center of the circle (south) (correct answer)
2. North, because the car is moving north there
3. Tangent to the track, in the direction of motion
4. Outward from the center because circular motion requires an outward force

Explanation: This question assesses understanding of net force in uniform circular motion. In uniform circular motion, the centripetal acceleration always points toward the center of the circle, changing the direction of the velocity while keeping the speed constant. The net force, according to Newton's second law, must also point toward the center to provide this acceleration. For the car at the northmost point, the center is to the south, so the net force points south. Choice D is incorrect because there is no outward force required; the inward net force causes the centripetal acceleration. To solve circular motion problems, always remember that the net force must provide the centripetal acceleration towards the center.

Question 16

1. It stays the same
2. It doubles
3. It quadruples (correct answer)
4. It becomes half as large

Explanation: This question tests understanding of how centripetal force depends on speed. The centripetal force formula is F = mv²/r, showing that force is proportional to the square of speed. When speed doubles, the required centripetal force increases by a factor of 2² = 4, or quadruples. This quadratic relationship means small speed increases require much larger force increases. Choice B incorrectly assumes a linear relationship between force and speed. To solve problems about changing circular motion parameters, use the centripetal force formula to identify which variables change and how they affect the required force.

Question 17

1. Tangential, in the direction of the satellite’s velocity
2. Radially outward, away from Earth, to prevent falling
3. Zero because the speed does not change
4. Radially inward, toward Earth’s center (correct answer)

Explanation: This question tests understanding of orbital motion as circular motion. A satellite in circular orbit requires centripetal acceleration directed toward Earth's center to maintain its curved path. By Newton's second law, the net force must point in the same direction—radially inward toward Earth. This force is gravity, which always points toward Earth's center. Choice C incorrectly assumes that constant speed means zero acceleration, ignoring that velocity direction changes continuously in circular motion. When analyzing orbital motion, recognize it as circular motion where gravity provides the required centripetal force directed toward the central body.

Question 18

1. Tangent to the circle in the direction of motion
2. Radially inward, toward the center of the circle (correct answer)
3. Radially outward, away from the center due to a centrifugal force
4. Zero because the speed is constant

Explanation: This question tests understanding of acceleration direction in circular motion. When an object moves in a circle at constant speed, it continuously changes direction, requiring an acceleration toward the center of the circle. This centripetal acceleration points radially inward at every point on the path, including point P. The common misconception in choice D assumes that constant speed means zero acceleration, but acceleration includes changes in direction, not just speed. For any circular motion problem, remember that acceleration always points toward the center when speed is constant.

Question 19

1. Along the string, toward the student’s hand (the center) (correct answer)
2. Along the direction of motion because speed is constant
3. Opposite the direction of motion to keep the speed constant
4. Radially outward due to centrifugal force from the motion

Explanation: This question addresses acceleration direction when tension provides the centripetal force. In horizontal circular motion at constant speed, the stopper requires centripetal acceleration directed toward the center (the student's hand). Since string tension is the only horizontal force, it must point along the string toward the center, causing acceleration in the same direction. The velocity is tangent to the circle, but acceleration points radially inward to change the velocity's direction. Choice D incorrectly references centrifugal force, which is not a real force in an inertial frame. For circular motion problems, identify which real force provides the centripetal acceleration—here it's the string tension pulling inward.

Question 20

1. It points east, because the yo-yo is farthest east
2. It points tangent to the circle, in the direction of motion
3. It points toward the center of the circle (westward) (correct answer)
4. It points outward (eastward) due to a centrifugal force

Explanation: This question assesses understanding of net force in horizontal circular motion with a string. The net force is centripetal, pointing toward the center to cause inward acceleration. Tension provides this force, pulling the yo-yo inward. At the easternmost point, the center is westward, so net force is westward. Choice D is a distractor claiming an outward centrifugal force, which is incorrect in the inertial frame. A useful strategy is to ignore fictitious forces and always direct the net force toward the center in circular motion analyses.