AP Physics 1 Quiz: Forces And Free Body Diagrams

What this quiz covers

This quiz focuses on Forces And Free Body Diagrams, 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. Weight
2. Normal force
3. Static friction (correct answer)
4. A force in the direction of motion

Explanation: This question assesses understanding of forces providing centripetal acceleration in circular motion on flat surfaces. Select the car as the system and identify forces causing the inward acceleration. Static friction between tires and road acts toward the center of the curve, providing the necessary horizontal force for turning at constant speed. Other forces like weight and normal are vertical and do not contribute horizontally. Choice D is incorrect because there is no 'force in the direction of motion'; centripetal force is perpendicular to tangential velocity. A key strategy is to recognize that centripetal force comes from real forces like friction, directed inward, and separate it from tangential forces affecting speed.

Question 2

1. Forces: $mg$ downward; normal force upward; tension 6.0 N to the right. (correct answer)
2. Forces: $mg$ downward; tension 6.0 N to the right. (No normal force.)
3. Forces: $mg$ downward; normal force upward; tension 6.0 N to the right; an extra force in the direction of motion.
4. Forces: $mg$ downward; normal force upward; friction to the left; tension 6.0 N to the right.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. To create a correct free-body diagram, select the book as the system and identify all external forces acting on it. The real forces include the gravitational force mg downward from Earth, the normal force upward from the table, and the tension force of 6.0 N to the right from the string. No frictional force is present since friction is negligible, and there is no additional force in the direction of motion as acceleration results from the net force, not an extra force. Choice C is incorrect because it includes an extra force in the direction of motion, which is a common misconception but not a real force; motion does not require a separate force beyond the existing unbalanced forces. Always isolate the object and list only contact and field forces acting on it, ensuring no fictitious forces like 'force of acceleration' are included.

Question 3

1. Forces: weight $mg$ straight down; normal force perpendicular to the slope. (correct answer)
2. Forces: weight $mg$ straight down; normal force straight up.
3. Forces: weight $mg$ straight down; normal force perpendicular to the slope; friction up the slope.
4. Forces: weight $mg$ straight down; normal force perpendicular to the slope; and a force down the slope because it is moving.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Select the sled as the system and account for its interaction with the environment on the inclined surface. The real forces are the gravitational force mg straight downward from Earth and the normal force perpendicular to the slope from the hill. No frictional force acts since friction is negligible, and the acceleration down the slope comes from the component of gravity parallel to the incline, not an additional force. Choice D is incorrect because it includes a separate force down the slope due to motion, which is not a real force; the downhill acceleration is solely due to the unbalanced component of gravity. To build accurate free-body diagrams on inclines, always draw gravity vertically downward and normal force perpendicular to the surface, resolving components as needed for analysis.

Question 4

1. Forces: $mg$ downward; normal force upward; a force to the right because it is moving.
2. Forces: $mg$ downward; normal force upward. (correct answer)
3. Forces: $mg$ downward only. (No normal force.)
4. Forces: $mg$ downward; normal force upward; kinetic friction to the left.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Isolate the puck as the system and consider its interactions while sliding. The only forces are the gravitational force mg downward from Earth and the normal force upward from the air table. Since friction is negligible and velocity is constant, there are no horizontal forces, and the net force is zero in both directions. Choice A is incorrect because it includes a force to the right due to motion, but constant velocity does not require a sustaining force; that's a misconception from Aristotelian thinking. Always remember that for objects in uniform motion, free-body diagrams should show balanced forces, with no 'momentum force' or 'force of motion' included.

Question 5

1. Forces: $mg$ downward; normal force upward; applied push 8.0 N right; static friction left. (correct answer)
2. Forces: $mg$ downward; normal force upward; applied push 8.0 N right; static friction right.
3. Forces: $mg$ downward; applied push 8.0 N right; static friction left. (No normal force.)
4. Forces: $mg$ downward; normal force upward; and a force left exerted by the box on the student.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Choose the box as the system and identify all external forces acting on it. The forces include the gravitational force mg downward from Earth, the normal force upward from the floor, the applied push of 8.0 N to the right from the student, and static friction to the left from the floor opposing the push and preventing motion. Since the box remains at rest, static friction equals the push in magnitude but opposite in direction, maintaining equilibrium. Choice D is incorrect because it includes a force left exerted by the box on the student, but free-body diagrams only show forces on the system, not forces exerted by the system on other objects, per Newton's third law. To avoid errors, focus solely on forces acting on the chosen object and use Newton's laws to infer magnitudes when needed.

Question 6

1. Forces: $mg$ downward; normal force upward; friction to the right; applied push to the right.
2. Forces: $mg$ downward; normal force upward; applied push to the right; kinetic friction to the left. (correct answer)
3. Forces: $mg$ downward; applied push to the right; kinetic friction to the left. (No normal force.)
4. Forces: $mg$ downward; normal force upward; kinetic friction to the left; and an additional force in the direction of motion.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Choose the crate as the system and consider all external interactions with it. The forces acting on the crate are the gravitational force mg downward from Earth, the normal force upward from the floor, the applied push of 10 N to the right from the person, and kinetic friction to the left from the rough floor opposing the motion. Since the crate moves at constant velocity, the net horizontal force is zero, with friction balancing the push, but no extra forces are needed. Choice D is incorrect because it adds an additional force in the direction of motion, which is not a real force; constant velocity means balanced forces, not an extra driving force. When constructing free-body diagrams, systematically identify gravitational, normal, frictional, and applied forces while avoiding pseudoforces related to motion or inertia.

Question 7

1. Forces: spring force upward; weight $mg$ downward; and an additional upward force because it accelerates upward.
2. Forces: spring force upward; weight $mg$ downward. (correct answer)
3. Forces: spring force upward only. (No weight.)
4. Forces: spring force downward; weight $mg$ upward.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Select the mass as the system and identify external forces from its environment. The real forces are the spring force upward from the spring and the gravitational force mg downward from Earth. At the instant it's momentarily at rest but accelerating upward, the spring force exceeds mg, resulting in a net upward force, but no additional force is present. Choice A is incorrect because it adds an extra upward force due to acceleration, but acceleration is the result of the imbalance between the real spring and weight forces, not a separate force. For oscillatory systems, include only elastic and gravitational forces in the free-body diagram and analyze net force for acceleration at specific points.

Question 8

1. Forces: $mg$ downward; normal force upward; tension to the right. (correct answer)
2. Forces: $mg$ downward; tension to the right; and a force to the left exerted by the hanging mass on the cart.
3. Forces: $mg$ downward; normal force upward; tension to the left.
4. Forces: $mg$ downward; tension to the right. (No normal force.)

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Isolate the cart as the system and list external forces interacting with it. The forces include the gravitational force mg downward from Earth, the normal force upward from the track, and the tension to the right from the string. The acceleration to the right is due to the tension, assuming a hanging mass on the other end, with negligible friction ensuring no opposing horizontal force. Choice B is incorrect because it includes a force to the left exerted by the hanging mass on the cart, but the hanging mass acts through the tension in the string, not directly; free-body diagrams should not include internal or fictitious forces. When dealing with connected systems, draw separate free-body diagrams for each object, representing connecting forces like tension accurately without duplicating interactions.

Question 9

1. Forces: weight down, normal up, applied force right, friction left
2. Forces: weight down, applied force right, and a smaller force left because it accelerates
3. Forces: weight down, normal up, applied force right (correct answer)
4. Forces: weight down, normal up, applied force right, and an equal-and-opposite applied force left on the same block

Explanation: This question tests your ability to identify forces in a free-body diagram on a frictionless surface. The block experiences only three forces: weight (downward), normal force (upward from the surface), and the applied force (to the right). Since the surface is frictionless, there is no friction force. Choice A incorrectly includes friction on a frictionless surface, while choice B incorrectly adds a "smaller force left" - forces don't automatically appear just because something accelerates. The key strategy is to carefully note given conditions (like "frictionless") and include only the forces that actually exist based on the physical interactions.

Question 10

1. Forces: weight $mg$ downward, normal force from wall horizontal, applied push into wall horizontal, static friction upward. (correct answer)
2. Forces: weight $mg$ downward, normal force from wall horizontal, applied push into wall horizontal, static friction downward.
3. Forces: weight $mg$ downward, applied push into wall horizontal, static friction upward (no normal force).
4. Forces: weight $mg$ downward, normal force from wall horizontal, and a force of rest upward because the box is not moving.

Explanation: This problem requires identifying forces in a free-body diagram for a box pressed against a wall at rest. When creating a free-body diagram, we select the box as our object and include only forces acting directly on it. The forces are: weight (mg) acting downward from gravity, normal force acting horizontally outward from the wall (perpendicular to the wall surface), applied force acting horizontally into the wall from the push, and static friction acting upward along the wall to prevent the box from sliding down. Choice D incorrectly includes a 'force of rest' upward, but being at rest is not a force—it's a state resulting from balanced forces. Since the box doesn't move, vertical forces balance (friction up equals weight down) and horizontal forces balance (normal force equals applied force). The key strategy is to identify all contact forces at each surface and remember that static friction acts to prevent motion, not because of motion.

Question 11

1. Weight downward, normal force upward, spring force to the left (correct answer)
2. Weight downward, normal force upward, spring force to the right because the block is moving right
3. Weight downward, spring force to the left, and a force of motion to the right
4. Spring force to the left only; vertical forces cancel and are omitted

Explanation: This question tests your ability to identify forces in a free-body diagram for a mass-spring system. The block experiences three forces: weight (downward), normal force (upward from the surface), and the spring force. Since the spring is stretched and pulls the block left, the spring force acts to the left. The block's rightward motion while slowing down confirms the net force is leftward. Choice B incorrectly states the spring force acts right because the block moves right - spring force depends on stretch/compression, not the object's motion direction. The key strategy is that spring force always acts to restore the spring to its natural length, regardless of the attached object's motion.

Question 12

1. Forces: $mg$ downward, normal force upward, a leftward tension force, and a rightward tension force. (correct answer)
2. Forces: $mg$ downward, normal force upward, a leftward tension force only (the opposite pull is the action-reaction pair on the other student).
3. Forces: $mg$ downward, a leftward tension force, and a rightward tension force (no normal force).
4. Forces: $mg$ downward, normal force upward, leftward tension, rightward tension, and an extra force of acceleration to the left.

Explanation: This problem requires identifying forces in a free-body diagram for a crate being pulled by two students. When creating a free-body diagram, we choose the crate as our object and include only forces that act directly on it. The forces are: weight (mg) acting downward from gravity, normal force acting upward from the floor, tension force to the left from the left rope, and tension force to the right from the right rope. Choice B incorrectly suggests including only one tension because the other is an action-reaction pair, but action-reaction pairs act on different objects—both rope tensions act on the crate. Since the crate accelerates left, the leftward tension must be greater than the rightward tension, creating a net force to the left. The crucial strategy is to include all forces acting on your chosen object, regardless of their action-reaction pairs, and remember that unequal opposing forces cause acceleration.

Question 13

1. The tension on the ball and the weight on the ball
2. The ball's weight and the normal force from the air
3. The force of the string on the ball and the force of the ball on the string (correct answer)
4. The ball's weight and the force of the ball on Earth are not an action-reaction pair

Explanation: This question tests the identification of action-reaction pairs according to Newton's third law in static systems. Select the ball as the primary object and examine pairwise interactions involving it. The force of the string on the ball (tension upward) and the force of the ball on the string (equal and opposite) form a true action-reaction pair. Other forces like weight act between the ball and Earth, not involving the string directly. Choice A is incorrect because tension and weight are not a pair; they arise from different interactions. When identifying pairs, always specify the two objects interacting and ensure the forces are equal, opposite, and of the same type, acting on each other.

Question 14

1. To the right, because the sled is accelerating right
2. To the left, opposite the sled's motion (correct answer)
3. Upward, opposite the rope's vertical component
4. Downward, because friction is caused by the normal force

Explanation: This question evaluates the understanding of force directions in free-body diagrams, particularly for friction in accelerating systems. Choose the sled as the system and consider all external forces from its surroundings. The friction force, being kinetic, acts opposite to the direction of the sled's velocity relative to the snow, which is to the right, so friction points to the left. Other forces include weight downward, normal upward, and tension along the rope at 30 degrees, with the horizontal component causing acceleration to the right despite friction. Choice A is incorrect because friction does not point in the direction of acceleration; it always opposes relative motion, not acceleration. When analyzing force directions, remember to determine friction's orientation based on the relative velocity between surfaces, then verify net force direction matches the observed acceleration.

Question 15

1. Weight downward, normal upward, applied push right, kinetic friction left (correct answer)
2. Weight downward, applied push right, kinetic friction right
3. Weight downward, normal upward, applied push right, kinetic friction right
4. Weight downward, applied push right, and a force of motion right

Explanation: This problem requires identifying forces on an object moving at constant velocity with friction. To draw a free-body diagram, we select the crate as our system and identify all forces acting on it. The crate experiences four forces: weight (mg) downward from gravity, normal force upward from the floor, the applied push to the right, and kinetic friction to the left opposing the motion. Since the crate moves at constant velocity, the net force is zero, meaning the push and friction forces are balanced. Choice C incorrectly shows kinetic friction in the same direction as motion - friction always opposes relative motion between surfaces. When an object slides across a surface, always draw friction opposite to the velocity direction.

Question 16

1. Weight downward, normal force upward, tension to the right, and a force of motion to the right
2. Tension to the right and friction to the left only
3. Weight downward, normal force upward, and tension to the right (correct answer)
4. Weight downward, normal force upward, and an equal-and-opposite tension to the left on the cart

Explanation: This question tests your ability to identify forces in a free-body diagram. For the cart on a frictionless track, we need to identify all real forces acting on it: weight (gravitational force downward), normal force (support force from the track upward), and tension (pulling force from the string to the right). Since friction is negligible, there is no friction force. Choice A incorrectly adds a "force of motion" which is not a real force - motion is the result of forces, not a force itself. The key strategy is to include only forces from physical interactions: gravity, contact forces (normal and friction), and forces from attached objects (tension).

Question 17

1. Weight and normal force only, since the book does not move
2. Weight, normal force, applied force, and static friction (correct answer)
3. Weight, applied force, and a force of motion; no normal force
4. Weight, normal force, and applied force; no friction because speed is zero

Explanation: This question tests the skill of identifying forces in static equilibrium with friction present. Select the book as the system and include only the external forces interacting with it. The forces are weight downward from gravity, normal force upward from the table, the applied horizontal force, and static friction horizontally opposing the applied force to keep the book at rest. Since the book does not move, static friction equals the applied force in magnitude but opposite in direction, and vertical forces balance. Choice D is incorrect because it omits friction, but friction is necessary to prevent motion despite the applied force. A useful strategy is to check for equilibrium by ensuring paired forces balance in each direction, including friction when surfaces are rough and motion is absent.

Question 18

1. Weight, normal force, kinetic friction, and applied push (correct answer)
2. Weight and applied push only
3. Weight, normal force, and applied push (no friction)
4. Weight, normal force, kinetic friction, applied push, and a force in the direction of motion

Explanation: This question assesses the skill of constructing free-body diagrams for objects experiencing friction and moving at constant velocity. To create the free-body diagram, select the box as the system and identify all external forces acting upon it. The real forces include the weight pulling downward due to gravity, the normal force pushing upward from the floor, the applied push horizontally in the direction of motion, and kinetic friction acting horizontally opposite to the motion. Since the box moves at constant velocity, the net force is zero, meaning the push balances the friction and the normal force balances the weight. Choice D is incorrect because it includes a fictitious 'force in the direction of motion,' which does not exist as motion itself does not produce a force. A transferable strategy is to always list only contact forces (like normal and friction) and field forces (like weight) acting on the chosen system, ensuring they align with Newton's first law for constant velocity scenarios.

Question 19

1. Forces: $mg$ downward; normal force perpendicular to ramp; tension up the ramp; kinetic friction up the ramp.
2. Forces: $mg$ downward; normal force perpendicular to ramp; tension up the ramp; kinetic friction down the ramp. (correct answer)
3. Forces: $mg$ downward; tension up the ramp; kinetic friction down the ramp. (No normal force.)
4. Forces: $mg$ downward; normal force perpendicular to ramp; tension up the ramp; kinetic friction down the ramp; plus a force up the ramp because it moves upward.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Choose the box as the system and list all external forces interacting with it on the ramp. The real forces are the gravitational force mg downward from Earth, the normal force perpendicular to the ramp from the surface, the tension upward along the ramp from the rope, and kinetic friction down the ramp opposing the upward motion. Since the box moves at constant velocity up the ramp, the net force parallel to the ramp is zero, with tension balancing the components of gravity and friction. Choice A is incorrect because it shows kinetic friction up the ramp, but friction always opposes the direction of relative motion, which is upward here, so it should be down the ramp. When dealing with inclined planes and friction, determine the direction of friction based on the object's motion relative to the surface and include only verifiable contact or field forces.

Question 20

1. Forces: $mg$ downward; normal force upward; 12 N right; 5.0 N left. (correct answer)
2. Forces: $mg$ downward; 12 N right; 5.0 N left. (No normal force.)
3. Forces: $mg$ downward; normal force upward; 12 N right; plus an extra force right because it accelerates.
4. Forces: $mg$ downward; normal force upward; 12 N left; 5.0 N right.

Explanation: This question assesses the skill of identifying forces and constructing free-body diagrams in AP Physics 1. Select the box as the system and identify forces from external agents. The forces acting are the gravitational force mg downward from Earth, the normal force upward from the floor, the 12 N pull to the right from Student 1, and the 5.0 N pull to the left from Student 2. The acceleration to the right results from the net horizontal force of 7 N right, with no friction mentioned. Choice C is incorrect because it omits the 5.0 N left and adds an extra force right due to acceleration, but acceleration is the outcome of unbalanced real forces, not a separate force itself. For multiple applied forces, represent each as a distinct vector in the free-body diagram and calculate net force separately for verification.