AP Physics C: Mechanics Practice Quiz: Forces and Free-Body Diagrams
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Test your understanding with short quizzes. This quiz has 15 questions to check your progress.
Question 1 of 15
All Questions (15)
A) An interaction between two objects or systems.
B) A property of a single object that causes it to accelerate.
C) A quantity measured in meters per second squared.
D) The energy stored within a system.
Correct Answer: A
The content explicitly states that a force is an 'interaction between two objects or systems.' The other options describe acceleration, a property of a single object, or energy, which are different physical concepts.
A) To calculate the speed of the textbook.
B) To show all the forces the textbook exerts on its environment.
C) To describe the forces exerted on the textbook by its environment.
D) To determine the mass of the textbook.
Correct Answer: C
The content specifies that a free-body diagram is used to 'describe the forces exerted on an object or system' and that it 'shows each of the forces exerted on the object or system by the environment.' It does not show forces exerted by the object itself.
A) They are scalar quantities.
B) They are vector quantities.
C) They always occur in pairs.
D) They are a form of energy.
Correct Answer: B
The content states, 'Forces are vector quantities that describe the interactions between objects or systems.' By definition, vector quantities have both magnitude and direction.
A) Forces are vector quantities.
B) The force of friction from the water is too large.
C) An object or system cannot exert a net force on itself.
D) The coordinate system was not aligned with the direction of motion.
Correct Answer: C
The person and the canoe form a single system. The push is an internal force within that system. The content states, 'An object or system cannot exert a net force on itself.' To move, the system needs an external force, like a paddle pushing on the water.
A) The gravitational force the satellite exerts on the Earth.
B) The force of the satellite's thrusters, even if they are turned off.
C) The gravitational force exerted on the satellite by the Earth.
D) The force of the satellite on itself.
Correct Answer: C
A free-body diagram shows forces exerted *on* the object or system by the environment. The gravitational force exerted by the Earth on the satellite is the primary external force. The force the satellite exerts on the Earth would be in the Earth's free-body diagram, not the satellite's.
A) With the x-axis horizontal and the y-axis vertical.
B) With one axis parallel to the direction of acceleration.
C) With the origin placed at the top of the inclined plane.
D) With one axis pointing in the direction of the gravitational force.
Correct Answer: B
The content explicitly states, 'A coordinate system with one axis parallel to the direction of acceleration of the object or system simplifies the translation from free-body diagram to algebraic representation.' For a block sliding down a ramp, this means tilting the axes.
A) A system exerting a net force on itself.
B) A scalar quantity.
C) An interaction between two objects.
D) A non-vector force.
Correct Answer: C
The content defines a force as 'an interaction between two objects or systems.' In this case, the two interacting objects are the player's foot and the ball.
A) The student forgot to include air resistance.
B) The diagram includes a force that the system exerts on itself.
C) The friction force should be pointing forward.
D) The normal force and gravity are not vector quantities.
Correct Answer: B
The label 'the car's own force' violates the principle that 'An object or system cannot exert a net force on itself.' All forces on a free-body diagram must be external forces exerted by the environment on the system (the car).
A) It includes all forces, both internal and external to the system.
B) It shows only the forces exerted on the object by the environment.
C) It must always have a coordinate system with a vertical y-axis.
D) It depicts the velocity and acceleration of the object.
Correct Answer: B
The content specifies that 'The free-body diagram of an object or system shows each of the forces exerted on the object or system by the environment.' It does not include internal forces or forces exerted by the object on the environment.
A) Because gravity always acts in the negative direction.
B) To ensure that all forces are positive values.
C) Because this choice aligns one axis with the direction of acceleration.
D) Because forces are scalar quantities and the coordinate system is arbitrary.
Correct Answer: C
The content states that aligning one axis with the direction of acceleration 'simplifies the translation from free-body diagram to algebraic representation.' Since the rocket is accelerating upward, aligning the y-axis with this direction is the most efficient choice.
A) The force the cart exerts on the horse.
B) The force the horse exerts on the cart.
C) The force the horse's hooves exert on the ground.
D) The internal forces holding the cart together.
Correct Answer: B
The free-body diagram for the cart must show all external forces exerted *on* the cart by its environment. The force the horse exerts on the cart is an external force from the perspective of the cart system. The other options describe forces exerted by the cart, forces on a different object (the horse), or internal forces.
A) are always constant.
B) are interactions between two objects.
C) can be represented by a single number.
D) have both a magnitude and a direction.
Correct Answer: D
The content identifies forces as vector quantities. The definition of a vector quantity is one that possesses both a magnitude (how strong the force is) and a direction (which way it pushes or pulls).
A) The gravitational force on block A from the Earth.
B) The force exerted by block A on block B.
C) The normal force on block B from the floor.
D) The gravitational force on block B from the Earth.
Correct Answer: B
When considering the combined A+B system, the forces between A and B are internal to the system. A free-body diagram shows only external forces exerted on the system by the environment. The force of A on B is an internal interaction and should not be included in the diagram for the combined system. The gravitational forces and the normal force from the floor are all external forces.
A) A single force vector pointing downwards.
B) A downward force vector and an upward force vector of equal magnitude.
C) A single force vector pointing in the direction of motion.
D) No force vectors, because the apple is not touching anything.
Correct Answer: A
The free-body diagram shows forces exerted on the object by the environment. The only object in the environment interacting with the falling apple is the Earth, which exerts a downward gravitational force. Therefore, the diagram shows a single downward vector representing gravity.
A) vector quantities.
B) interactions between two different objects or systems.
C) simplified by the choice of coordinate system.
D) always balanced by a frictional force.
Correct Answer: B
Because a force is defined as an interaction *between two* objects or systems, any force an object exerts must be on a *different* object. Internal forces within a system cancel each other out (as per Newton's 3rd Law, which is a related concept), so a system cannot produce a *net* force on its own center of mass.