AP Physics C: Mechanics Practice Quiz: Kinetic and Static Friction
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Test your understanding with short quizzes. This quiz has 13 questions to check your progress.
Question 1 of 13
All Questions (13)
A) When the surfaces are at rest relative to each other.
B) When one surface is sliding relative to the other.
C) Only when an object is accelerating.
D) Before an object starts to move.
Correct Answer: B
The provided content explicitly states that kinetic friction is described as friction between two surfaces when one is sliding relative to the other.
A) To cause an object to accelerate.
B) To maintain an object's motion at a constant velocity.
C) To prevent an object from slipping or sliding.
D) To be a constant force regardless of the situation.
Correct Answer: C
The content describes static friction as the force that 'adopts the value and direction required to prevent an object from slipping or sliding on a surface.'
A) It is inversely proportional to the normal force.
B) It is directly proportional to the normal force.
C) It is equal to the normal force.
D) It is independent of the normal force.
Correct Answer: B
The equation shows that the kinetic friction force is the product of the coefficient of kinetic friction ($\mu_k$) and the normal force ($F_N$). If $\mu_k$ is constant, the friction force is directly proportional to $F_N$.
A) It is a constant value determined solely by the coefficient of static friction.
B) It is always greater than the kinetic friction force.
C) It is an adjustable force that opposes an applied force, up to a certain maximum.
D) It is always equal to the product $\mu_{s}F_{N}$.
Correct Answer: C
The content states that 'Static friction adopts the value and direction required to prevent an object from slipping,' and the inequality $|\vec{F}_{f,s}|\le\mu_{s}F_{N}$ shows it can be any value up to a maximum, not a fixed value.
A) The force of gravity.
B) The applied force pushing the object.
C) The normal force the surface exerts on the object.
D) The net force acting on the object.
Correct Answer: C
The provided text explicitly defines the equation as 'the product of the normal force the surface exerts on the object and the coefficient of kinetic friction.'
A) Greater than 100 N.
B) Less than 100 N.
C) Exactly 100 N.
D) It cannot be determined without knowing $\mu_s$ and $F_N$.
Correct Answer: C
According to the description, 'Static friction adopts the value and direction required to prevent an object from slipping.' Since the box is not moving, the static friction force must be equal in magnitude and opposite in direction to the applied force of 100 N.
A) The exact value of the static friction force at all times.
B) The minimum force required to start motion.
C) The maximum possible magnitude of the static friction force.
D) The kinetic friction force that acts after motion begins.
Correct Answer: C
The inequality $|\vec{F}_{f,s}|\le\mu_{s}F_{N}$ indicates that the magnitude of the static friction force, $|\vec{F}_{f,s}|$, can be any value less than or equal to the product $\mu_{s}F_{N}$. Therefore, $\mu_{s}F_{N}$ represents the upper limit or maximum value of this force.
A) By finding the value needed to prevent motion.
B) By multiplying the coefficient of kinetic friction by the normal force.
C) By finding the maximum value of the static friction.
D) By setting it equal to the applied force.
Correct Answer: B
The content provides the specific equation for the magnitude of kinetic friction: $|\vec{F}_{f,k}|=\mu_{k}F_{N}$, which is the product of the coefficient of kinetic friction and the normal force.
A) Its magnitude is equal to $\mu_{s}F_{N}$.
B) Its magnitude is zero because the object is not moving.
C) Its magnitude is equal to the magnitude of the applied force.
D) Its magnitude is equal to the kinetic friction force.
Correct Answer: C
The content states that 'Static friction adopts the value...required to prevent an object from slipping'. If the object is at rest despite an applied force, the static friction force must be equal in magnitude to that applied force to maintain equilibrium. It is only equal to $\mu_{s}F_{N}$ at the exact moment before slipping.
A) It is halved.
B) It remains the same.
C) It is doubled.
D) It is quadrupled.
Correct Answer: C
The formula for kinetic friction is $|\vec{F}_{f,k}|=\mu_{k}F_{N}$. Since the kinetic friction force is directly proportional to the normal force ($F_N$), doubling the normal force will also double the magnitude of the kinetic friction force, assuming $\mu_k$ is constant.
A) It is the same in both scenarios.
B) It is doubled in the second scenario.
C) It is halved in the second scenario.
D) It is zero in both scenarios.
Correct Answer: B
Static friction adjusts its magnitude to be equal and opposite to the applied force to prevent motion. Since the applied force was doubled and the block still did not move, the static friction force must also have doubled to counteract it.
A) The normal force, $F_N$.
B) The coefficient of kinetic friction, $\mu_k$.
C) The coefficient of static friction, $\mu_s$.
D) The gravitational force.
Correct Answer: B
The equation for kinetic friction is $|\vec{F}_{f,k}|=\mu_{k}F_{N}$, which uses $\mu_k$. The equation for the maximum static friction is $|\vec{F}_{f,s}|_{max}=\mu_{s}F_{N}$, which uses $\mu_s$. Therefore, $\mu_k$ is used for kinetic friction but not for static friction.
A) The force has a magnitude of $\mu_{k}F_{N}$ and is directed to the right.
B) The force has a magnitude of $\mu_{k}F_{N}$ and is directed to the left.
C) The force has a magnitude less than or equal to $\mu_{s}F_{N}$ and is directed to the left.
D) The force has a magnitude that depends on the object's speed.
Correct Answer: B
The content provides the magnitude of kinetic friction as $|\vec{F}_{f,k}|=\mu_{k}F_{N}$. Friction is a force that opposes motion. Since the object is sliding to the right, the kinetic friction force must be directed to the left.