Kinetic and Static Friction AP Physics C Mechanics MCQ – Exam Style Questions etc.
Kinetic and Static Friction AP Physics C Mechanics MCQ
Unit 2: Force and Translational Dynamics
Weightage : 20-15%
Question
An experiment is set up using a hanging mass to pull a cart along a horizontal track. A light string is attached from the hanging mass over a pulley to a force sensor. A second light string is attached from the force sensor to the cart. There is a motion sensor that can be used to measure different features of the motion of the cart. The pulley is ideal, and there is negligible friction between the cart and the track.
Block A of mass m is at rest on top of block B of mass M when a horizontal force of magnitude F is exerted on block A, for a time Δt as shown in the figure. Block A moves across block B. The coefficient of friction between blocks A and B is μ. There is negligible friction between block B and the surface. Which of the following is a correct expression for the acceleration of block B ?
A \(\frac{μmg}{m}\)
B \frac{μmg}{M}\)
C \(\frac{μ(m+M)g}{(M)}\)
D \(\frac{μMg}{M+m}\)
E \(\frac{μ(m+M)g}{(m+M)}\)
Answer/Explanation
Ans:B
According to Newton’s third law, if there is a force of friction from block B on block A, this force of friction is also exerted from block A on block B. Applying Newton’s second law to block B yields \(ΣF=f=m_Ba\) so, \(μN=Ma\) . The normal force is the force between blocks B and A. This will be equal to the weight of block A. Substituting this into the above equation yields \(a=\frac{μN}{M}=\frac{μmg}{M}\).
Question
A horizontal force F pushes a block of mass m against a vertical wall. The coefficient of friction between the block and the wall is μ. What value of F is necessary to keep the block from slipping down the wall?
(A) mg
(B) μmg
(C)\(\frac{mg}{μ}\)
(D) \(mg (1 – μ)\)
(E)\( mg (1+ μ)\)
Answer/Explanation
Ans:C
Question
Two blocks rest on a table, as shown above. The bottom block is pulled to the right by an applied force \(\vec{F}\) F that is strong enough so that the two blocks do not move together (i.e., they do not have the same acceleration or velocity). There is friction between the blocks, but the tabletop is frictionless. When the top block leaves the bottom block, where does it land and why?
(A) The top block will land directly below where it starts because objects at rest tend to stay at rest.
(B) The top block will land to the left of where it starts because of the static friction between the blocks.
(C) The top block will land to the left of where it starts because of the kinetic friction between the blocks.
(D) The top block will land to the right of where it starts because of the static friction between the blocks.
(E) The top block will land to the right of where it starts because of the kinetic friction between the blocks.
Answer/Explanation
Ans:E
The top block accelerates to the right, but does not move as fast as the bottom block. The force exerted on the top block is friction from the bottom block. Since the blocks do not move together, the friction is kinetic friction. Since the blocks are moving toward the right, the top block will land to the right of where it started.
Question
A cart is moving to the right with a constant speed of 20 m s. A box of mass 80 kg moves with the cart without slipping. The coefficient of static friction between the box and cart is 0.30, and the coefficient of kinetic friction between the box and cart is 0.15. What are the magnitude and direction of the frictional force acting on the box as it moves with the cart?
(A) Zero
(B) 120 N to the right
(C) 120 N to the left
(D) 240 N to the right
(E) 240 N to the left
Answer/Explanation
Ans:A