Newton’s Third Law AP Physics C Mechanics MCQ – Exam Style Questions etc.
Newton’s Third Law Diagrams AP Physics C Mechanics MCQ
Unit 2: Force and Translational Dynamics
Weightage : 20-15%
Question
A small sphere hangs from a string attached to the ceiling of a uniformly accelerating train car. It is observed that the string makes an angle of 37° with respect to the vertical. The magnitude of the acceleration a of the train car is most nearly
The figure shows a train car with a sphere hanging from its ceiling down and to the left. Above the train car there is an arrow pointing horizontally to the right. The arrow is labeled vector a.
A \(6.0 m/s^2\)
B \(7.5 m/s^2\)
C \(8.0 m/s^2\)
D \(10 m/s^2\)
E \(13 m/s^2\)
Answer/Explanation
Ans:B
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
The figure shows blocks C and D on a horizontal surface in contact with each other. A force F is applied to block C. In which of the following situations does block D apply a force of magnitude \(\frac{1}{3}F\) to block C ?
Answer/Explanation
Ans:C
Using Newton’s second law on the two-block system yields \(a=\frac{F}{m}=\frac{F}{(2m+m)}=\frac{F}{3}m\) . Then, substituting this into an equation for block D to calculate the force block C exerts on block D yields \(F_{CD}=m_Da\)=\((m)(\frac{F}{3m})=\frac{1}{3}F\). According to Newton’s third law, the force block D exerts on block C has this same magnitude.
Question
Two blocks are connected by a light string, as shown in Figure 1. There is friction between the blocks and the table. The system is released from rest, and the blocks accelerate. The tension in the string is \(T_1\). Then the setup is returned to its starting position, and a third block is attached as shown in Figure 2. The masses of the blocks are related as follows: \(M_1>M_2>M_3\). The system is again released from rest and allowed to accelerate. The tension in the string on the left is \(T_2\). Which of the following gives a correct relationship between the tensions in the string on the left in the two situations?
A \(T_1<T_2\)
B \(T_1=T_2\)
C \(T_1>T_2\)
D The relationship cannot be determined without knowing the actual masses of the blocks.
E The relationship cannot be determined without knowing the coefficient of friction between the blocks and the table.
Answer/Explanation
Ans:A
Applying Newton’s second law to the block of mass yields \(∑F=M_1g−T=M_1a\). Thus, the less the acceleration, the greater the tension. In Figure 2, the additional block on the right will slow down the system. Since the acceleration is less in Figure 2, the tensions must be greater in Figure 2, and \(T_1<T_2\).