Torque and Work AP Physics C Mechanics MCQ – Exam Style Questions etc.
Torque and Work AP Physics C Mechanics MCQ
Unit 6: Energy and Momentum of Rotating Systems
Weightage : 10-15%
Question
A square piece of plywood on a horizontal tabletop is subjected to the two horizontal forces shown above. Where should a third force of magnitude 5 newtons be applied to put the piece of plywood into equilibrium?
Answer/Explanation
Ans:A
Solution: To balance the forces (Fnet=0) the answer must be A or D, to prevent rotation, obviously A would be needed
Question
A uniform rigid bar of weight W is supported in a horizontal orientation as shown above by a rope that makes a 30° angle with the horizontal. The force exerted on the bar at point O, where it is pivoted, is best represented by a vector whose direction is which of the following?
Answer/Explanation
Ans:B
Solution: 
Since the rope is at an angle it has x and y components of force.
Therefore, H would have to exist to counteract Tx.
Based on Ʈnet = 0 requirement, V also would have
to exist to balance W if we were to chose a pivot
point at the right end of the bar
Question
The arm is held in the horizontal position and the hand is bent at the wrist so the fingers point up, as shown in the figure above. The torque exerted by the weight of the hand with respect to the shoulder is most nearly
(A) 6 N .m
(B) 10 N .m
(C) 30 N. m
(D) 60 N. m
(E) 70 N .m
Answer/Explanation
Ans:A
Question
A homogeneous bar is lying on a flat table. Besides the gravitational and normal forces (which cancel), the bar is acted upon by exactly two other external forces, \(F_1\) and \(F_2\) , which are
parallel to the surface of the table. If the net force on the rod is zero, which one of the following is also true?
(A) The net torque on the bar must also be zero.
(B) The bar cannot accelerate translationally or rotationally.
(C) The bar can accelerate translationally if \(F_1\) and \(F_2\) are not
applied at the same point.
(D) The net torque will be zero if \(F_1\) and \(F_2\) are applied at the
same point.
(E) None of the above
Answer/Explanation
Ans: D
Since \(F_{net}=F_1 + F_2 = 0\), the bar cannot accelerate translationally, so (C) is false. The net torque does not need to be zero, as the following diagram shows, eliminating (A) and (B).
However, since \(F_2 = -F_1\), (D) is true; one possible illustration of this given below: