SAT Subject Physics Practice Question 255: Answer and Explanation

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Question: 255

26. A uniform bar is lying on a flat table. Besides the gravitational and normal forces (which cancel), the bar is acted upon by exactly two other forces, F1 and F2, which are parallel to the surface of the table. If the net force on the rod is zero, then which one of the following is true?

A. The net torque on the bar must also be zero.
B. The bar can accelerate translationally if F1 and F2 are not applied at the same point.
C. The net torque will be zero if F1 and F2 are applied at the same point.
D. The bar cannot accelerate translationally or rotationally.
E. None of the above

Correct Answer: C


Since Fnet = F1 + F2 = 0, the bar cannot accelerate translationally, so B is false. The net torque does not need to be zero, as the following diagram shows (eliminating A and D).

However, since F2 = −F1, C is true; one possible illustration of this is given below.

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