Physic question

Question

A 1.0-m long plank of wood is allowed to pivot around its center of mass, located exactly at the midpoint of the plank; the center of mass is fixed in place. A downward force is applied on one side of the plank. Which of the following is true?

A. Neither the net force nor the net torque are zero.

B. The net force is non-zero while the net torque equals zero.

C. The net force is zero but the net torque is non-zero.

D. Both the net force and the net torque are zero.

Answer 1

Since the center of mass does not accelerate, the net force must be zero. Whatever the plank is anchored to, it must apply a force to perfectly cancel any other forces. Since the anchor is in the middle, it cannot counteract any torque. Therefore net torque is simply the sum of all external torques.