Final answer:
The net torque about the pivot at the bottom of a drawbridge in mechanical equilibrium is zero. This ensures there is no angular acceleration and the drawbridge maintains its position. The condition for mechanical equilibrium requires that all external torques cancel out.
Step-by-step explanation:
The net torque about the pivot at the bottom of the drawbridge, if the drawbridge is in mechanical equilibrium, is zero. This is because for an object to be in mechanical equilibrium, the second condition that must be met is that the net external torque on the system must be zero (net T = 0). In this state, the torques that cause counterclockwise rotation (considered positive) and those that cause clockwise rotation (considered negative) must balance each other out. If the drawbridge is not moving, it means that there is no net torque causing it to rotate in either direction, thus the answer is (c) Zero.
An object will continue spinning at the same angular velocity when the net torque acting on it is zero. When the net torque is zero, the object has no angular acceleration and therefore maintains its current state of rotation under Newton's first law for rotational motion. The moment of inertia and angular momentum must be constant for this condition to hold true.