When a body is weighed on an ordinary balance we demand that the arm should be horizontal if the weights on the two pans are equal. Suppose equal weights are put on the two pans, the arm is kept at an angle with the horizontal and released. Is the torque of the two weights about the middle point (point of support) zero? Is the total torque zero? If so, why does the arm rotate and finally become horizontal?

Question

Philoid · Accepted Answer

The torque on A

The torque on B

So, the net torque is zero.

But this is all about equilibrium. When the angle was initially constructed, we had to do some work against the gravity. When we release the hindrance the system tries to attain its stability back to remain at minimum potential energy. This is why the balance attains its equilibrium again.