Principle of moments

When a bar resting on a pivot is balanced, the moment of all the forces tending to make the bar rotate in a clockwise direction is balanced by an equal moment in an anticlockwise direction.

Therefore, for objects in equilibrium:

sum of anticlockwise moments = sum of clockwise moments

This is called the principle of moments.

In this example, we can use the principle of moments to check that the bar is in equilibrium:

sum of anticlockwise moments = 3.0 x 2.0 + 1.0 x 4.0 = 10 Nm

sum of clockwise moments = 2.0 x 3.0 + 4.0 x 1.0 = 10 Nm

This confirms that the bar is in equilibrium.