Physics – 1.5.2 Turning effect of forces | e-Consult

An everyday situation where the principle of moments is used is when tightening a bolt with a spanner (wrench). The spanner acts as a lever, and the bolt's resistance to turning is the force we are trying to overcome. The bolt's resistance acts at the nut, which is the pivot point (the fulcrum) of the lever.

To tighten the bolt, we apply a force to the spanner's handle. The distance from the fulcrum (the nut) to where we apply the force on the spanner handle is the effort arm. The distance from the fulcrum to the bolt is the resistance arm. The force we apply to the spanner handle is the effort, and the tightening force on the bolt is the resistance.

The principle of moments states that the effort moment must be greater than or equal to the resistance moment for the bolt to tighten. Effort Moment = Effort Force x Effort Arm Distance and Resistance Moment = Resistance Force x Resistance Arm Distance.

Calculation Example:

Let's say we want to tighten a bolt. The resistance force acting on the bolt is 50 N, and the resistance arm distance is 0.2 m. The effort force applied to the spanner handle is 20 N, and the effort arm distance is 0.5 m.

Resistance Moment = 50 N x 0.2 m = 10 Nm

Effort Moment = 20 N x 0.5 m = 10 Nm

In this case, the effort moment (10 Nm) is equal to the resistance moment (10 Nm). Therefore, the bolt will remain in the same position and will not tighten. To tighten the bolt, we need to increase either the effort force or the effort arm distance. For example, if we increase the effort force to 40 N, the effort moment becomes 40 N x 0.5 m = 20 Nm, which is greater than the resistance moment of 10 Nm, and the bolt will tighten.