Key Points

• When an object is attached to a pivot it is attached to a fixed point about which the object can rotate:
• If a force is applied along a line that passes through the pivot, the object will not rotate and remains stationary.
• If there is a distance between the pivot and the line of action of the force:
• The object will rotate about the pivot in the direction of the applied force.
• If the force is not applied perpendicularly to the object, consider the perpendicular distance from the pivot to the line of force to determine the rotational effect.

The turning effect of a force is called a moment. Moments describe how a force causes an object to rotate around a pivot point.

The size of a moment determines the extent of the rotation:

• A larger moment results in a greater rotational effect.
• A smaller moment results in less rotation.

Moments can be increased. We can increase the size of a moment by either increasing the force applied, or increasing the perpendicular distance from the pivot. Moments can be decreased. We can decrease the size of a moment by either decreasing the force applied, or decreasing the perpendicular distance from the pivot.

Formula

In certain situations, moments can balance each other out and the object will stay still (instead of turning). For this to be the case, the total clockwise moment will be equal to the total anticlockwise moment about a pivot.