Couple and Moment of Couple and example

 ICSE PHYSICS Couple and Moment of Couple 

Couple

When a force is applied in a pivoted body it causes that body to rotate about the axis passing through the pivoted point.

Actually the rotation of the body is not only due to the force applied but also due to the equal and opposite force of reaction at the pivoted point. This pair of forces formed by the external force and the force of reaction is called couple.

When you open or shut a door by applying force at the handle an equal and opposite force of reaction is produced at the pivoted point.

If you need a large turning effect to turn an object pivoted at a point then you have to apply two forces of equal magnitude but opposite to each other which turns the body in the same direction.

To turn a water tap we have to apply two forces of equal magnitude but in opposite direction.

Other examples:

1. Tightening the cap of a bottle.

2. Turning a key in a lock.

3. Turning a steering wheel.

 

Moment of couple

 Let us take a bar of length 'd' and it is pivoted at a point ‘O’.

Two equal forces are applied at points A and B at the same time but in opposite direction and the bar turns in clockwise direction.

The moment of force at ‘O’ due the force applied at A = FxOA

The moment of force at ‘O’ due the force applied at B = FxOB

The total moment of both forces = FxOA + FxOB = Fx(OA + OB) = FxAB = Fxd

hence the moment of couple = F x d