Moment of a force

It makes calculations easier to try to measure the perpendicular distance between the line of action of the force and the pivot. For example, if you apply a force to a spanner it rotates. The pivot is at the bolt.

If you want to undo a very tight nut, would you choose a spanner with a short handle or a long handle?

Why is a door handle on a door placed far away from the hinges where the door pivots?

Activity 1

What you will need:

a rod

different hanging weights

What you will do:

1. Hold a rod horizontally and hang a weight on the rod near your hand.

2. Move the weight to different distances from your hand keeping the rod horizontal. 

What happens?

3.  Try it with heavier weights. 

You can feel the turning effect on your hand. The turning effect depends on the size of the force (the weight) and the distance from your hand. 

The turning effect of the force is called a moment or a torque.

This is calculated by:

The distance used is always the shortest (perpendicular) distance. Moments are measured in newton-metres, written Nm.

Moments act about a pivot in a clockwise or anticlockwise direction.

Example:

Perpendicular distance from pivot to force d = 0.50 m.

Force F = 10 N

Moment = Fd

Moment = 10 N x 0.50 m

Moment = 5 Nm.  This is a clockwise moment.

The force will rotate the object in a clockwise direction about the pivot.

It is important to remember that the distance d is the perpendicular distance from the pivot to the line of action of the force (see diagram).

Example:

What is the turning effect of the force in the diagram, about the nut at point P.

Perpendicular distance from the force to P = 20 cm = 0.20 m

Moment of force = force x perpendicular force

                            = 10 N x 0.20 m

                            = 2 Nm turning clockwise 

Spanners and levers both use moments.

Spanners are used to turn nuts and bolts. If you need to undo a nut that is very tight, you can:

use a short spanner and apply a large force

or

use a long spanner and apply a small force

Using the longer spanner increases the distance from the pivot. This reduces the amount of force needed to undo the nut from the bolt.

Removing the lid from a can of paint requires a large lifting force on the lid. A screwdriver acts as a lever.  The pivot is the edge of the can, and this is very close to where the strong push is needed to lift the lid to open the can.  

A screwdriver with a long handle means that you can push down on the handle of the screwdriver with a small force and still open the can.