Spark image


A force can make things move along but it can also TURN things. Think about the door handle, a screwdriver, a steering wheel, a wheelbarrow or a spanner. They all use a force to turn something, even levering off the top of a bottle. But why is the door handle far away from the hinges and why is it much easier to undo a nut with a long spanner than a short one? Obviously the distance from the pivot that the force acts must be important as well as the size of the force itself

The turning effect of a force is large if:
(a) the force itself is large;
(b) the distance from the pivot is large.

The turning effect of a force is called the MOMENT of the force, measured in units of Newton metres (Nm).

It is defined as:

moment of a force = force x perpendicular distance of force from pivot (fulcrum)


There is also an upward force at the pivot but since it goes through the pivot it has no moment about the pivot and therefore no turning effect. This upward force will be equal and opposite to the force F.

N.B.. We have ignored the effect of the weight of the lever in all the diagrams.

You can show the turning effect of forces very easily with a door. If one of your friends goes on one side of the door and pushes near the door handle with their little finger then you push on the other side near the hinge with your whole weight your friend will be able to close the door against you!

A force of 30 N on the end of a spanner is used to turn a nut. If the spanner is 25 cm long, what is the moment of the force about the centre of the bolt?
Moment = force x distance from the pivot
= 30 x 0.25 = 7.5 Nm

Using a spanner that is too long may tighten a nut too much and it will then be very difficult to undo it again. Think about trying to change the wheel on a car if the nuts on it have been tightened up using a long spanner!

The following diagrams show some more examples of levers.

© Keith Gibbs 2020