Moments around us
Moments are not something that only happens in a classroom. Almost every tool you pick up works by producing one.
A spanner for loosening a nut
Figure to be added
The axis of rotation is the line through the centre of the nut. When you pull on the handle of the spanner, the perpendicular distance is the length of spanner between the nut and your hand.
Try to turn the nut with your fingers and the perpendicular distance is less than a centimetre. The spanner multiplies that distance many times over, so the same pull from your hand delivers a far bigger moment.
Pushing on a bicycle pedal
Figure to be added
Here the axis is the central axle the pedals turn about. The perpendicular distance is the length of the pedal crank — the distance from that axle out to where your foot rests.
When the pedal is right at the top
With the pedal at its highest or lowest point, the line of action of a downward push runs straight through the axle. The perpendicular distance is then zero, so there is no moment however hard you stamp. Cyclists know this — it is why they push the pedal forward rather than straight down.
Using a wheelbarrow
Figure to be added
The axis here is the axle of the wheel. The load acts downwards and produces a moment one way; the upward force you apply at the handles produces a moment the other way.
That is why the handles are long. Your force acts much further from the axle than the load does, so a force smaller than the load is enough to lift it.
One pattern behind all three
Whatever the tool, it is doing the same job: increasing the perpendicular distance so that a small force delivers a large moment.
Work these out yourself
For each of the following, say what the axis of rotation is and between which two things the perpendicular distance is measured.
- Cutting paper with a pair of scissors
- Using a long-handled tool to loosen a screw
- Turning a key in a door lock