Moments
Why a small weight far out balances a big one close in.
A moment is a force multiplied by how far from the pivot it acts. A weight of one newton a metre out and a weight of two newtons half a metre out make the same moment, so a beam carrying both balances. This is the whole of the seesaw, the crowbar, the spanner and the crane.
The distance that counts is the perpendicular distance from the pivot to the line the force acts along, not the distance to where it is applied. Push straight at a pivot and the distance is nothing, so the moment is nothing, which is why a door will not open if you push at the hinge.
| Quantity | Symbol | How it is worked out | Unit |
|---|---|---|---|
| A moment | M | force × perpendicular distance | newton metres |
| Balance | the moments one way equal the moments the other |
In the app, put a plank on a pin and hang weights off it. A meter reading the plank's angle settles at nothing when the moments agree. Move one weight out and watch the angle go. The centre of mass of a shape can be moved, which changes the moment its own weight makes about a pin somewhere else.
Open the scales scene in the app to watch this one run.