A moment is the turning effect of a force. Change the weights and how far out they sit, then find the balance point where the two turning effects are equal.
Balance the seesaw: make the left and right moments equal.
Left moment
Right moment
The moment equation
Moment = force × distance from the pivot. The distance is measured from the pivot (the balance point) to where the force acts. A seesaw balances when the total anticlockwise moment equals the total clockwise moment. That is why a small child far from the pivot can balance a heavier adult sitting close in.
Check yourself
What are the two things a moment depends on?
The size of the force and the distance from the pivot. Moment = force × distance, so moving a weight further out increases its turning effect.
A 200 N child sits 2 m from the pivot. How far out must a 400 N adult sit to balance?
1 m. The child's moment is 200 × 2 = 400 N m, so the adult needs 400 N × distance = 400 N m, which gives a distance of 1 m.
Why is a long spanner easier to turn a stiff bolt than a short one?
The longer spanner puts your force further from the pivot (the bolt), so the same force gives a bigger moment and a greater turning effect.