The turning effect of a force
A force can make an object turn about a fixed point called the pivot (or fulcrum). The turning effect is called the moment of the force. Opening a door, using a spanner or balancing a see-saw all depend on moments. The further the force is from the pivot, the larger the turning effect.
Key idea
moment = force × perpendicular distance from the pivot. Its unit is the newton metre (N m). A clockwise moment turns one way; an anticlockwise moment turns the other.
The principle of moments
When an object is balanced (in equilibrium) about a pivot, the total clockwise moment equals the total anticlockwise moment. This is the principle of moments. For full equilibrium, the resultant force must also be zero.
Worked example
A child of weight 300 N sits 1.5 m from the pivot of a see-saw. Where must a 450 N child sit to balance it?
Clockwise = anticlockwise: 300 × 1.5 = 450 × d, so 450 = 450 × d, giving d = 450 ÷ 450 = 1.0 m from the pivot.
Centre of mass and stability
The centre of mass is the point where the whole weight of an object seems to act. An object is more stable when it has a low centre of mass and a wide base. It topples once its centre of mass passes beyond the edge of its base.
Remember
- moment = force × perpendicular distance.
- Balanced: clockwise moments = anticlockwise moments.
- Low centre of mass + wide base = more stable.