Moments, levers and gears
Moments (Separate Physics only)
A force can make an object turn about a . The turning effect is called the of the force. You must recall this equation:
, or M = Fd
| Quantity | Symbol | Unit |
|---|---|---|
| Moment | M | newton-metres (N m) |
| Force | F | newtons (N) |
| Distance | d | metres (m), the perpendicular distance from the pivot to the line of the force |
Example: you push a door with a force of 20 N, 0.8 m from the hinge.
- M = Fd = 20 × 0.8
- M = 16 N m
Pushing further from the pivot gives a bigger moment for the same force. That's why door handles are on the side away from the hinges.
Key terms
The principle of moments (Separate Physics only)
If an object is balanced, the about the pivot equals the total anticlockwise moment about the pivot.
Example: on a seesaw, child A (weight 300 N) sits 2 m to the left of the pivot. Where must child B (weight 400 N) sit to balance it?
- Anticlockwise moment = 300 × 2 = 600 N m
- For balance, clockwise moment = 600 N m
- 400 × d = 600
- d = 600 ÷ 400 = 1.5 m to the right of the pivot
The heavier child sits closer to the pivot.
Exam tip
Distances must be in metres. If a diagram gives centimetres, convert first, or use N cm for every moment.
Key terms
Levers and gears (Separate Physics only)
and pass on turning effects. Both can act as .
Levers: a small force applied makes a large moment, which can produce a bigger force close to the pivot. A crowbar, a spanner, a wheelbarrow and scissors all use this.
Gears: wheels with teeth that lock together. Each gear turns the next one the opposite way.
- A small gear driving a larger gear: the larger gear turns more slowly, but with a . This helps a bike climb hills in a low gear.
- A large gear driving a smaller gear: the small gear turns faster, with a smaller moment.