Unit 8: Pressure, Moments and Their Effects
Forces have real consequences: they can cause objects to turn, and they act differently depending on the area they push on. This unit covers pressure and the turning effect of forces (moments).
01 Pressure
pressure (Pa) = force (N) ÷ area (m²)
For the same force, a smaller area produces a bigger pressure. This is why a sharp knife cuts more easily than a blunt one (smaller contact area, same force), and why wide tyres or skis prevent sinking into soft ground (larger area, lower pressure).
02 Moments (turning effect)
moment (N·m) = force (N) × perpendicular distance from pivot (m)
A moment is the turning effect of a force around a pivot point. This is why it is easier to open a door by pushing far from the hinge than close to it — a larger distance from the pivot gives a larger moment for the same force.
03 Balanced moments
An object is in rotational equilibrium (balanced, not turning) when the total clockwise moment equals the total anticlockwise moment. This is the principle behind a simple seesaw or a balance beam.
Example: A 40 kg child sits 1.5 m from the pivot of a seesaw. A 60 kg child must sit closer to the pivot (1 m) to balance, because they produce the same moment with less distance.
V Key vocabulary
| Term | Meaning |
|---|---|
| Pressure | Force per unit area, measured in pascals (Pa). |
| Moment | The turning effect of a force, measured in newton-metres (N·m). |
| Pivot | The fixed point around which an object turns. |
| Equilibrium | A balanced state where the resultant force and resultant moment are both zero. |
? Quick check
Try to answer before you tap. Answers are hidden underneath.
A force of 200 N acts on an area of 0.5 m². Calculate the pressure.
400 Pa (P = F ÷ A = 200 N ÷ 0.5 m² = 400 Pa).
A spanner applies a 50 N force at 0.2 m from a bolt's pivot. Find the moment.
10 N·m (moment = force × distance = 50 N × 0.2 m = 10 N·m).
Why do snowshoes stop a person sinking into snow?
They increase the contact area, which reduces the pressure on the snow for the same body weight (force).