ZIMSEC O Level · Physics · Form 3
Moments
Moment = force × perpendicular distance from the pivot (N m).
Principle of moments: for balance, sum of clockwise moments = sum of anticlockwise moments.
Worked example: A 40 kg child sits 1.5 m from the pivot of a see-saw. Where must a 60 kg child sit to balance it?
40 × 10 × 1.5 = 60 × 10 × d → 600 = 600d → d = 1.0 m
Centre of mass and stability
The centre of mass is the point where the weight seems to act. Objects are more stable with a low centre of mass and a wide base (buses, chairs).
Pressure
Pressure = force ÷ area (P = F/A), measured in pascals (Pa = N/m²).
- Sharp knives and nails have small areas, so large pressure. Tractor tyres and wide straps spread force, so small pressure.
- Liquid pressure increases with depth: P = ρgh. That is why dam walls are thicker at the bottom.
- Atmospheric pressure is measured with a barometer; it falls with altitude.
- Hydraulic systems pass pressure through a liquid (car brakes, jacks).
Exam tip: Convert cm² to m² for pressure: 1 cm² = 0.0001 m².
Check yourself
- A 600 N box rests on an area of 0.2 m². Find the pressure.
- Find the pressure 10 m below the surface of water (ρ = 1000 kg/m³, g = 10 N/kg).
- Why is a dam wall thicker at the bottom?
Show answers
- 3 000 Pa
- 100 000 Pa
- Water pressure increases with depth.
Questions about this topic (0)
No questions about this topic yet.
Sign in or create a free account to join in.