Module 9
Motion & pressure
Quick-reference revision notes for parents.
9.1 Speed
speed = distance ÷ time (m/s, km/h, mph)
Worked example
A runner covers 200m in 25s. Speed?
200 ÷ 25 = 8 m/s
Rearranged forms:
- distance = speed × time
- time = distance ÷ speed
9.2 Motion graphs (distance–time)
- Straight line going up = moving at constant speed.
- Steeper line = faster.
- Flat (horizontal) line = stationary.
- Line going down = moving back towards the start.
- Gradient = speed.
Reading off a graph
Pick two points on the line. Speed = (rise) ÷ (run) = (distance change) ÷ (time change).
9.3 Pressure in gases
- Gas particles move randomly and collide with the walls of the container.
- Each collision exerts a tiny force; together they cause pressure.
- Heating a gas → particles move faster → harder collisions → higher pressure.
- Reducing the volume → particles hit walls more often → higher pressure.
9.4 Pressure in liquids
- Liquids exert pressure in all directions.
- Pressure increases with depth — more liquid weight pushing down.
- This is why dam walls are thicker at the bottom and submarines need strong hulls.
- Upthrust: an upward force from the liquid; if upthrust ≥ weight, the object floats.
9.5 Pressure in solids
pressure = force ÷ area (Pa = N ÷ m²)
Worked example
A box weighing 200N rests on the floor. Its base is 4 m². Pressure?
200 ÷ 4 = 50 Pa
- Smaller area + same force → bigger pressure (drawing pin, knife).
- Larger area + same force → smaller pressure (snowshoes, tractor tyres).
9.6 Turning forces (moments)
moment = force × perpendicular distance from pivot (Nm)
Principle of moments: when a system is balanced, total clockwise moments = total anticlockwise moments.
Worked example — see-saw
A 20N child sits 3m from the pivot. To balance, where must a 30N child sit on the other side?
20 × 3 = 30 × d
60 = 30d
d = 2m
Quick reference
- Speed = distance ÷ time
- Distance–time graph: gradient = speed; flat = stopped
- Pressure = force ÷ area (Pa = N/m²)
- Liquid pressure increases with depth; gas pressure rises with temperature or smaller volume
- Moment = force × distance from pivot. Balanced when clockwise = anticlockwise