Speed
- Speed = distance ÷ time. Distance = speed × time. Time = distance ÷ speed.
- 150 km in 2.5 hours is an average speed of 60 km/h.
- Change minutes into hours by dividing by 60: 45 minutes = 0.75 hours. 1 hour 30 minutes is 1.5 hours, not 1.3 hours.
Density
- Density = mass ÷ volume. Mass = density × volume. Volume = mass ÷ density.
- A 500 g block with a volume of 200 cm³ has a density of 2.5 g/cm³.
Pressure
- Pressure = force ÷ area. Force is in newtons (N), area in m², and pressure in N/m² (also called pascals).
- A force of 600 N on an area of 2 m² gives a pressure of 300 N/m².
Converting compound units
- Convert each unit separately. 72 km/h = 72 000 m in 3600 seconds = 20 m/s.
- 1 m/s = 3.6 km/h: multiply by 3600 (seconds in an hour), then divide by 1000 (metres in a kilometre).
- Rates of pay and unit prices are compound measures too, such as £9.50 per hour or 60p per kg.
Key terms
- Compound measure
- A measure made from two other measures, such as speed from distance and time.
- Speed
- Distance travelled ÷ time taken.
- Density
- Mass ÷ volume.
- Pressure
- Force ÷ area.
- km/h
- Kilometres per hour, a unit of speed.
- g/cm³
- Grams per cubic centimetre, a unit of density.
- N/m²
- Newtons per square metre, a unit of pressure.