Compound units — speed, density, pressure

A compound unit combines two basic measurements. WJEC tests three:

• Speed = distance / time (m/s, km/h, mph)
• Density = mass / volume (g/cm³, kg/m³)
• Pressure = force / area (N/m² = pascals; N/cm²)

The standard triangle method

Each formula has three quantities. Cover the one you want to find:

• speed = distance ÷ time
• distance = speed × time
• time = distance ÷ speed

Apply the same idea to density (mass, density, volume) and pressure (force, pressure, area).

Speed unit conversions

To convert m/s to km/h: multiply by 3.6 (since 1 m/s = 3600 m/h = 3.6 km/h). To convert km/h to m/s: divide by 3.6.

Example: 72 km/h → 72 ÷ 3.6 = 20 m/s.

Density worked example

A metal block has mass 540 g and volume 60 cm³. Density = 540 ÷ 60 = 9 g/cm³.

If you needed kg/m³: 9 g/cm³ × 1000 = 9000 kg/m³.

Pressure worked example

A force of 240 N acts on an area of 0.2 m². Pressure = 240 ÷ 0.2 = 1200 N/m² (= 1200 Pa).

Mixed-unit traps

A car travels 150 km in 2 hours 30 minutes. Average speed?

Convert time to a single unit: 2.5 hours. Speed = 150 ÷ 2.5 = 60 km/h.

If asked for m/s: 60 ÷ 3.6 = 16.67 m/s (3 s.f.).

WJEC exam tip

State the formula EXPLICITLY before substituting M1. On Foundation/Intermediate the M1 is awarded for "speed = distance / time" written down even before any numbers go in. Many candidates lose this mark by jumping straight to division.