Key Terms
- Speed: Distance travelled per unit time (D ÷ T)
- Density: Mass per unit volume (M ÷ V)
- Pressure: Force per unit area (F ÷ A)
- Average speed: Total distance ÷ total time (NOT average of individual speeds)
- Formula triangle: Visual aid — cover the quantity you want to find
Must-Know Facts
- Speed: S = D ÷ T; Distance: D = S × T; Time: T = D ÷ S
- Density: D = M ÷ V; Mass: M = D × V; Volume: V = M ÷ D
- Pressure: P = F ÷ A; Force: F = P × A; Area: A = F ÷ P
- m/s to km/h: multiply by 3.6; km/h to m/s: divide by 3.6
- g/cm³ to kg/m³: multiply by 1000
- Always convert to consistent units before calculating
Key Formulas
- Speed: S = D/T
- Density: D = M/V
- Pressure: P = F/A
- Units: speed (m/s or km/h), density (g/cm³ or kg/m³), pressure (N/m² or Pa)
- 1 Pa = 1 N/m²; 1 kPa = 1000 Pa
Common Mistakes
- Formula triangle confusion: Always write D = M/V then rearrange — don't guess from the triangle
- Unit mismatch: Mixing km and m, or g and kg gives wrong answers — convert units first
- Average speed: Total distance ÷ total time (not average of two speeds)
- Time as decimals: 1 hour 30 min = 1.5 hours, NOT 1.3 hours
- Pressure area: Use consistent units — force in N, area in m² gives Pa
This topic summary covers Knowledge Organiser: Compound Measures within Compound Measures for GCSE Mathematics. Revise Compound Measures in Number for GCSE Mathematics with 12 exam-style questions and 22 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Compound Measures
Which formula correctly gives speed in terms of distance and time?
A car travels 80 km at 40 km/h and then 80 km at 80 km/h. Explain why the average speed for the whole journey is NOT 60 km/h.
Quick recall flashcards
Convert 72 km/h to m/s
Divide by 3.6
72 ÷ 3.6 = 20 m/s
Check: 72 km/h means 72000 m per 3600 s
72000 ÷ 3600 = 20 m/s
Convert 25 m/s to km/h
Multiply by 3.6
25 × 3.6 = 90 km/h
Check: 25 m/s means 25 metres per second
25 × 3600 ÷ 1000 = 90 km/h