GCSE-ready- Home
- Lessons
- GCSE & IGCSE Physics
- Getting started: using equations and units
Getting started: using equations and units
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
GCSE Physics papers are full of calculations. Most marks come from a clear routine: write the equation, convert units, substitute, calculate, add the unit.
Quantities and units
Every physical quantity has a unit. Use the standard (SI) units in equations.
Convert, then substitute
Convert before you substitute:
g to kg: ÷ 1000. km to m: × 1000.
minutes to s: × 60. hours to s: × 3600.
Then: write the equation, put in the numbers, calculate, and give the unit.
g to kg: ÷ 1000. km to m: × 1000.
minutes to s: × 60. hours to s: × 3600.
Then: write the equation, put in the numbers, calculate, and give the unit.
Rearranging
If a = b × c, then b = a ÷ c and c = a ÷ b.
Example: distance = speed × time, so time = distance ÷ speed and speed = distance ÷ time.
Round your final answer sensibly, usually to the same number of significant figures as the data (often 2 or 3).
Example: distance = speed × time, so time = distance ÷ speed and speed = distance ÷ time.
Round your final answer sensibly, usually to the same number of significant figures as the data (often 2 or 3).
A cyclist travels at 6 m/s for 5 minutes. How far does she travel?
- Convert: 5 minutes = 5 × 60 = 300 s
- distance = speed × time = 6 × 300
Answer: 1800 m
Convert to SI units, write the equation, substitute, calculate, give the unit. If a = b × c, then b = a ÷ c.
The interactive lesson includes the diagrams for this topic.
Check you have got it
Answer 7 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.