Kinetic energy
Ek = 1/2 m v2
The half and the squared are two separate marks to lose — and most people lose at least one.
Know the equation
| Symbol | Quantity | Unit |
|---|---|---|
| Ek | kinetic energy | J |
| m | mass | kg |
| v | speed | m/s |
Rearrangements
- v = sqrt(2 Ek / m)
- m = 2 Ek / v2
Apply it — mark your own working
Work each one out on paper first, then reveal the mark scheme and tick the marks you actually earned. That is exactly how you should mark past papers.
A car of mass 1200 kg is travelling at 15 m/s. Calculate its kinetic energy.
Do the calculation on paper first — then mark it.
A ball of mass 0.40 kg has 20 J of kinetic energy. Calculate its speed.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Multiplying the speed by 2 instead of squaring it (15 x 2 = 30, not 152 = 225).
- Forgetting the 1/2 — halving is a whole mark.
- When rearranging for v, taking the square root of only part of the expression, or forgetting the root altogether.
Exam tip: Write the equation, then substitute, then evaluate on a new line. If the mass is given in grams, convert to kg (divide by 1000) BEFORE substituting.
Still losing marks on the calculations?
I'll go through your working line by line and show you exactly where the marks are — your first lesson is free.