Topic 8 – Energy – forces doing work — Edexcel GCSE Physics
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Topic 8 – Energy – forces doing work explained
This equation gives the energy in an object's kinetic store because it is moving.
Read the full explanation
Multiply half the mass in kilograms by the square of the speed in metres per second: KE = ½ × m × v². Squaring the speed means doubling speed quadruples the kinetic energy. A 4 kg trolley moving at 3 m/s has KE = ½ × 4 × 3² = ½ × 4 × 9 = 18 J. Square the speed before multiplying by the mass, and keep mass in kilograms and speed in metres per second so the answer is in joules. Convert grams to kilograms and km/h to m/s first, because the squared unit (m/s)² depends on the speed being in the correct unit.
Your focus
- Recall the equation KE = ½ × m × v² and the units of each quantity.
- Calculate the kinetic energy of a moving object using mass and speed.
- Explain how changing the speed affects the kinetic energy because the speed is squared.
Topic 8 – Energy – forces doing work exam tips
Marking Points
- Squares the speed in metres per second before multiplying by half the mass in kilograms.
- Multiplies half the mass by the squared speed and states the result in joules.
- Converts mass to kilograms and speed to metres per second before substituting into the equation.
- Recognises that doubling the speed increases the kinetic energy by a factor of four because the speed is squared.
Examiner Tips
- 💡Square the speed as a separate step so the arithmetic is easy to check.
- 💡Write the unit of the answer as J and confirm mass and speed are in kg and m/s.
- 💡Use the factor-of-four relationship to sense-check answers when speed changes.
Common Mistakes
- Multiplies by the speed rather than its square; square the speed first.
- Doubles the mass instead of halving it; use half the mass in the equation.
- Leaves the speed in km/h or the mass in grams; convert to m/s and kg before calculating.
- Assumes doubling the speed doubles the energy; recognise that the squared term makes it four times larger.