When a car brakes, what happens to its kinetic energy?

It is transferred to thermal energy in the brakes and surroundings

On a graph of distance against speed, which shape does braking distance have?

A curve getting steeper (proportional to v^2)

Braking distance, speed and work done by brakes

When brakes stop a car, they do work to remove all its kinetic energy. This explains why braking distance rises so steeply with speed.

GCSE · 1PH0 · 2.32, 2.33 (Physics only)
About 15 minutes

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Work done by the brakes

Friction between the brake pads and discs does work on the car, transferring its kinetic energy to thermal energy in the brakes (they get hot).
work done by brakes = braking force × braking distance
To stop, this must equal the kinetic energy: F × d = ½ × m × v²

Why speed matters so much

For the same braking force, braking distance ∝ v².
Double the speed: kinetic energy is 4 times bigger, so braking distance is 4 times longer.
Thinking distance only doubles when speed doubles (distance = speed × reaction time).
So on a graph of stopping distance against speed, thinking distance is a straight line, but braking distance curves upwards.

Calculating braking distance

Example: a 1200 kg car at 20 m/s brakes with a force of 6000 N.
KE = ½ × 1200 × 20² = 240 000 J
d = KE ÷ F = 240 000 ÷ 6000 = 40 m

Speed (m/s)	Thinking distance at 0.7 s (m)	Braking distance, 1200 kg car, 6000 N (m)
10	7	10
20	14	40
30	21	90

Worked example

A car's braking distance is 15 m at 13 m/s. Estimate its braking distance at 26 m/s with the same braking force.

The speed has doubled.
Braking distance ∝ v², so it is multiplied by 2² = 4.
15 × 4 = 60 m

Answer: About 60 m

Key idea

Brakes do work equal to the car's kinetic energy: F × d = ½mv². Braking distance is proportional to speed squared, so doubling speed makes it four times longer, while thinking distance only doubles.

Key words

Braking distance: The distance a vehicle travels after the brakes are applied until it stops.
Work done: Energy transferred when a force moves an object: force × distance moved in the direction of the force. Unit joule (J).
Kinetic energy: Energy in the store of a moving object: KE = ½ × m × v^{2}.
Thinking distance: The distance travelled during the driver's reaction time, before the brakes are applied.
Friction: A force that opposes motion between surfaces in contact.
Time: 1 hour = 3600 s. Convert minutes and hours to seconds before using P = E ÷ t or Q = I × t.

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Extra practice

2 more questions

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A 1200 kg car travels at 20 m/s. The brakes apply a force of 6000 N. Calculate the braking distance.
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Answer: 40 m
KE = ½ × 1200 × 20² = 240 000 J. Work done = KE, so d = 240 000 ÷ 6000 = 40 m
Brakes apply a force of 6000 N over 25 m. How much work do they do?
Show the answer
Answer: 150000 J
W = F × d = 6000 × 25 = 150 000 J

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Sources and references

Pearson Edexcel GCSE Physics (1PH0) specification, topic 2.32, 2.33 (Physics only)

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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