Stopping Distances

GCSE Physics · Forces

Stopping a vehicle

The stopping distance is the total distance a vehicle travels from the moment a driver spots a hazard to when the vehicle stops completely. It has two parts:

stopping distance = thinking distance + braking distance

Thinking distance — the distance travelled during the driver's reaction time (before the brakes are applied).
Braking distance — the distance travelled while the vehicle is braking (slowing to a stop).

What affects thinking distance?

Thinking distance depends on the driver's reaction time and the speed:

Higher speed → further travelled while reacting.
Tiredness, alcohol, drugs, and distractions (e.g. phones) → slower reactions → longer thinking distance.

What affects braking distance?

Braking distance depends on speed and the braking force / grip:

Higher speed → much longer braking distance (see below).
Poor road/tyre conditions — wet, icy, gravel, or worn tyres → less grip → longer braking distance.
Worn or faulty brakes → less braking force → longer braking distance.
Greater mass (heavily loaded vehicle) → longer braking distance.

The effect of speed

Thinking distance is proportional to speed (double the speed → double the thinking distance).
Braking distance increases with the square of the speed (double the speed → four times the braking distance), because the kinetic energy (½mv²) that the brakes must remove increases with v².

So stopping distance rises sharply at higher speeds — a key road-safety message.

Braking, energy and forces

When braking, work is done by the friction force between the brakes and wheels. This transfers the car's kinetic energy into thermal energy — the brakes get hot. A larger braking force decelerates the car faster but too large a force can cause skidding and loss of control.

Worked example

If a car's speed doubles, what happens to its thinking and braking distances?

Thinking distance doubles (proportional to speed).
Braking distance becomes four times greater (proportional to speed²). ✓

Common mistakes

Forgetting stopping distance = thinking + braking (not just braking).
Saying braking distance doubles when speed doubles — it becomes four times greater.
Putting alcohol/tiredness under braking distance — they affect thinking distance (reaction time).

Exam tips

Learn which factors affect thinking distance (reaction time: tiredness, alcohol, distraction) vs braking distance (speed, road/tyre condition, brakes, mass).
Emphasise the square relationship for braking distance with speed.
Link braking to kinetic energy → thermal energy in the brakes.

Key facts to remember

Stopping distance = thinking distance + braking distance.
Thinking distance ∝ speed (and worsened by tiredness, alcohol, drugs, distraction).
Braking distance ∝ speed² (and worsened by wet/icy roads, worn tyres/brakes, greater mass); braking converts kinetic energy to heat in the brakes.