Reaction Time and Braking

This lesson covers human reaction time, stopping distances and the factors that affect them — as required by the Edexcel GCSE Physics specification (1PH0), Topic 2: Motion and Forces. You need to understand the concepts of thinking distance and braking distance, how to measure reaction time experimentally, and the dangers of large decelerations.

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Stopping Distance

When a driver sees a hazard and needs to stop, the stopping distance is the total distance the vehicle travels from the moment the driver first sees the hazard to the moment the vehicle comes to a complete stop.

Stopping distance = Thinking distance + Braking distance

Thinking Distance

Thinking distance is the distance the vehicle travels during the driver's reaction time — the time between seeing the hazard and pressing the brake pedal.

During this time, the car continues at its original speed (no braking force yet).

Thinking distance = Speed × Reaction time

Braking Distance

Braking distance is the distance the vehicle travels after the brakes are applied until the vehicle stops.

During this time, the braking force decelerates the car.

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Reaction Time