IGCSE Physics / Forces and movement
Stopping distance: thinking and braking
Edexcel International GCSE Physics (4PH1), chapter 3: Forces and movement, specification point 1.19. Free lesson.
Read the revision notes · IGCSE Physics past papers
A vehicle’s stopping distance is the total distance it travels from seeing a hazard until it stops. It has two parts: thinking distance and braking distance.
What makes up stopping distance
Stopping a car happens in two stages. First, the driver reacts and moves their foot to the brake. The car keeps travelling at the same speed during this time. Then the brakes slow the car to rest. The stopping distance is the total distance across both stages. Thinking distance is travelled during the driver’s reaction time. Braking distance is travelled while the brakes are on. Stopping distance = thinking distance + braking distance.
How to calculate thinking distance
During the reaction time, the car’s speed stays constant. So you can use distance = speed × time to find how far it travels before braking begins. Thinking distance = speed × reaction time. Use speed in metres per second (m/s) and reaction time in seconds (s). The answer is in metres (m). Convert a speed in km/h to m/s, or a reaction time in milliseconds to seconds, before substituting.
How to calculate stopping distance
Find the thinking distance first, then add the braking distance. The car has already travelled the thinking distance before the brakes start slowing it down.
What a stopping-distance chart shows
The chart compares thinking and braking distances for one car and one alert driver on a dry road. Speed is along the bottom, and distance in metres is shown on the vertical axis. Thinking distance rises in proportion to speed because the reaction time is unchanged. Braking distance rises much faster. At 10, 20 and 30 m/s, the thinking distances are 7, 14 and 21 m. At the same speeds, the braking distances are 7.5, 30 and 67.5 m. From 10 to 20 m/s, braking distance becomes four times as large. Doubling speed doubles thinking distance, but more than doubles braking distance.