Estimate the overall stopping distance of a car travelling at about 13 m/s in good conditions.

About 23 m, made up of roughly 9 m of thinking distance and 14 m of braking distance.

Estimate the overall stopping distance of a car travelling at about 9 m/s in good conditions.

About 12 m, roughly 6 m thinking distance and 6 m braking distance.

Estimate the overall stopping distance of a car travelling at about 31 m/s in good conditions.

About 96 m, roughly 21 m thinking distance and 75 m braking distance.

What happens to thinking distance when a car's speed is doubled?

It doubles, because the reaction time is unchanged and the car covers twice the distance in it.

What happens to braking distance when a car's speed is doubled?

It becomes roughly four times as long.

OCR GCSE Physics A J249

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Global Challenges

Topic overview Typical speeds and everyday accelerations Reaction time, stopping distance and the dangers of large decelerations Estimating stopping distances and the forces in a collision Energy resources, and trends in their use The national grid, and step-up and step-down transformers Why the national grid is efficient, and the transformer equation Mains electricity: a.c., d.c. and the domestic supply Live, neutral and earth wires, and electrical danger Red-shift and the Big Bang The formation of the Sun, and radiation from bodies The solar system Circular orbits and stable orbits Radiation and the temperature of a body, and seismic waves and sonar

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Global Challenges

Estimating stopping distances and the forces in a collision

3 things to know
Paper 4

Overview

Estimated stopping distances at typical speeds, how thinking and braking distance change when speed doubles and why, calculating overall stopping and thinking distances, estimating the braking, driving and collision forces on cars, lorries and passengers, and why collision forces are so much larger than in ordinary driving.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain why braking distance grows so much faster than speed.

Key term

A driver's reaction time is 0.7 s and the car is travelling at 20 m/s. Estimate the thinking distance.

Key term

A car of mass 1000 kg decelerates at 8 m/s². Estimate the braking force acting on it.

Questions from this subtopic

Each one with the model answer underneath.

1
Estimate the overall stopping distance of a car travelling at about 13 m/s in good conditions.
Model answer
About 23 m, made up of roughly 9 m of thinking distance and 14 m of braking distance.
2
Estimate the overall stopping distance of a car travelling at about 9 m/s in good conditions.
Model answer
About 12 m, roughly 6 m thinking distance and 6 m braking distance.
3
Estimate the overall stopping distance of a car travelling at about 31 m/s in good conditions.
Model answer
About 96 m, roughly 21 m thinking distance and 75 m braking distance.
4
What happens to thinking distance when a car's speed is doubled?
Model answer
It doubles, because the reaction time is unchanged and the car covers twice the distance in it.
5
What happens to braking distance when a car's speed is doubled?
Model answer
It becomes roughly four times as long.
6
Explain why braking distance grows so much faster than speed.
Model answer
The energy in the car's kinetic store depends on the square of its speed, so at double the speed four times as much energy must be removed by the same braking force, over four times the distance.

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