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Motion and Forces

Braking distance, speed and work done

  • 3 things to know
  • Paper 1

Overview

How stopping distance grows with speed, thinking distance estimates, the work done equation, and braking distance found from the kinetic energy a braking force must remove.

What you need to know

Answer each one before you reveal it.

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Key term

Describe how the stopping distance of a car changes as its speed increases.

Key term

Estimate the thinking distance for a car travelling at 20 m/s if the driver's reaction time is 0.7 s.

Key term

Estimate the thinking distance for a car travelling at 30 m/s if the driver's reaction time is 0.8 s.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe how the stopping distance of a car changes as its speed increases.

    Model answer

    The stopping distance increases, and it grows much faster than the speed itself does.

  2. 2

    A car needs about 23 m to stop from 13 m/s. Estimate whether the stopping distance from 26 m/s is more or less than double this, and give a reason.

    Model answer

    More than double, because the braking part of the distance grows with the square of the speed.

  3. 3

    Estimate the thinking distance for a car travelling at 20 m/s if the driver's reaction time is 0.7 s.

    Model answer

    20 × 0.7 = 14 m.

  4. 4

    Estimate the thinking distance for a car travelling at 30 m/s if the driver's reaction time is 0.8 s.

    Model answer

    30 × 0.8 = 24 m.

  5. 5

    A typical car stops from 13 m/s in about 23 m. Estimate the stopping distance from 31 m/s.

    Model answer

    It is roughly 100 m, about four times as far, because the braking distance grows with speed squared.

  6. 6

    Explain why a lower speed limit outside a school reduces the risk of a serious injury.

    Model answer

    A lower speed gives a much shorter stopping distance and smaller forces in any impact that does occur.

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