Explain that the stopping distance of a vehicle is affected by a range of…

The idea

Stopping distance is the total distance a vehicle travels from the moment the driver sees a hazard until the vehicle comes to rest. It is the sum of the thinking distance (travelled while the driver reacts) and the braking distance (travelled while the brakes slow the vehicle). A heavier vehicle carries more momentum at the same speed, so more work must be done by the braking force and the braking distance grows. A faster vehicle has a larger braking distance because kinetic energy depends on the square of the speed. A longer reaction time, caused by tiredness, alcohol or distraction, increases thinking distance. Worn brakes, a wet or icy road, and reduced friction between tyre and road all lengthen braking distance.

What earns the marks

• Identifies stopping distance as thinking distance plus braking distance.
• States that a more massive vehicle has a longer braking distance for the same speed.
• Explains that greater speed increases braking distance because kinetic energy rises with the square of speed.
• Recognises that a longer reaction time increases thinking distance.
• Links worn brakes, poor road state or low tyre–road friction to a longer braking distance.

Common mistakes and how to put them right

• Treats stopping distance as only the braking distance.

  Put it right: Add the thinking distance to obtain the total.

• Assumes mass has no effect on braking distance.

  Put it right: Recognise that a heavier vehicle needs more work to stop.

• Claims speed doubles braking distance.

  Put it right: Note that kinetic energy depends on speed squared, so the effect is larger.

Exam tips

• Separate thinking distance from braking distance before choosing an answer.
• Ask which factor changes reaction time and which changes the braking force or friction.
• Use the kinetic energy relationship to judge how speed affects braking distance.