FRICTION AND BRAKING
3 marks · NormalA cyclist travels along a road on a foggy morning. Suddenly, a dog runs out in front of them. The cyclist has good brakes and new tyres, but still almost hits the dog.
Explain two reasons, using your knowledge of physics, why the cyclist nearly failed to stop in time. (3)
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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Poor visibility in fog increases reaction time, increasing thinking distance; the cyclist travels further before starting to brake. Although brakes and tyres are in good condition (maximising friction, so braking distance is minimised), the increased thinking distance can still cause a near-miss.
Also accept: Mention of the speed being too high for the conditions
Common mistakes: Ignoring the effect of visibility (reaction time); Thinking tyres alone always prevent stopping issues
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Identifies that fog increases reaction time (poor visibility).
B1 · 1 mark · Explains that greater reaction time leads to a longer thinking distance.
B1 · 1 mark · States that even with good friction, increased thinking distance can cause stopping issues.
Worked solution
One reason is that the fog made it harder for the cyclist to see the dog, so their reaction time was increased. This meant a longer thinking distance, so the cyclist travelled further before braking. Even though the brakes and tyres were good, only the braking distance is minimised; the thinking distance was still large, nearly causing an accident.