FRICTION AND BRAKING
4 marks · HardA car travels at 50 mph on a wet road surface and an identical car travels at the same speed but on a dry road. Both drivers apply the brakes at the same time when they see an obstacle ahead.
Explain why the car on the wet road will have a longer stopping distance. Refer to the forces involved in your answer.
State two factors, apart from the road surface, that could increase the braking distance for both cars.
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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(a) Wet roads decrease the friction between the tyres and the road, reducing the maximum braking force. This results in a smaller deceleration, so the car takes longer and travels further before stopping. · must show reference to friction being lower on wet surface, reference to resultant smaller braking force or deceleration
(b) e.g. worn brake pads, worn tyres, increased vehicle mass, under-inflated tyres, brake failure. · must show any two valid factors not related to road surface
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that there is less friction or less grip between tyres and road when wet
B2 (a) · 1 mark · Relates this to reduced braking force/deceleration or longer stopping distance
B3 (b) · 1 mark · Gives one valid factor affecting braking distance (not road surface, e.g. worn brakes, worn tyres, heavy load)
B4 (b) · 1 mark · Gives a second valid factor (as above)
Worked solution
a) On a wet road, there is less friction between the tyres and the road, so the braking force is reduced. Because the car cannot decelerate as quickly, it travels further before stopping, so the stopping distance increases.
b) Possible factors:
• Worn brake pads (reduce braking force)
• Worn tyres (less grip/friction)
• Heavier vehicle mass
• Under-inflated tyres (reduce contact/friction)