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A car moving at 60 km/h is able to come to rest in 20 m after braking. If the same car is moving at 120 km/h, what stopping distance will it require?

1. 60 m
2. 40 m
3. 20 m
4. 80 m

Correct answer: 80 m

Solution

For constant braking deceleration, d = v^2/(2a), so d is proportional to v^2. Doubling the speed (60 -> 120 km/h) multiplies the stopping distance by 4: 20 m x 4 = 80 m.

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