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Question

# A car, moving at 1.5 msâˆ’1 applies brakes and comes to rest in 2s. If the same car travels at double the speed, what time would it take to come to rest after applying brakes?

A
4s
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B
8s
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C
2s
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D
3s
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Solution

## The correct option is A 4sGiven,Initial velocity u1=1.5ms−1Final velocity v1=0Time taken t1=2sAccording to first equation of motion,v1=u1+a1t10=1.5+2a1a1=−1.52=−0.75 ms−2Now, for the second case, Initial velocity u2=3ms−1Final velocity v2=0The braking system is still the same. So it provides the same retarding acceleration of −0.75 ms−1So acceleration a2 is same as a1=−0.75 ms−1Using first equation of motion again,v2=u2+a1t20=3−0.75×t2t=30.75=4s

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