Question

A car is running at a steady speed of 72km/hr . If it's speed is reduced to 18km/hr in 2 seconds on applying brake . Find (1)retardation ,(2) time in which the car comes to a stop ,(3) distance traveled by car before coming to a rest ?
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Answer 1

Given: u = 72 km/h = 20 m/s; v = 18 km/h = 5 m/s; t = 2 s

1) a = (v – u) / t

= (5 – 20) / 2

a = – 7.5 m/s²

2) Let, u = 5 m/s

–u = at

t = –u / a

= | –5 / –7.5 |

t = 0.67 s

So, the total time = 2.67 s

3) s = (v²–u²)/2a

= ((0)²–(20)²) / 2(–7.5)

= (–400) / 2(–7.5)

s = 26.67 m