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Question

A car is moving in a straight line with speed 18 km h1. It is stopped in 5 s by applying the brakes. Find:

(i) the speed of car in m s1,
(ii) the retardation of car
(iii) the speed of car after 2 s of applying the brakes.

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Solution

(i) Initial speed of the car, u=18km/h

To convert km/h into m/s we need to multiply it by 5/18.

Therefore speed in m/s = 18×518 =5m/s

(ii) Final speed, v=0 , u = 5 m/s , time taken to stop t = 5s
From the first equation of motion, v=u+at0=5+a×5a=1
Hence, the retardation is 1 m/s2.

(iii) Velocity after 2 seconds of applying brakes, u = 5 m/s , a = -1 m/s2 , t = 2s

using first equation of motion we get:

v=u+at=51×2=52=3 m/s


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