Question

A car moving along a straight highway with a speed 72 km /h os bought to a stop within a distance of 100m. What is the retardation of the car and how long does it take for he car stop ?

Answer 1

initial speed of car u=72km/h=20m/s
the car stops with a velocity v=0
the distance would be covered s=100m
let retardation be a
v^2= u2-2as
or, a=u2/2s=400/200=2m/s
time taken be t
a=(v-u)/t
or,t=20/2=10s

Answer 2

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Initial velocity of the car, u = 126 km/h = 35 m/s

Final velocity of the car, v = 0

Distance covered by the car before coming to rest, s = 200 m

Retardation produced in the car = a

From third equation of motion, a can be calculated as:

V^2 - U^2 = 2aS

(0)^2 - (35)^2 = 2 . a . 200

a = ( 35×35)/(2× 200)

a= 3.06 m/ s^2

From first equation of motion, time (t) taken by the car to stop can be obtained as:

V = U + at

t = (V- U )/a = (-35)/(-3.06) = 11.44 sec

I hope, this will help you

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