Fewpal
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A 20KN automobile is moving at a speed of 70Kmph when the brakes are fully applied causing all four wheels to skid. Determine the time required to stop the automobile. 

(i) on concrete road for which µ = 0.75 

(ii) On ice for which µ = 0.08

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Given data: 

W = 20KN, u = 70Kmphr = 19.44m/sec, v = 0, t = ?

Consider FBD of the car as shown in fig,

∑V = 0, R = W ...(i)

∑H = 0, Fr = 0 Fr = µR ...(ii) 

Here net force is the frictional force 

i.e. F = Fr ma = µR = µmga = µg ...(iii)

(1) on concrete road for which µ = 0.75

a = µg = 0.75 X 9.81 = 7.3575 

a = 7.35 m/sec2 ...(iv)

Using the relation v = u – at

0 = 19.44 – 7.35t

t = 2.64 seconds

(1) On ice for which µ = 0.08

a = µg = 0.08 X 9.81 = 0.7848

a = 0.7848m/sec2

Using the relation v = u – at

0 = 19.44 – 0.7848t

t = 24.77 seconds

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