A driver of car moving at `20 ms^(-1)` finds a child on the road 50 m ahead and stops the car 10 m earlier to the child. If the mass of the car with the driver is 1000 kg. Calculate the force exerted by the breaks on the car and the time taken to stop the car.

Explanation

Given `v_0 = 20 ms^(-1)` , Effective distance s = (50 – 10)m
` m= 1000 kg, F = ma = ?, v= 0, s = 40 m`
wkt, ` v^2 = v_0^2 + 2as`
` 0 = (20)^2 + 2 xx a xx 40`
` therefore a = - ( 400)/(80) = - 5 ms^(-2)`
retarding force `F= mg = 1000 xx 5 ....... = 5 xx 10^3 N`
time required to stop the car `t = (0 - v_0)/(a) = (-20)/(-5) =4 s `