Question

a bus is moving with speed of 72 km per hour can be stopped by breaks after at least 10 m. what will be the minimum stopping distance if the same bus is moving at a speed of 144 km per hour?

Answer 1

the distance will be (144/72)squre *10m = 40m

Answer 2

let 
u be the initial velocity
v be the final velocity
a be the acceleration
s be the distance

For case 1
u = 72 km/hr =20 m/s
v = 0 m/s
s = 10 m
a = ?

using v^2 - u^2 = 2as

a = (v^2 -u^2)/2a
a = (0 - 20 X 20)/2 X 10
a = - 20 m/s^2                                          -------(i)

For case 2
u = 144 km/hr = 80 m/s
v = 0 m/s
a = - 20 m/s^2                                        ----------( from i)
s = ?

using v^2 - u^2 = 2as
s = ( v^2 -u^2)/2a
s = (0 - 80 X 80)/ 2 X (-20)
s = 80 m