a car moving with a speed of 40km/hr can be stopped by applying brakes after at least 2m.If the same car is moving with 80km/hr.What is the least stopping distance? 

Initial velocity, u = 40 km/h = 40 × (5/18) = 200/18 m/s

Final velocity, v = 0

Distance travelled before coming to rest, s = 2 m

Using, v² = u² + 2as

=> a = -u²/(2s)

=> a = -30.864 m/s²

Again,

u = 80 km/h = 400/18 m/s

v = 0

a = -30.864m/s²

Now, v² = u² + 2as

=> s = -u²/(2a) = 8 m