A car moving at a speed v is stopped by a retarding force F in a distance s. If the speed of the car was 3v, the force needed to stop it within the same distance s will be

Let m be the mass of the car and a be the deceleration produced by force F, then F= ma, where a is given by 2as=v2 or a=v22s.Therefore, F=mv22s.Thus F∝v2. If v is increased by 3 times, F will increase by 9 times.