A car moving at 40km/h is to be stopped by applying brakes in the next 4.0 m. if the car weight 2000kg what average force must be applied on it?

Question

A car moving at 40km/h is to be stopped by applying brakes in the next 4.0 m. if the car weight 2000kg what average force must be applied on it?

Ans : Since car moving at speed 40km/h with mass = 2000kg its kinetic energy is 123456.7901234568 joule . its displacement is given which is 4 m . Force is = Energy/displacement. = 30864.1975308642 N .

Ans2: Given initial velocity = 100/9 m/s . Displacent is 4.0m and final velocity is 0 ( since body have to stop at last. )
using formula V² = u² + 2as acceleration is equal to -15.4320987654321m/s² . Mass is 2000kg using formula F=ma force = -30864.1975308642N Which indicate that force is applying in opposite direction of motion of car.

 

Which is correct answer or correct way of solving equation and why? or why not? 

Answer 1

Tony,

Both answers contain the same magnitude for the force, but answer 2 has the correct sign. Notice that the car would not have a "weight" of 2000kg since that is a mass unit!  I like Ans 2.