Kinetic energy problem?

Question

A toy car has a kinetic energy of 12 Joules. What is the kinetic energy after a frictional force of 0.6 newtons has acted on it for 5 minutes?

Answer 1

Hi Greg P

There seems to be something missing from your question. The change in kinetic energy (KE) is equal to the work (W) done against the car's motion (by the work-energy theorem). The work done is equal to the the friction force (F) multiplied by the distance (s) over which it is applied:

del.KE = W = int|F.ds

For constant force, W = F*s

You're given the initial kinetic energy and the friction force, and the *time* for which it is applied. But we need the distance over which it is applied. If we had the car's mass we could work out the initial velocity and the deceleration, and hence the distance s via kinematics. Or if we had the co-efficient of friction between car and surface we could work out the mass, and hence the initial velocity, deceleration, and distance s. But we don't have these. Is there another piece to the problem you've overlooked: mass, co-efficient of friction, initial velocity or distance over which force is applied?


The Chicken

Answer 2

0.3 J