Physics question on forces! when a person is driving in a car and applies the brake...?

Question

...the car loses kinetic energy to do work against friction. does the person lose kinetic energy as well? according to my teacher, the person loses kinetic energy too, but i don't see how.

and the person is wearing a seat belt. if that is anyhow related. :)

Answer 1

Kinetic energy is the energy that matter has as it's moving. When you're in the car and it's moving, you've got more kinetic energy - as you slow down, you loose kinetic energy - you're travelling slower.

Relative to the car, you're stationery, but relative to road, you're moving at the same speed as the car, so when the car slows down, so do you - and you thus loose kinetic energy.

Answer 2

First, the answer is yes:

The driver had a kinetic energy of 0.5*M*V^2, and as the car stopped he lost this kinetic energy, so he must have lost kinetic energy.

Anytime a force acts on an object in a direction opposite to it's motion, it will lose kinetic energy, as a negative work is performed on it (work=force*length, where both are vectors).

What's confusing here is that you see how kinetic energy is dependent on your reference frame rather than being a real absolute and solid element of reality. From a standpoint that's moving with the cars initial velocity, the story is quite different:

The car and driver started at rest, and were accelerated by latching on to the moving ground. They gained kinetic energy, by lending some from the moving Earth.

Both viewpoints are equally valid, and kinetic energy is in the eye of the beholder.....

Anytime an object loses kinetic energy, it is always possible to find a reference frame in which it actually gained kinetic energy (the reference frame in which the body was originally static)

Answer 3

Yes the person loses kinetic energy.  If he has lost velocity, he has lost kinetic energy.  If he was not wearing a seat belt, then it is possible the he wouldn't lose kinetic energy, since while the car stops, he keeps going (for a while).  In fact, you can say that the seat belt causes the person to lose energy, since it ties his movement to that of the car.  (Assume the seat is slippery and by itself won't keep the person in place.)

Another way to look at it is to examine the work done by friction.  If the car mass is Mc and the person mass is Mp, the frictional force is proportional to Mc + Mp;  frictional work is frictional force times distance, so the presence of Mp increases the work by friction which goes into decreasing the kinetic energy of the person.