# Physics 12 Momentum and work, energy question

1. Jun 28, 2013

### gdhillon

Q:A boy of mass 45 kg is travelling without a seat belt, in a car with a velocity of 14 m/s.
When the car stops suddenly, he is stopped by the windshield in a distance of 0.024m.
What force does he exert on the windshield?

i set the impulse of the vehicle (mv)= to FD (because (delta)E=W=Fd) and got a force of 2.6*10^4N. That seems to high of a force to me.

Calculations: p=mv ∇E=p=45(14)=630N*s ∇E=Fd 630=F*.024 solving for F i got 26250

Last edited by a moderator: Jun 28, 2013
2. Jun 28, 2013

### Staff: Mentor

Momentum is not the same as energy. The units are not the same. So ∇E=p is false.

However, you could use kinetic energy. Set the change in kinetic energy to the work done making the change (F x d).

3. Jun 28, 2013

### haruspex

mass * velocity cannot be equated with force * distance; the first gives momentum, the second energy.
Since the question provides a distance and asks for the force acting over that distance, energy is the right quantity to consider. The answer should be much higher.
(However, it is not true that the force will be constant over the impact distance. The question ought to ask for a lower bound on the peak force.)