# Conservation of momentum, Elastic car collision.

1. Apr 16, 2013

### Zsmitty3

A 1689 kg car collides head on with a 2000 kg truck. The collision is elastic. If the velocity of the truck is 17km/h in the same directions as the car's initial velocity, what is the initial speed of the car in km/h?

I'm getting an answer but doesn't seem to make sense.

So I'm looking at the conservation of momentum equation to set this up.

(1689-kg)(x)+(2000-kg)(0 km/h)=(1689-kg)(0)+(2000-kg)(17 km/h)

Then: (1689)(x)=(34000) or x= 20.13

I'm assuming that the final velocity of the car is 0 but it doesn't specifically say in the questions. Can I assume that since it's head on? Also it's a multiple choice question and the closest answer I can find is 19 km/h.

2. Apr 16, 2013

### haruspex

There's not enough information. Below, you assume the truck is initially stationary. Is that true? (Not what I would call 'head on'.)
You should not. You are told the collision is elastic. What additional conservation law does that allow you to use?

3. Apr 16, 2013

### Zsmitty3

The Truck is parked so yes its initial is 0. Sorry I forgot to add that. Conservation of KE?

Last edited: Apr 16, 2013
4. Apr 17, 2013

Yes.