# Homework Help: Perfectly Elastic Collision

1. Dec 2, 2007

### Dorney

[SOLVED] Perfectly Elastic Collision

1. A particle of mass m and speed V collides at a right angle with a very massive wall in a perfectly elastic collision. What is the magnitude of the change in momentum of the particle?

2. P before colision = P after collision.

3. I think the answer is zero. Is this correct?

2. Dec 3, 2007

### shawshank

sorry, wrong

Last edited: Dec 3, 2007
3. Dec 3, 2007

### ddrtrinity

Well, it does change direction. :) So it's -1 x P (for the new velocity).

4. Dec 3, 2007

### Bill Foster

Momentum prior to collision: $$m\vec{v_1}$$
Momentum after collision: $$m\vec{v_2}$$

$$\vec{v_1}=v_x \hat{x}$$
$$\vec{v_2}=-v_x \hat{x}$$

$$\Delta{p}=m(\vec{v_1}-\vec{v_2})$$
$$\Delta{p}=m(v_x\hat{x}- - v_x\hat{x})$$
$$\Delta{p}=2m v_x \hat{x}$$

$$2mv$$

5. Dec 3, 2007

### shawshank

Sorry, bill is right, i was wrong.
Perfectly elastic collision occurs in isolated systems tho no? so you can't really consider this a collision, this is like magnitude of velocity in equals to magnitude of velocity out.

6. Dec 3, 2007

### Dorney

Thank you!! That makes so much more sense!