# Homework Help: Unit-vector momentum?

1. Feb 1, 2009

### FlukeATX

[Resolved] Unit-vector momentum?

1. The problem statement, all variables and given/known data
A 0.200 kg ball hits a wall with a velocity of (7.60 m/s)i + (8.60 m/s)j + (3.50 m/s)k. It rebounds from the wall with a velocity of (1.80 m/s)i + (3.40 m/s)j + (-4.70 m/s)k. In unit-vector notation, what are (a) the change in the ball's momentum, (b) the impulse on the ball, and (c) the impulse on the wall?

2. Relevant equations
p=mv

3. The attempt at a solution
I attempted subtracting each component of the vectors multiplied by the mass, like this:
Pi = ((7.6)-(1.8))*0.2
Pj = ((8.6)-(3.4))*0.2
Pk = ((3.5)+(4.7))*0.2

P=(1.16)i+(1.04)j+(1.64)k

This was wrong, and I'm not sure why it is or how to go about it. I submit with the wileyplus system, which said that none of my vector components were correct. Anyone know why?

Last edited: Feb 1, 2009
2. Feb 1, 2009

### LowlyPion

Your calculations are initial - final?

3. Feb 1, 2009

### FlukeATX

That was it, thanks!

4. Feb 1, 2009

### LowlyPion

You solved it yourself.

All you needed was a sign.