# Conservation of momentum and energy problem (Please check if my setup is right)

1. Feb 8, 2012

### Kinermatics

1. The problem statement, all variables and given/known data

2. Relevant equations
m1v1i + m2v2i = m1v1f + m2v2f (Conservation of momentum)
(1/2)m2*(v2i)^2 + (1/2)m1*(v1i)^2 = (1/2)m2*(v2f)^2 + (1/2)m1*(v1f)^2 (Conservation of energy)

3. The attempt at a solution
I separated the momentum into x and y components and got 2 equations
I used the conservation of energy (can be used since its elastic) and got another equation
-I have 3 unknowns and three equations...which gives me hope that this is solvable lol
1. The problem statement, all variables and given/known data
Here are my equations:
Momentum for x:
0 = m2*v2f*sin(theta) + m1*v1f*sin(75)
unknowns here: theta, v2f, v1f

Momentum for y:
m2*v2i = m2*v2f*cos(theta) + m1*v1f*cos(75)
unknowns here: theta, v2f, v1f

Conservation of energy equation:
(1/2)m2*(v2i)^2 = (1/2)m2*(v2f)^2 + (1/2)m1*(v1f)^2
unknowns here: v2f, v1f

Can you guys see if these equations are right before I start using the tedious algebra involved in this.

Last edited: Feb 8, 2012
2. Feb 8, 2012

### vela

Staff Emeritus
The x-momentum equation should have a minus sign before the first term, but other than that, your set-up looks fine.

3. Feb 8, 2012

### Kinermatics

oh so..
0 = -m2*v2f*sin(theta) + m1*v1f*sin(75)?
Minus sign just because it goes to the left right?
And thank you!

4. Feb 9, 2012

### vela

Staff Emeritus
Right.