# Homework Help: Perfectly elastic collision

1. Dec 3, 2008

### southernbelle

1. The problem statement, all variables and given/known data
In an isolated system, cart1 (with mass = 1 kg and vi1 = 2) has a perfectly elastic collision with cart2 (with mass = 2 kg and vi2 = 0). Find the velocity of cart1 and the velocity of cart2 after the collision.
I have to solve this using kinetic energy and momentum equations.

2. Relevant equations
m1vi1 + m2vi2 = m1v1f + m2v2f
1/2m1vi12 + 1/2m2vi22 = 1/2m1v1f2 + 1/2m2v2f2

3. The attempt at a solution
I have gotten to this point:
2= V1F + 2V2F
4= V1F2 + 2V2F

but I cannot get the numbers to work out correctly.
Using another equation I know that V1f = 2/3 and V2f = 4/3

I am doing substitution wrong or something.

2. Dec 3, 2008

### Staff: Mentor

This makes sense.
How did you get this?

3. Dec 3, 2008

### southernbelle

I got that by solving the kinetic energy equation.
I mutiplied both sides by 2 to get rid of the halves.

It is actually supposed to read:

4 = Vif^2 + 2V2f^2

4. Dec 3, 2008

### Staff: Mentor

That's good.

Take the first equation and solve for one of your variables. Then substitute that into the second equation. Solve the quadratic.