Elastic Collision and speed of a ball

1. Nov 20, 2009

Atlos

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

A 120 g ball moving to the right at 4.5 m/s catches up and collides with a 420 g ball that is moving to the right at 1.2 m/s.

If the collision is perfectly elastic, what is the speed of the 420 g ball after the collision?

2. Relevant equations

$$m_1v_{1i}+}m_2v_{2i} = m_1v_{1f}+m_2v_{2f}$$

3. The attempt at a solution

I manipulated the formula a little bit to solve for V2_f and came up with this:
$$(m_1v_{1i}+}m_2v_{2i} - m_1v_{1f}) / m_2$$

then I plugged in my numbers:
(.12*4.5 + .42*1.2 - .12*.63) / .42

simplified to this:
(.54 + .504 - .0756) / .42

and got 2.3057. Mastering physics said that is wrong however and I'm not sure what I'm doing wrong. Is the conservation of momentum the correct equation for this problem? I found the velocity of the 120 gram ball fine, which was .63 m/s, but am stuck on this one. Any help would be appreciated.

2. Nov 20, 2009

rl.bhat

Hi Atlos, welcome to PF.
How did you get v1f = 0.63 m/s?

3. Nov 20, 2009

Atlos

I came to find v1f by using the following equation:

$$[(m_1-m_2) / (m_1+m_2) ]v_{1i} + [2m_2 / (m_1+m_2]v_{2i}$$

4. Nov 21, 2009