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## Homework Statement

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?

## Homework Equations

[tex]m_1v_{1i}+}m_2v_{2i} = m_1v_{1f}+m_2v_{2f} [/tex]

## The Attempt at a Solution

I manipulated the formula a little bit to solve for V2_f and came up with this:

[tex](m_1v_{1i}+}m_2v_{2i} - m_1v_{1f}) / m_2[/tex]

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.