Physics
posted by Brittany .
A ball of mass 0.640 kg moving east (+x direction) with a speed of 3.00 m/s collides headon with a 0.740 kg ball at rest. If the collision is perfectly elastic, what will be the speed and direction of each ball after the collision?

Your question and those of your classmates keep using the word "prefectly". A reaction is either elastic, or not elastic. One cant be imperfectly elastic, nor perfectly elastic.
momentum:
.640*3= .640V1 + .740V2
energy:
1/2 .640*9=1/2 .640 V1^2+ 1/2 .740 V2^2
use the first equation to solve for V1 in terms of all else, then put that expression into the second equation, and have fun expanding it and collecting the terms. You will have a quadratic, use the quadratic equation to solve it. 
actually they can be inelastic.....