Momentum and Impulse. find final velocity

[maxjohnfresh]

Homework Statement

A 0.10-kg object moving initially with a velocity of 0.20 m/s eastward makes an elastic head-on collision with a 0.15-kg object initially at rest. What is the final velocity of the .10 kg object after the collision ?

Homework Equations

im not sure which equation to use :/

The Attempt at a Solution

i tried using m1v1+m2v2=(m1+m2) V
but it didnt come out right.

and i know the answer is -0.045 m/s

 

[rl.bhat]

Hi maxjohnfresh, welcome to PF.
i tried using m1v1+m2v2=(m1+m2) V
This formula is true if both objects move together after collision.
So use the formula
m1v1i+m2v2i=(m1v1f+m2v2f) .
Apply the conservation of energy and solve for v1f and v2f.