# Conservation of Momentum Problem

## Homework Statement

Objects A and C are made of different materials, with different
“springiness,” but they have the same mass and are initially at
rest. When ball B collides with object A, the ball ends up at rest.
When ball B is thrown with the same speed and collides with
object C, the ball rebounds to the left. Compare the velocities of
A and C after the collisions. Is vA greater than, equal to, or less
than vC?

Pi = Pf

## The Attempt at a Solution

A: mb(Vbi) = mVAf
B: mbVBi = mbVBf + mvCf

Now...I can see that:
VAf= mb(Vbi)/m
VCf = (mb(Vbi) - mbVBf )/m

I don't think I can say, from the above equations which is bigger (VAf or VCf)...do I have to sub one into the other now? How do I continue the solution? Thanks for your help!

## Answers and Replies

kuruman
Science Advisor
Homework Helper
Gold Member
First off equation B is incorrect. If the ball rebounds to the left, its final velocity should be negative.

Change the sign as indicated above and proceed as you did. Then take the ratio VAf/VCf. Is it greater or less than one?