What Is the Final Speed of Two Colliding Discs?

[chicubs08]

Homework Statement

A disc of mass m is moving horizontally to the right with speed v on a table with neglibible friction when it collides with a second disc of mass 2m. The second disc is moving horizontally to the right with speed v/2 at the moment of impact. The two disc stick together upon impact. The speed of the body immediately after the collision is:

A)v/3
B)v/2
C)2v/3
D)3v/2
E)2V

Homework Equations

p = mv (that wasn't given, but that's the equation I used)

The Attempt at a Solution

mv = mv + 2mv/2

m(v) = m(v+2v/2)

v= v+2v/2

v= 2v/2 + 2v/2

for the final answer, I got: v= 2V, according to my answer sheet, the correct answer is 2v/3. I can't seem to figure out what I'm doing wrong. Please help.

 

[LowlyPion]

Welcome to PF.

mv +2m(v/2) = Total Momentum = 2*mv

After collision you have a total mass of 3m and you have total momentum = 2mv

So ... 3m*V = 2mv

V = 2v/3