# Homework Help: Collision question - Simple one

1. Nov 8, 2011

### nukeman

1. The problem statement, all variables and given/known data

Something I am doing wrong?

"A 65kg boy and his 40kg sister both wearing roller blades, face each other at rest. The girl pushes the boy hard, sending him backwards with a velocity of 2.90 m/s toward the west. Ignore friction.

What speed does the girl go?"

I cam up with m1(v1i - v1f) = m2(v2f - v2i)

Solved for v2f and got 3.9 m/s

But the correct answer is 4.71 m/s

?? Any help?

2. Relevant equations

3. The attempt at a solution

2. Nov 8, 2011

### DaveC426913

m1 * v1 = m2 * v2
where
m1 = 65
v1 = 2.9
m2 = 40
v2 = ?

3. Nov 8, 2011

### nukeman

what the heck, I am still getting 3.9 :(

im going (65)(2.4)=(40)(x) solve for x, which = 3.9

4. Nov 9, 2011

5. Nov 9, 2011

### DaveC426913

I imagine he's beating his head on his keyboard right about now...:tongue2:

6. Nov 9, 2011

### nukeman

Haha, Dave, yes I was!

Thanks, I guess my 4's look way too much like 9's :)

7. Nov 9, 2011

### DaveC426913

on ... your ... keyboard ?