# Collision question - Simple one

• nukeman
In summary, the conversation was about a physics problem involving a 65kg boy and a 40kg girl on roller blades. The girl pushes the boy with a velocity of 2.9 m/s, and the question is asking for the speed of the girl after the push. The attempted solution using the equation m1(v1i - v1f) = m2(v2f - v2i) resulted in a speed of 3.9 m/s, but the correct answer is 4.71 m/s. After some back and forth, it was discovered that the error was due to mistaking a 4 for a 9 on the keyboard.

## Homework Statement

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?

nukeman said:

## Homework Statement

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?

## The Attempt at a Solution

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

what the heck, I am still getting 3.9 :(

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

JHamm said:

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

Haha, Dave, yes I was!

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

nukeman said:
Haha, Dave, yes I was!

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

on ... your ... keyboard ?