# Homework Help: Find the coefficient of restitution between the balls (should be easy!)

1. May 9, 2007

### daletron

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

i'm retaking a test question for half credit and this is the 3rd part to the question. i know the velocities are correct because the teacher gave us the answers.

this SHOULD be simple... but okay.. (velocity) V1 = 20ft/s, V2 = 0ft/s. after the collision V1 = 16ft/s, V2 = 4ft/s. I'm supposed to find the coefficient of restitution. The formula for that (Partially elastic i assume) is (V1 - V2) = -e(V1-V2). I get -.6 for e. but isn't e supposed to be between 0 and 1? the e is SUPPOSED to be 1/3 (given by the teacher). and i'm pretty sure i didn't copy that down wrong so it just doesn't make sense to me

2. Relevant equations
(V1 - V2) = -e(V1-V2)

3. The attempt at a solution

(16ft/s - 4ft/s) / -20ft/s = -.6

Picture of the problem
http://i11.photobucket.com/albums/a162/daletron/untitled.jpg

2. May 9, 2007

### andrevdh

First use momentum conservation to determine the speeds of the balls along the given directions.

3. May 9, 2007

### daletron

i did, hence when i said "after the collision V1 = 16ft/s, V2 = 4ft/s"

4. May 10, 2007

### andrevdh

For the restitution you need to consider the speeds of the objects along the line of action. Collisions without friction changes the velocity components of the objects along the line of action. The components perpendicular to these are unaffected by the collision.

Last edited: May 10, 2007