# Colision with a wall/platform and coefficient of restitution

1. May 19, 2014

### kohlerb

1. A ball is launched against a solid wall with straight trajectory and a velocity of Vo. It hits the wall on a 3ft height and it falls on the ground at a 2ft distance (by the left of the wall). Then, an elastic platform is positioned on the exact height of the collision of the ball, and it is throwed once again, with the same velocity. But now, it falls on the ground at a distance of 0,83ft.
a) Coefficient of restitution = ?
b) Vo = ?

g = 32,17; No drag;

My attempt was to calculate the velocity right after the wall colision, and right after the platform colision, using kinematics equations for velocity (free fall and horizontal movement), and then divided both to find the coefficient, but that doesn't sound right to me. And I have no idea on how to calculate the Vo.

2. May 20, 2014

### Staff: Mentor

Hi kohlerb. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

With the elastic platform, the ball rebounds far less distance? That doesn't sound right. (Maybe the data has been accidently swapped around?)

If this were an exam question, you should still "go through the motions" for solving it, even though the data values won't give a logical answer. At least demonstrate that you know the correct method for determining coefficient of restitution.

What would the rebound speeds be, for each throw?

Last edited by a moderator: May 6, 2017
3. May 20, 2014

### kohlerb

I'm sorry, I wrote the question poorly, the platform is a "soft" platform, it really makes the ball rebounds less. But I still dont know if the way I calculated the coefficient is the correct way. Can I divide the two rebound speeds to find it?

Rebound speeds:

Without the platform: 4,65ft/s
With the platform: 1,93ft/s

Last edited by a moderator: May 6, 2017
4. May 21, 2014

### Staff: Mentor

With one of the rebound speeds being that of an elastic collision, you should be able to divide them. As you have not shown how you calculated them, I can't say whether your working looks right or not.