# Collision and friction

1. Nov 22, 2013

### Simon666

Theoretical question with practical implications (trying to model yarn as mass points connected with spring systems and need to get collision response with objects right):

A mass point approaches a plate under an angle. You have the coefficient of restitution β and the friction coefficient μ. You know the initial speed v of the point and can decompose it in normal component vN and tangential component vT to the surface.

What is the collision response of the mass point?

Just inverting the sign of vN and multiplying with β?

Or is there also a friction response since technically the surface and mass point exert forces on each other in an infinitely small amount of time, so an impulse reaction, and I also need to add -μ*vN to the tangential component or something?

2. Nov 22, 2013

### CWatters

Not really my field. Whole books are written on impacts, Google books suggests "Impact Mechanics" By W. J. Stronge

If the mass points bounce off without sliding over the surface then I don't think any energy is lost due to friction (work = force * displacement but the displacement is zero?).

So I think you just need to use the normal component of the velocity and the coefficient of restitution to work out the normal component of velocity after the impact.

Page 97 appears to discuss this....

http://books.google.co.uk/books?id=...oefficient of restitution at an angle&f=false

Edit: However having read that bit again ... Can the springs between the point masses store energy?