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## Homework Statement

I want to calculate the function R(z) (R=radius, z=altitude) of a funnel in which a coin( if it has no friction and only gets a horizontal speed v) always stays on the same z-coordinate.

the radius of the coin is r and r<<R.

What function has the funnel and how would a sphere behave if it has the same mass and the same radius and gets the same speed.

You can see how they work when u watch this:

http://www.youtube.com/watch?v=Loc48W0VC6o&feature=rec-LGOUT-exp_fresh+div-1r-3-HM

## Homework Equations

I think it is F_z=F_g.

pls help me

The function R(z) for a pointmass is e-function but what is it for a coin?