# Homework Help: Circular motion with decreasing radius

1. Nov 1, 2008

### psychicist

1. The problem statement, all variables and given/known data
A ball is tied around a vertical cylinder with radius R by an unstrechable rope, moving around the cylinder horizontally with initial velocity v0 and initial length of rope from the tangent of cylinder L0. How long the ball takes to strike the cylinder? (the friction is constant)

2. Relevant equations
Conservation of angular momentum, $$v_0 r_0 = v_1 r_1$$

3. The attempt at a solution

In my steps, by taking the tangent of the rope on the cylinder as centre, the initial radius of rotation is L0, but the final one is 0 by this way? What is the wrong of my concept?