# Homework Help: Tetherball problem

1. Jan 29, 2014

### ybgl

1. The problem statement, all variables and given/known data
A .3 kg ball is attached to a string that is 50.0 cm in length, and completes 2 circles every second. Find the tension required to whirl the ball with the given constraints and also find the angle θ

2. The attempt at a solution
I only have that Ftcosθ = Mg and that mv^2/r = ftsinθ
I don't know where to go from there since all I have is the length and mass because to find the radius it is r=Lsinθ which I cannot find without θ...

2. Jan 29, 2014

### Andrew Mason

Welcome to PF!

This is just an algebra problem now. You have done the physics!

2 equations. 2 unknowns. Solve. Hint: try dividing the two equations.

AM

Last edited: Jan 29, 2014
3. Jan 29, 2014

### ybgl

I have tried, it gives me (g)(r)sinθ/cosθ=v^2 which is 3 unknowns since I don't know radius, velocity, or θ

4. Jan 30, 2014

### Andrew Mason

You can work out r from L and θ. θ is the downward angle that the tether makes as measured from the horizontal, so r = Lcosθ. Just substitute into your equation.

v can be determined from the period and radius. Or you can just use mω2r for centripetal force.

AM