# Ball on a string help?

1. Sep 10, 2011

### vl108

A tether ball of mass m is suspended by a rope of length L from the top of a pole. A youngster gives it a whack so that it moves with some speed v in a circle of radius r = L sin(θ) < L around the pole.
a) Find an expression for the tension T in the rope as a function of m, g, and θ.
b) Find an expression for the speed v of the ball as a function of θ.

F = mv^2 / r

I do not know what to do. All I have is Tcosx = mv^2/r - mg

2. Sep 10, 2011

### vl108

+ mg sorry

3. Sep 11, 2011

### BruceW

That's not right. There are essentially two equations. One for the vertical forces and one for the horizontal forces.

You've written the equation for the vertical forces, but the centripetal force shouldn't be in it. You should have: Tcos($\theta$) = mg For the vertical forces.
From this you get the tension in the rope.

Then you can use the equation for the horizontal forces to get the speed of the ball.