# Homework Help: Rotational Motion

1. Jan 22, 2005

### thenewbosco

I have a diagram for this at http://snipurl.com/diagr [Broken]

There is a uniform disk with radius R, mass M, it has a string wrapped around it and is attached to a fixed rod.
Part 1: Show the tension in the string is 1/3 the weight of the disk

what i have done for this is

$$Torque=Fd$$
and setting the force equal to Tension
$$Torque=TR$$
then $$\sum Torque=I\frac{a}{R}$$ and i used $$I=\frac{1}{2}MR^2$$for the moment of inertia

Then $$TR=\frac{1}{2}MRa$$
$$T=\frac{1}{2}Ma$$
and then acceleration is just g, so i get the tension equals 1/2 the weight. where have i gone wrong????

Last edited by a moderator: May 1, 2017
2. Jan 22, 2005