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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

[tex] Torque=Fd [/tex]

and setting the force equal to Tension

[tex] Torque=TR [/tex]

then [tex] \sum Torque=I\frac{a}{R}[/tex] and i used [tex]I=\frac{1}{2}MR^2[/tex]for the moment of inertia

Then [tex] TR=\frac{1}{2}MRa[/tex]

[tex]T=\frac{1}{2}Ma[/tex]

and then acceleration is just g, so i get the tension equals 1/2 the weight. where have i gone wrong????

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

[tex] Torque=Fd [/tex]

and setting the force equal to Tension

[tex] Torque=TR [/tex]

then [tex] \sum Torque=I\frac{a}{R}[/tex] and i used [tex]I=\frac{1}{2}MR^2[/tex]for the moment of inertia

Then [tex] TR=\frac{1}{2}MRa[/tex]

[tex]T=\frac{1}{2}Ma[/tex]

and then acceleration is just g, so i get the tension equals 1/2 the weight. where have i gone wrong????

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