Posted by **Stacu** on Tuesday, December 16, 2008 at 6:19pm.

YOU DID

The tension in the string is the force exerted down by the falling disk and up by whatever is holding the disk. They are equal and opposite. If the disk is a yo-yo, that force is exerted by your finger on the string to give the tension.

Force up on disk = T

Force down on disk = m g

a = acceleration down = (mg -T)/m

Torque on disk = T r

Torque = I alpha

so

alpha = T r/I

but alpha = a/r by geometry

T r/I = (m g - T)/(m r)

m T r^2 = (m g-T) I

T (m r^2 + I) = m g I

but I = .5 m r^2

T (1.5 m r^2) = .5 m^2 g r^2

T = (.5/1.5) m g = (1/3) m g

now you should be able to do the rest

for the energy part

KE = .5 m v^2 + .5 I omega^2

but omega = v/r

_____________

for the part where u have

T r/I = (m g - T)/(m r)..

where did that come from?? (the r in "mr"?

I get that a = mg - T / m

but the r?

## Answer this Question

## Related Questions

- phys - A disk with a rotational inertia of 2.0 kg.m^2 and a radius of 0.40 m ...
- physics - A string is wrapped around a uniform disk of mass M = 1.6 kg and ...
- mechanics - A string is wound around a uniform disk of radius R and mass M. The ...
- Physics - what are the forces acting on a string that is wounded around a ...
- physics - A force of 100 N is applied tangentially to a disk of radius 10 cm ...
- Physics College - Experiment in this week (week 8) is to study rotational ...
- Physics - A solid cylindrical disk has a radius of 0.18 m. It is mounted to an ...
- phys - A disk rolls down a slope. It rotational axis is its center of mass. It ...
- Physics helppp - The figure below shows an arrangement in which four disks are ...
- physics - A solid cylindrical disk has a radius of 0.17 m. It is mounted to an ...

More Related Questions