# Homework Help: Torque/work problemneed help as soon as possible can anyone help?

1. Jun 15, 2009

### jmb07

torque/work problem..need help as soon as possible...can anyone help??!

How much work is done on a motor of a cd player to make a cd sin, starting from rest?
The cd has a diameter of 12.8 cm and a mass of 16.1g. The laser scans at a constant tangential speed of 1.26 m/s. Assume that the music is first detected at a radius of 20.3 mm from center of disk. Ignore the small circulr hole at the center of the cd.

I know that the change in kinetic rotation = work
Just not sure how to apply this to start this problem...

2. Jun 15, 2009

### LowlyPion

Re: torque/work problem..need help as soon as possible...can anyone help??!

Welcome to PF.

Well then what is the formula for rotational kinetic energy of the disk?

You know tangential speed at the radius of first detection of the music, so that should give you the rotational rate of displacement shouldn't it?

3. Jun 15, 2009

### jmb07

Re: torque/work problem..need help as soon as possible...can anyone help??!

Okay i figured out the v=r(angular velocity). I get the angular velocity is equal to 62.07 m/s.
I think I have come up with an equation that 1/2(m(r^2)(angular speed^2)) ...and I am just not sure where to go from here because I plug in the numbers but the answer I get is way to big compared to the correct answer (0.0635 J)

4. Jun 15, 2009

### LowlyPion

Re: torque/work problem..need help as soon as possible...can anyone help??!

Check you units.
Also check I for a solid disk = 1/2*m*r2

This should yield KE = 1/2*I*ω2 = 1/4*m*r22 = 1/4*(.0161)*(.064)2*(62.07)2

Note: the 62.07 is rad/s not m/s as you wrote. Units matter.