# Rotational motion about a fixed axis

1. Apr 17, 2005

### cavery4

A CD has a mass of 17 g and a radius of 6.0 cm. When inserted into a player, the CD starts from rest and accelerates to an angular velocity of 20 rad/s in 0.65 s. Assuming the CD is a uniform solid disk, determine the net torque acting on it.

I thought to use: torque = I * alpha (acceleration)
To find Inertia, I did I = 1/2 MR^2
to find acceleration: alpha = (wf - wi)/time or 20 rad/s / .65 seconds
Also, I converted 17 g to .17 kg and 6.0 cm to .06 meters.

My answer was .942. This was marked wrong by webassign. Obviously, I am missing something here. Wrong formula? Missing an important concept? Possibly wrong calculation?

If you point me to my error, I would appreciate this greatly!
I used the above methods to find the correct answer to this problem:

The circular blade on a radial arm saw is turning at 256 rad/s at the instant the motor is turned off. In 17.0 s the speed of the blade is reduced to 80 rad/s. Assume the blade to be a uniform solid disk of radius 0.160 m and mass 0.400 kg. Find the net torque applied to the blade.

What is different about these two problems? What does the radial arm have to do with it?

2. Apr 17, 2005

### dextercioby

It can't be.It should be approx. $9.4\cdot 10^{-4} \mbox{Nm}$.

So check your arithmetics again.And those units,too.

Daniel.

3. Apr 17, 2005

### cavery4

thank you!

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