Rotational motion about a fixed axis

[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?

 

[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.

 

[cavery4]

thank you!