# Re: Physics (check)

Posted by COFFEE on Friday, March 30, 2007 at 4:16am.

A sanding disk with rotational inertia 1.1 x 10^-3 kg*m^2 is attached to an electric drill whose motor delivers a torque of 6 Nm about the central axis of the disk. What are the following values about the central axis at the instant the torque has been applied for 185 milliseconds?

(a) the angular momentum of the disk
_______ kg*m^2/s
(b) the angular speed of the disk

--------
my submitted answers were wrong.. and this is what i did/come up with.

185 ms = 0.185 seconds

L = Iwf
L = (1.1 x 10^-3)(33.96)
L = 0.037356

.. am i missing something? .. what am i doing wrong? please show me, thanks!

* Physics - bobpursley, Friday, March 30, 2007 at 7:54am

You are missing the basic laws of motion.

Torque= inerita* angular acceleration
solve for angular acceleration, then.

wf= wi+ angular acceleration*time

angular momentum= I*wf

=====================

ok, so..

6 N*m = 1.1x10^-3 kg*m x Angular Acceleration
Angular Acceleration = 5454.54

wf = (0) + (5454.54)(0.185)
wf = 1008.09 [which answers part b]

thus

Angular Momentum = (1.1x10^-3)(1008.09)
= 1.108899

You are totally ignoring precision of digits in your answers. I would never put more significant digits (precision) in the answer than in the input data. However, in your defense, I see that the input torque is to only one significant digit, and moment of inertia is two sig digits, and time is three. One wonders. However, putting the answers to seven is a feat of imagination. Otherwise, the work looks ok, I didn't punch the calc on it.

i get what you mean..

and oh!! my answers were right :D
thanks!

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posted by Fahd

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