Posted by Emily on Friday, July 30, 2010 at 3:36pm.
I have two questions, both are about angular momentum so I thought I'd post them together. I'm really confused on anything regarding rolling wheels and things like that (help me please!. It seems as though the whole chapter was amassed of all of the topics I cant wrap my head around. For both of these, the only work I have down is angular momentum l=rXp, or m(rXv), or =rmvsintheta. Any help would be so great and much appreciated. Thanks so much in advance!
1. A particle is acted on by two torques about the origin: τ1 has a magnitude of 4.5 N·m and is directed in the positive direction of the x axis, and τ2 has a magnitude of 1.4 N·m and is directed in the negative direction of the y axis. What are the magnitude and direction of d/dt, where is the angular momentum of the particle about the origin?
2. A sanding disk with rotational inertia 1.3 103 kg·m2 is attached to an electric drill whose motor delivers a torque of 9 N·m 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 140 ms?
(a) the angular momentum of the disk
in kg·m2/s
(b) the angular speed of the disk
in rad/s

Physics  bobpursley, Friday, July 30, 2010 at 5:37pm
You failed to mention moment of inertia. I recommend you reread that section, it is what imposes resistance to change of angular motion.
Torque=momentinertia*alpha (alpha is angular acceleartion, in radians/sec^2)
Torque*time=momentinertia*change angular velocity
angular speed final=initialangular speed +alpha*time
angular displacement=intial angular speed*time+1/2 alpha *Time^2
MEMORIZE these, understand them. Make flash cards, these are the equations of motion.
2.a)angular momentum =momentinertia (intial w+alpha*time)
but alpha=torque/moment of inertia (the first equation you should memorize, above).
solve.
b wfinal=alpha*time
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