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 Advanced Mechanics Advanced Mechanics Physics Help Forum Apr 30th 2009, 05:36 PM #1 Junior Member   Join Date: Feb 2009 Posts: 23 conservation of angular momentum the problem is: a horizontal vinyl record of mass .10 kg and radius .10m rotates freely about a vertical axis through its center with an angular speed of 4.7rad/s. the rotational inertia of the record about its axis of rotation is .0005kgm^2. a wad of wet putty of mass .02kg drops verticaly onto the record from above and sticks to the edge of the record. what is the angular speed of the record right after the putty sticks to it? ok, so conservation of angular momentum would be: Iwi = Iwf (I= inertia, w=omega, i=initial, f=final) so the initial momentum is given basically, (.0005)*(4.7) what i think final momentum is: .5(.1+.02)*(.1)^2 w because the formula for I for a disk is .5mr^2. and the mass would be the mass of the disk plus the mass of the putty. solving for w would give 3.92. i've been given the answer, which is 3.4. i've tried several different ways, none of which are getting me the correct answer. this is the method which i was most confident in, but still incorrect. can anyone help me out? where am i going wrong?   May 1st 2009, 01:21 AM #2 Physics Team   Join Date: Feb 2009 Location: India Posts: 365 The final moment of inertia would be the moment of inertia of the disk + moment of inertia of the wad = 0.5mr^2 + m'r^2 where m' is the mass of the wad. ( wad can be considered as the point mass hence its moment of inertia will be m'r^2)   May 1st 2009, 02:49 AM   #3
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 Originally Posted by Parvez The final moment of inertia would be the moment of inertia of the disk + moment of inertia of the wad = 0.5mr^2 + m'r^2 where m' is the mass of the wad. ( wad can be considered as the point mass hence its moment of inertia will be m'r^2)

Aint this what he did?   May 10th 2009, 09:58 PM #4 Senior Member   Join Date: May 2009 Location: Mumbai,India Posts: 102 initial Angular momentum=Iw final angular momentum={I+mr^2}w1 w1=Iw/{I+mr^2}=0.0005(4.7)/{0.0005+0.0002}=5/7(4.7)=nearly 3.4 rad/s u have to use the given value of MI of the record....how did u assume that it is a disc.?if u do, then u have to use the calculated value in both expressions for AM. uve used the given value of I in the initial case and calculated 0.5mr^2 for the second, so ur answer is bound to be wrong!! Last edited by Akshay; May 10th 2009 at 10:12 PM.   Sep 1st 2009, 01:18 AM #5 Junior Member   Join Date: Sep 2009 Posts: 18 the ans is 3.357. I'=final moment of inertia is I+mr^2 where m is the mass of the putty. pls check.   Oct 13th 2009, 10:38 PM #6 Banned   Join Date: Oct 2009 Posts: 2 I agree with you.  Tags angular, conservation, momentum Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post assaftolko Advanced Mechanics 1 Sep 2nd 2012 08:26 AM amirshrestha Kinematics and Dynamics 2 Apr 28th 2012 11:49 AM physicsquest Advanced Mechanics 10 Jun 13th 2009 12:31 AM monomoco Advanced Mechanics 1 Mar 15th 2009 10:45 PM orgoistough Advanced Mechanics 1 Oct 31st 2008 09:14 PM