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 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum Jun 23rd 2012, 07:21 AM #1 Junior Member   Join Date: Jun 2012 Posts: 1 Inertial moment of pendulum watch Hello guys, how are you? Well, here is my doubt: "The pendulum of a watch is formed by a thin rod of 207g and 20cm lenght and by a disc of 398g and 11cm of diameter. When the pendulum rotates around the extremity of the rod, with the plane of the disc perpendicular to the rotate axis, it's inertial moment is 0.016kg.m^2. How far from the rotate axis, is the center of the disc attached to the rod?" First, I have to say that I found the question a little confusing... but if I understand well, I think it is this what it wants we to find: http://s13.postimage.org/6qpmdwzzb/pendulum.jpg As I said, this is only my interpretation. This figure wasn't on the question. Well, my attempt: m: mass of the rod M: mass of the disc Using the Steiner's Theorem: I=I(disc)+I(rod)+(m+M)x^2 I=((MR^2)/2))+((mL^2)/12)+(m+M)x^2 Solving this equation I found x=7.13cm. I'd like to know if I did the correct resolution, because I'm not very confident with the way I did the inertial moment and all the stuff. Thanks in advance, Frank.   Jun 25th 2012, 10:18 AM #2 Physics Team   Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,347 You're on the right track, but I would offer a couple of pointers: 1. The formula that you used for the rod, I = mL^2/12, is valid if the pivot point is at the center if the rod. But for a rod whose pivot point is at one end (like on a pendulum), the moment of inertia is much larger: I = mL^2/12. 2. You need to apply the Parallel Axis theorem to the disc only, as the dimension 'x' is the distance from the pivot point to the center of the disc (the rod has a given length of 20 cm and teg pivot is at one end, so its moment of inertia is not affected by 'x'). FYI, I get an answer this is a over twice the value of yours.  Tags inertial, moment, pendulum, watch 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 jkepler Kinematics and Dynamics 8 Aug 6th 2016 05:53 AM Nimmy Advanced Electricity and Magnetism 0 Oct 27th 2011 10:00 AM C.E Special and General Relativity 11 Apr 16th 2009 11:14 AM HassanZahid Special and General Relativity 1 Dec 24th 2008 04:35 PM evabern Special and General Relativity 0 Sep 29th 2008 12:12 PM 