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 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum Jun 9th 2010, 01:40 PM #1 Junior Member   Join Date: Jun 2010 Posts: 3 lump disk mass to a point hello imagine a disk rotating by an axis that passes through the center of the disk how can we find the analogue of a mass rotating by an axis? I mean, how can we lump the whole mass of the wheel to a point? how much will be the distance of this point from the axis? for example to rotate a disk with radius 10 and mass 100 you need the same force as to rotate a point of mass 100 and rotation radius = ?? I suppose the radius will be some less than 10 (since not all parts of the disk are at this radius), but how much exactly? thanks PS: we need to do this in order to calculate other things, eg angular acceleration, etc   Jun 10th 2010, 06:59 AM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 Just as force = mass x linear accln, torque = moment of inertia x angular accln The Moment of inertia of a disk = 0.5 MR^2 and that of a rod with a point mass is MR^2 . Equate the two by adjusting the eqns to give what you want ;same torque? or same angular accln?   Jun 10th 2010, 12:57 PM   #3
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 Originally Posted by physicsquest Just as force = mass x linear accln, torque = moment of inertia x angular accln The Moment of inertia of a disk = 0.5 MR^2 and that of a rod with a point mass is MR^2 . Equate the two by adjusting the eqns to give what you want ;same torque? or same angular accln?
same torque
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