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- Thread starter Amru123
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Ibix

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I'm not sure what this is meant to mean. The mass of a body is the same whatever axis you rotate it about. Why would it change?the centre of mass has all the mass concentrated in it

Did your teacher mention the parallel axis theorem? https://en.m.wikipedia.org/wiki/Parallel_axis_theorem

- #3

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Yeah,he did mention.Centre of mass is where the all the mass is assumed to be concentrated.Just look up for it's definition.I'm not sure what this is meant to mean. The mass of a body is the same whatever axis you rotate it about. Why would it change?

Did your teacher mention the parallel axis theorem? https://en.m.wikipedia.org/wiki/Parallel_axis_theorem

- #4

Ibix

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The parallel axis theorem is the rigorous answer to your question - take a look at the derivation in the wiki link. Intuitively, the centre of mass is the place where the average distance to all the little elements of mass making up the body is minimised. If one tries to rotate about an axis that does not pass through the center of mass then the average distance to all the little bits of mass is increased. So the ##mr^2## is increased because the ##r^2## increase, not because the m changes. And the total ##mr^2## is just the moment of inertia.

- #5

mfb

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Mass contributes to moment of inertia no matter where your axis is - but mass closer to the axis contributes less (it scales with the squared radius), so a lot of mass close to the axis leads to the smallest moment of inertia. Anyway, you cannot replace an object by a point-mass if you want to calculate its moment of inertia, as discussed in the previous post.

- #6

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Finally turn the bat through a third axis perpendicular to the first two, but this time turn the bat as though the bat were on a wood lathe. This time much less effort would be required to turn the bat at the same rate. This axis of turning on the wood lathe would be the minimum axis of inertia.

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