# I need information on centres of mass

Hi I am doing an assignment where I roll a tin can filled with car oil. This will affect its rolling because it distorts its centre of mass, resulting in an awkward rolling motion. I have already done the experiment, the first revolution is the slowest one by a lot because it slows down a lot when it's halfway through the revolution. Afterwards it speeds up by a lot. I assume this happens because when the tin can is released, all of the oil is at the bottom (it's not full) and as it reaches half a revolution the oil is on top. Moving it upwards slows down the tin can and then it accelerates because the oil is moving forwards. In any case, I would find any link or formula regarding centres of mass to be very useful. Thanks for the help.

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Simon Bridge
Homework Helper
Consider: if the tin can is full of oil, the center of mass is unaffected, but it's motion will still be affected.
If the oil is not very sticky, then it will not, in general, turn with the can. Think how water in the can would behave... wouldn't it stay mostly at the "bottom" with some getting dragged up the surface and falling down... and some sloshing about? You are better served to think of the situation in terms of conservation of angular momentum.

Per your question: there is no formula for the COM of your setup - you will have to use your knowledge of com to derive one.
You know the com of a hollow cylinder (the can) and for solid cylinders (the ends of the can) and you'll need to work out the com for how you want to model the oil part ...

There's a big list of moments of inertia here:
http://en.wikipedia.org/wiki/List_of_moments_of_inertia

Thank you