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[問題] 唔明centre of gravity同centre of mass

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唔明centre of gravity同centre of mass

兩者分別在何= =?點解要take moment???
area同mass係正比麼???
完全不明白
   
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Volume 同 mass 先係正比

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Take moment 係要計下件object有無rotational motion

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only for uniform gravitational field cog=com=geometric centre

only for uniform object
com=geometric centre

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for center of mass if you apply a force to an extended object along the line of its CM, only transitional motion takes place.
for center of gravity it is a point of an object that the gravitational force acts upon it.

in uniform gravitational field, position of CM = position of CG
imagine a very huge object  that the size is comparable to the earth. if it is placed above some part of the earth and some part  the space, it is clear that it is not in uniform gravitational field, therefore the CG shifts away from its CM. note that CM remains stationary

you can easily figure out the above by the definition of center of mass
xCM = weighted average of positions (weighted by mass)
which is independent of gravity

[ 本帖最後由 tangyuetying 於 2012-6-17 02:06 PM 編輯 ]

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