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 Aug 4th 2008, 07:21 PM #1 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 Force Why force must exist in opposite direction than increasing gravitational potential energy??
Aug 5th 2008, 07:11 AM   #2

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 Originally Posted by werehk Why force must exist in opposite direction than increasing gravitational potential energy??
Sure it can. So I can only assume that you mean a specific force, but I can't think of what would prompt this question. So what force are you talking about?

-Dan
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 Aug 5th 2008, 08:46 PM #3 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 For example, we are always pulled by gravity
Aug 7th 2008, 03:56 PM   #4

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 Originally Posted by werehk For example, we are always pulled by gravity
Barring any other forces in the problem and any internal changes in energy we have
$\displaystyle \vec{w} \cdot \vec{s} = mg (- \Delta h) \equiv - \Delta PE$

The basic idea is that when we increase the height we are "winding up" the potential energy because when released the object has more energy to give.

-Dan
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