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Old Aug 9th 2019, 03:14 AM   #11
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Sorry if I'm a bit blur at this topic...

The definition of GPE for a mass at a point is 'the work done in bringing a mass from infinity to the point'. But when you bring a mass from infinity towards the gravitational body, isn't the work done always positive?

Originally Posted by neila9876 View Post
Actually, we can start from the infinite or the center of the gravitatioal body. It's only the difference between negative and positive values. If we start from the infinite, we can get an approximate feasible way to set out. If we start form the center of the gravitational body, we set out from a nearly impossible mission. In another hand, in the infinite the gravity is zero, if we take potential energy is also zero, then we feel more natural in logic.
Sorry but I really don't understand why starting from the centre of the gravitational body is inapplicable.
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Old Aug 9th 2019, 04:19 AM   #12
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Work done can be considered negative if the work is being done by something (the gravitational field) on something else (the test mass).

I wouldn't worry too much about the details regarding certain conventions. They probably exist just to make the energy book-keeping tasks more convenient. The GPE convention, although rather odd, allows for the mechanics of objects in free-fall to simply convert GPE to KE (one decreases, the other increases), making this set of mathematics useful and meaningful.

Last edited by benit13; Aug 9th 2019 at 04:23 AM.
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