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# When is the gravitational energy 0? Watch

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1. At what point does an item have 0 gravitational potential energy?

Is it:

1. At the centre of the Earth or
2. When an object is lying on ANY surface (table floor...)
3. At sea level

Or in other words:

What is the h (height) measured relative to?
2. Is nobody doing Physics / Mechanics?
3. One moment.
4. When we calculate potential energy we are actually calculating CHANGES in PE. Therefore we measure the PE with respect to a set origin which might be the floor or a certain height above the floor etc.

For example, if I measure the potential energy of an object 10 m above the floor I would use mgh and find the value.

One would also think the PE of the object at the ground is zero, right ? However, this is only a relative measure.

Relative to the ground in this case

Get it ?
5. gravitation energy is zero when there is no gravitation force acting on you - ie if you are floating in space a zillion miles from the nearest object.

When you talk about h, all you are talking about is the difference in gravitational energy between one location and another.
6. I have just been to check my notes from earlyer this year when we studied Gravity, according to my notes

The gravitational potential at a point in a field is equal to the work done in bringing unit mass of 1kg from infinity to that point in the field

Why from infinity?
At infinity GPE is zero because the gravitational field is assumed to have no effect.

Formula.
V = -GM/r
(V = gravitational potential)
(G = gravitational constant,)
(M = mass creating the field,)
(r = distance from mass)

>this is an "inverse law" NOT an "inverse squared law".
>the minus sign indicates that the GPE at a point is less than at infinity.
>we get to the formula by integrating newtons law. (can't remember which one)
>to find the work done in moving mass (m) between points of different potential we multiply (m) by the difference in potential, i.e change in potential X mass

I guess the bit in bold answers your question, I added the rest for anyone looking over this.

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Updated: November 30, 2010
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