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- Thread starter harini_5
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Doc Al

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Potential energy is a scalar, so you just add them.

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In a sense you are right - you do "add them", however only once you've carefully defined your coordinate system such that the P.E components from each will be of different signs.

The sea on the earth is a perfect example of this. when the moon gets close to it its gravitational P.E decreases, hence it rises (less effective pull from the earth).

Please someone correct me if I'm wrong.

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Doc Al

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The gravitational PE between two masses is given by:Hang on, P.E is the work done against the gravitational field to take your test particle there against the field, once you choose your coordinates the other mass is working against the Gravitational field, hence you subtract them not add them.

In a sense you are right - you do "add them", however only once you've carefully defined your coordinate system such that the P.E components from each will be of different signs.

[tex]{PE} = - \frac{Gm_1 m_2}{r}[/tex]

Where r is the distance between them. (Note that when they are infinitely far apart the PE is taken to be 0.)

If you have two massive bodies, M1 & M2, the change in PE of the system when you bring a small mass close to them is:

[tex]{PE} = (-\frac{GM_1 m}{r_1}) + (-\frac{GM_2 m}{r_2})[/tex]

The reason for the rising of the sea is the difference in the moon's gravitational field strength acting on the sea compared to the earth.The sea on the earth is a perfect example of this. when the moon gets close to it its gravitational P.E decreases, hence it rises (less effective pull from the earth).

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