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gravitational potential energy (1 Viewer)

Premus

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if they ask us to calculate this for an object.... using the formula
E = - G m1 m2
--------------
r

then do we leave the answer as a negative?
 

sub

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yeah, unless they ask magnitude, in which case u want the positive :)
 

velox

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yes because the GPE is always negative. Only do the modulus if it is talking about the change in GPE. Remember the GPE is 0 at an infinite distance from the mass, so anything closer will be negative.
 

Premus

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Ok cool thanks :)

"if it is talking about the change in GPE" - what do u mean by this wrx ? can u pls give an example?

Thanks
 

tempco

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does r in the formula refer to the distance between the centres of the objects?
 

helper

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yes r is distance between centre of mass
 

gordo

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PremusDog said:
Ok cool thanks :)

"if it is talking about the change in GPE" - what do u mean by this wrx ? can u pls give an example?

Thanks

if an object is x metres away from a planet and u move it to z distance away from the planet where z>x, then u increase its potential energy although it will still be a negative value, the change in potential energy is postivie
 

jumb

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Hey so is r the distance moved?
 

lucyinthehole

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no, don't you find the change by working out the original value, then the new value, then fidig the difference? thus r is always the distance between the centres of gravity of the 2 objects in question
 

Jase

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unless they ask for change in energy in raising an object... then its positive.
 

jumb

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lucyinthehole said:
no, don't you find the change by working out the original value, then the new value, then fidig the difference? thus r is always the distance between the centres of gravity of the 2 objects in question
That made a whole lot of no sense.

So, ignoring your 2nd sentence, 'r' is the change in distance from the center's of gravity?
 

gordo

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no, u will get the wrong answer
u have to work out the Ep at the first point and the Ep at the second and minus them
 

tempco

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(-G<sup>Mm</sup>/<sub>r</sub>) - (-G<sup>Mm</sup>/<sub>R</sub>)

-[(G<sup>Mm</sup>/<sub>r</sub>) - (G<sup>Mm</sup>/<sub>R</sub>)]

is equal to

-GMm[<sup>1</sup>/<sub>(r-R)</sub>]

so it's the same thing if you sub in the difference, or do it separately.

EDIT: It's all wrong.
 
Last edited:

tempco

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Actually, thinking about it, you have to calculate the GPE of two positions separately and find the difference, since directly subbing in the difference (say, R-r) in height of the two would give you the GPE of an object R-r metres high, and not the difference between the two GPEs.
 

lucyinthehole

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that's what i meant! *sulks* no one understands me...

ok, rephrasing my previous post: you have object at x distance, you move it to z distance. so to calculate change in potential energy, you find the E at x, then the E at z, and then find the difference.

and my second sentence was just to correct your saying that r is the distance between the 2 points x and z. no, it's not: r is always the distance between the object and whatever the second object is tha it's all in reference to.

please tell me i make sense?
 

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