gravitational potential energy (1 Viewer)

[Premus]

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]

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

 

[velox]

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]

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]

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

 

[helper]

yes r is distance between centre of mass

 

[gordo]

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]

Hey so is r the distance moved?

 

[lucyinthehole]

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]

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

 

[jumb]

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]

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]

(-G^Mm/_r) - (-G^Mm/_R)

-[(G^Mm/_r) - (G^Mm/_R)]

is equal to

-GMm[¹/_(r-R)]

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

EDIT: It's all wrong.

 

[Jase]

whoaho slow down there.. 1/R - 1/r is not equal to 1/(R-r) ^^

 

[tempco]

whoaho slow down there.. 1/R - 1/r is not equal to 1/(R-r) ^^

haaaaaaah. damn. lol forgot maths too early ^^U

 

[gordo]

too early?

 

[sub]

dont ask gordo...i dont think u want to know

 

[tempco]

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]

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?

 

[tempco]

yep, that makes sense :uhhuh: