Definition of gravitational potential

[Rohan1997]

Homework Statement

The definition of gravitational potential at a point in my textbook is "the work done per kg to move a small test mass from infinity to that point"

I am having difficulty grasping this concept,

how is work done bringing an object closer to earth??

shouldn't work be done in lifting an object hence moving it from distance=0 to that point, and not the other way round??

Homework Equations

The Attempt at a Solution

 

[PWiz]

You're right - work is not done by you - rather, it's the gravitational field that does work in bringing the object closer. That's exactly why the formula has a -ve sign.

 

[AlephNumbers]

The definition of gravitational potential at a point in my textbook is "the work done per kg to move a small test mass from infinity to that point"

I think Wikipedia's definition is much clearer. http://en.wikipedia.org/wiki/Gravitational_potential The first sentence of that article is probably all you need to know to start solving gravitational potential problems. While your textbook's definition is not wrong, I would have described it differently for an intro to physics student. Also, PWiz speaks the truth.

 

[Rohan1997]

You're right - work is not done by you - rather, it's the gravitational field that does work in bringing the object closer. That's exactly why the formula has a -ve sign.

ahh that makes a bit more sense that i can cope with, thank you

But since work is a change of energy would the energy change be GPE---> KE?

 

[PWiz]

The correct way to write it will be $ΔPE=-ΔKE$, because their sum has to remain constant.

 

[Rohan1997]

The correct way to write it will be $ΔPE=-ΔKE$, because their sum has to remain constant.

No you're wrong, I am talking about the energy changes qualitatively not quantitatively, so the way I have wrote it is also correct.

 

[AlephNumbers]

No you're wrong, I am talking about the energy changes qualitatively not qualitatively, so the way I have wrote it is also correct.

Okay, maybe so, but when you actually have to start solving problems, you are going to need some equations.

 

[Rohan1997]

Okay, maybe so, but when you actually have to start solving problems, you are going to need some equations.

sound then.

 

[PWiz]

If the potential energy of an object reduces by 5J, then its kinetic energy increases by 5J. I didn't explicitly say you were wrong. What I meant was that its generally suitable to use the equation I gave above to avoid confusion, so that $KE + PE$ stays invariant.

 

[Rohan1997]

If the potential energy of an object reduces by 5J, then its kinetic energy increases by 5J. I didn't explicitly say you were wrong. What I meant was that its generally suitable to use the equation I gave above to avoid confusion, so that $KE + PE$ stays invariant.

I understand what you were saying but again, I'm not talking about anything in quantitative format... I was describing energy changes in a descriptive way. If an exam question came up asking me to describe the energy changes I would get no marks for writing what you said...

 

[PWiz]

But since work is a change of energy would the energy change be GPE---> KE?

The last part looked like a transformation equation to me... nevermind. If a description is asked, then don't leave room for error by using any loose words. Of course, this varies depending on what is expected from you during an exam, but I have a tendency to use equations even for these kind of questions in my exams - no room for misunderstanding is then left. In the end however, follow what advice your teacher gives you prior to appearing for the examination (as it will agree with the format in which you're expected to give answers).