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-   -   How to explain there cannot be a case where r=0 in F=G(Mm/r^2) (http://www.physicsforums.com/showthread.php?t=674199)

WaaWaa Waa Feb24-13 03:51 PM

How to explain there cannot be a case where r=0 in F=G(Mm/r^2)
 
Hi. This is my first post here. In one of our science groups in Facebook, a member is asking about a case where r=0 in Newton's Equation F=G(Mm/r^2)

The best i could do was to state that there cannot be two point masses with a distance r=0 between them. He seems to accept my explanation but his intuition that it should be 'infinity' still remains. I would like to explain it better. Can you please help?

I have searched the site but could not find the answer. If there is already a thread, I would be glad if you could point me towards it.

rock.freak667 Feb24-13 04:25 PM

Re: How to explain there cannot be a case where r=0 in F=G(Mm/r^2)
 
The gravitational force of attraction between two point masses, M & m2, separated by a distance 'r' is given by:

F= GMm/r2

If r=0, then you don't have two masses anymore but one mass. In which cases, gravity would vary with depth of the planet. Read more here.

KeithPickering Feb24-13 04:40 PM

Re: How to explain there cannot be a case where r=0 in F=G(Mm/r^2)
 
Quote:

Quote by WaaWaa Waa (Post 4284196)
Hi. This is my first post here. In one of our science groups in Facebook, a member is asking about a case where r=0 in Newton's Equation F=G(Mm/r^2)

The best i could do was to state that there cannot be two point masses with a distance r=0 between them. He seems to accept my explanation but his intuition that it should be 'infinity' still remains. I would like to explain it better. Can you please help?

I have searched the site but could not find the answer. If there is already a thread, I would be glad if you could point me towards it.

When r=0, you no longer have two masses, you have one. This condition actually does occur at a black hole, where all mass is (believed to be) contained in a singularity, i.e., a single point.

Ryoko Feb25-13 12:33 AM

Re: How to explain there cannot be a case where r=0 in F=G(Mm/r^2)
 
I don't think the gravity law works well on a quantum scale since the nuclear forces and electromagnetic forces become very strong at small distances.

WaaWaa Waa Feb25-13 01:29 AM

Re: How to explain there cannot be a case where r=0 in F=G(Mm/r^2)
 
Thank you guys for your insights, we managed to resolve the question.

When we are talking about classical physics, I think we sometimes tend to grab ideas from Relativity and Quantum Mechanics and get confused and this seems to be the root of the problem.


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