# How is the attraction of a small mass to a bigger mass

1. Oct 7, 2015

### netqwe

explained by the particle level ?

2. Oct 7, 2015

### ZapperZ

Staff Emeritus
Why do you think this would be any different at the "particle level"? It would help if you elaborate a bit more on the impetus of your question.

Zz.

3. Oct 7, 2015

### netqwe

The intention was not an explanation based on
the attraction and repulsion terms but how it
translated to the particle level

Last edited: Oct 7, 2015
4. Oct 7, 2015

### ZapperZ

Staff Emeritus
But that was my question. Why do you think such a thing would be different at the particle level than what we already know at the macroscopic level? Presumably, you already know this at the macroscopic level. So what made you ask this for the "particle level" that caused you to think that it might be different?

Zz.

5. Oct 7, 2015

### netqwe

I put the question another way and gradually:
What makes the attraction of a small mass by a big mass ?

6. Oct 7, 2015

### ZapperZ

Staff Emeritus
Gravity, if we are only considering gravitational interaction.

So why is asking you why you came up with this question like pulling teeth?

Zz.

7. Oct 7, 2015

### netqwe

OK , what is gravity or what gravity does to the masses ?

8. Oct 7, 2015

### Staff: Mentor

They are mutually attracted - by Newton's third law, the small mass exerts a force on the large mass that is equal and opposite to the force the large mass exerts on the small one. Now look at Newton's second law $F=ma$ and ask yourself how the acceleration of the large mass will compare with the acceleration of the small mass, given that the magnitude of the force is the same. (It's a good exercise to actually work through the numbers for an extreme case, such as the gravitational force between the earth and a falling person - the person has a mass of maybe 100 kilograms, the earth has a mass of maybe $6\times{10}^24$ kilograms - how much does each move?).

The gravitational force between two masses is just the sum of the gravitational force between every single pair of particles in the two masses. However, there are something like $10^{80}$ particle pairs involved in the interaction between between the earth and a falling person so it is completely impractical to actually carry out such a computation. Fortunately, it can be proven that the result of this calculation will always be the same as what we get if we assume that all the mass of both objects is concentrated in a single point at the center of mass; this allows us to just do one $F=Gm_1m_2/r^2$ caculation to find the force on each mass.

9. Oct 7, 2015

### Staff: Mentor

Gravity is the force described by Newton's law of gravity: $F=Gm_1m_2/r^2$.

As for why that force exists... We don't know. Science is about describing how the universe we live in works, and it's not so good at explaining why we live in a universe that works that way instead of some other way.

10. Oct 7, 2015

### netqwe

What material gravitational force built from ?

11. Oct 7, 2015

### ZapperZ

Staff Emeritus
Please spend some time browsing the forum. Even in this General Physics forum itself, there is already another thread title "what is gravity?" This topic has been addressed numerous times.

Zz.

12. Oct 7, 2015

### netqwe

I moved on search results for query: 'what is gravity' but didn't found an answer to :
What material gravitational force built from ?

13. Oct 7, 2015

### ZapperZ

Staff Emeritus
Why would you think that a force, any force, is "built" from some sort of "material"?

Zz.

14. Oct 7, 2015

### netqwe

So how you expect the interaction between the masses will be performed ?

15. Oct 7, 2015

### ZapperZ

Staff Emeritus
Good question to ask Al Einstein. He only required that a mass causes space-time "distortions" nothing else.

Zz.

16. Oct 8, 2015

### Staff: Mentor

None of the fundamental forces are 'built' from anything. They simply describe how particles interact with each other.

17. Oct 8, 2015

### netqwe

In other words for now the science far from explaining the nature forces .

18. Oct 8, 2015

### Staff: Mentor

No, science does exactly that. It tells us the rules that the fundamental forces work by, when they apply, etc. It also tells us that they aren't made up of anything. That is your assumption.

19. Oct 8, 2015

### netqwe

But you can't describe something without explaining its structure .
Whether it will be a ' space time ' sheet or a spot body the interaction
between it and other body should be base on attraction force ,
otherwise the bodies will not cling together .
The answer for the question 'Which particles the force built form' is still open .

20. Oct 8, 2015

### Staff: Mentor

You can if no structure exists.