# I Gravity as a force

1. Aug 8, 2016

### petrushkagoogol

Why is gravity always attractive in nature ?

2. Aug 8, 2016

### Staff: Mentor

Physics cannot answer "why" questions on a fundamental level. If you ask "why" long enough, you'll always get to "because we observe that the universe is like that".

As simplified description, gravity acts on the energy of objects - most objects have their mass as largest contribution to their total energy. Energy is always positive (this is an observation - but a universe with negative energies would look completely different), so gravity always attracts.

3. Aug 9, 2016

### David Lewis

For the same reason time always moves from past to future.

4. Aug 15, 2016

### Andrew Mason

I agree that, at least for now, gravity appears to be a fundamental phenomenon. I think I know what you are saying about energy being positive (as in $E = \sqrt{(pc)^2 + (mc^2)^2}$) but of course energy itself is often expressed as a negative quantity (e.g. binding energy, potential energy).

I think we know why time always moves from the past to the future. I don't think we yet fully understand why (or perhaps even if) mass always attracts other mass by gravity.

AM

5. Aug 17, 2016

### TahirGorgen

Maybey because their is no other attraction force in our galaxy and not only because the Gravity pull of one is more powerfull than the other.

Last edited by a moderator: Aug 17, 2016
6. Aug 17, 2016

### Khashishi

Because masses are always positive.

7. Aug 17, 2016

### Andrew Mason

So are positive charges. But they don't attract each other.

AM

8. Aug 17, 2016

### phinds

This makes no sense. Can you clarify what you are saying? Do you not believe that positive and negative charges attract each other?

9. Aug 18, 2016

### lychette

expressing potential energy as a negative quantity is a convention....not a requirement

10. Aug 19, 2016

### Andrew Mason

Yes. But unless you want to treat gravitational potential energy as decreasing with increasing separation distance, gravitational potential energy has to be expressed as a negative. So it is not an arbitrary convention.

AM

11. Aug 19, 2016

### jbriggs444

The point being made is that only differences in gravitational potential energy are physically significant. The numeric value is irrelevant. One is free to choose a baseline potential energy of zero at the surface of the gravitating object. Then gravitational potential energy is positive everywhere [above the surface] and is still increasing with increasing separation.

12. Aug 19, 2016

### Andrew Mason

Then it is negative below the surface. The point is that energy is often considered negative for real physical reasons - hence the need to at least qualify the statement "a universe with negative energies would look completely different".

AM

13. Aug 19, 2016

### Staff: Mentor

Gravitational potential energy can be expressed as negative value, but it does not have to be - just add mc2 and everything is fine. This choice also gives the total energy added to the system by adding the object, which is always positive.

14. Oct 6, 2016

### Cres Huang

Free fall and inclined plane experiments since Galileo show the uniform acceleration of falling bodies regardless of their compositions, shapes, sizes, and distances. The important fact that has been overlooked is, gravitational acceleration is independent of composition, shape, size, and distance. However, the paradox is, attracting acceleration is dependent upon composition, shape, size, and distance. Therefore, isn't gravity not attraction force, or it does not pull?

15. Oct 6, 2016

### Staff: Mentor

It does depend on distance.
There is no paradox. It would be odd if the force on an object would not depend on the total mass and the mass distribution of the other object.
What?
Gravity is clearly an attractive force.

16. Oct 6, 2016

### Cres Huang

Doesn't the hammer and feather drop performed by David Scott in Apollo 15 show the independent of mass and gravitational acceleration?

17. Oct 6, 2016

### Staff: Mentor

(a) the experiments showed nothing new, countless other experiments have shown the same before (we have vacuum chambers on Earth... but other experiments are much more sensitive). The Apollo spacecraft could not have reached the Moon otherwise, for example.
(b) the experiment gave an example of the general principle that the gravitational acceleration of an object is independent of its own mass. The acceleration still depends on the mass of the Moon and the distance of the objects to it, but those two things were the same for both falling objects.

18. Oct 6, 2016

### PeroK

If he had dropped another moon what would have happened?

19. Oct 6, 2016

### Andrew Mason

Just to follow up on the subtle point made by PeroK that Galileo's principle that gravitational acceleration of mass m toward a body M is independent of m is really an approximation. It applies only where M>>>>m (such that the acceleration of M toward the centre of mass of M and m is negligible). Newton's law of universal gravitation is the correct law. Perhaps Mr. Huang could clarify what concerns he has about Newton's formulation.

AM

20. Oct 6, 2016

### Staff: Mentor

The acceleration of the individual objects does not depend on their mass - this is exact in Newtonian gravity. The acceleration relative to the ground can depend on mass, if we don't neglect the acceleration of this ground.