# Another gravity question

1. Sep 30, 2008

### Landru

Why is gravitational pull an "acceleration" rather than linear?

In other words, why is it that gravity turns out to be " distance = gravity*time2 " rather than simply " distance = gravity*time " ?

2. Sep 30, 2008

### atyy

Newton's 2nd law: F=ma (Determines acceleration of m)
Newton's gravity law: F=GMm/r2 (Determines force between M and m).

Together:
ma=GMm/r2
a=GM/r2

Gravity is dependent on the distance r between two masses. The strength of gravity is approximately constant over a short distance, say near the surface of the earth, and we call "GM/(radius of earth)2" the acceleration due to gravity.

3. Sep 30, 2008

### Staff: Mentor

The question really doesn't make any sense as posed. "acceleration" and "linear" are not similar things for gravity to be one or the other of. "Linear" just describes the shape of a function. In fact, you can build plenty of functions where something about gravity is expressed linearly, such as acceleration or force vs time.

Second, the word "gravity" in "distance= gravity*time" does not fit. Gravity is a type of force or acceleration.

4. Sep 30, 2008

### Landru

Why does space-time force things together?

5. Sep 30, 2008

### Nabeshin

Figure that one out and you'll be looking at a nobel prize.

6. Sep 30, 2008

### Landru

I though special theory of relativity explained why gravity is a "force" (in the sense that it accelerates objects like ordinary "force" does) but I simply haven't come across the explanation yet.

I am puting effort into understanding these things, but I'm still early on in higher math and I'd just like a sneak peak for a taste of whats to come.

7. Sep 30, 2008

### cepheid

Staff Emeritus
No, the special theory of relativity doesn't say anything at all about gravity. In fact, the reason why it's "special" is because it is only valid in the special case where gravity is not present. Special relativity only deals with inertial reference frames. If you are accelerating, then you are NOT in an inertial reference frame.

Einstein's General Theory of Relativity applies (as the name suggests) generally. It explains gravity as arising due to curvatures in spacetime itself. (Curvatures which are created by the presence of bodies with mass).

8. Oct 1, 2008

### Nabeshin

Einstein's General Relativity describes the force of gravity and says the cause is curved spacetime. I don't know, but to me this isn't a very deep understanding of the problem. The question is shifted to why does mass curve spacetime.

As to why things do not follow straight paths in curved space I've always wondered. Intuition in earthly observations (rubber sheet etc.) shows that things will curve, but this is due to gravity. In a system devoid of gravity is the same behavior experienced? Because surely there is no gravity to cause objects moving in 4-D spacetime to sink to lower energy states.