Also, gravity is not a force, it's acceleration. It just appears as a force on a co-ordinate system fixed to local gravitational center...
A-ha! No-one expects the Physics inquisition!
It is not actually acceleration either, if you are careful with the terms. It is curvature of the local space time, that manifests itself as acceleration / force towards the center of mass.
If my General Relativity memories are accurate, you can actually make acceleration disappear with a suitable choice of coordinate system...
Mika
Yes, in free fall on homogenous gravity field (or space-time) neither acceleration or force can be measured.
However, in co-ordinates fixed to the center of mass, the local space-time will cause an acceleration of objects towards the center. Since the acceleration is due to space and time and since acceleration is completely dependant on definitions of time and space, being d
2x/dt
2 (second derivate of change of location in relation to change of time), it follows that the most simple way to interpret this is as simple space-time induced acceleration (in a fixed reference frame).
Whether it can be considered a force in General Relativity isn't really even a relevant question, since interpreting gravity as a force actually complicates things in that model.
Newton's gravity law sees the gravity as a force because it handles things in a fixed rigid reference frame, but the law
G = gamma * m * M / R
2, where gamma is gravitational constant of unit Nm
2/kg
2 and the G is weight (or gravitational force)
could be just as easily written as
g = gamma
2 * M / R
2 where gamma
2 is gravitational constant of unit m
3/kgs
2 and the g is actually the direct acceleration caused by gravitation, in relation to the center of gravity. M is of course the combined mass of the system.
For example, in case of Earth and stuff on it's surface, we can agree that the CoG is close enough to center of Earth to make no difference, and we can use the Earth's mass as M since the mass of single objects is proportionately ridiculously small, and the equation works as follows:
g = 9.80665 m s
-2 (experimentally easy to confirm)
gamma = g * R
2 / M
R = 6,378,100 m -> R
2 = 4.06801596 * 10
13 m
M = 5.9742*10
24 kg
gamma = 9.80665 m s
-2 * 4.06801596 * 10
13 m / 5.9742*10
24gamma = 6.67764867 × 10
-11 m
3 kg
-1 s
-2And this works just as well as Newton's formulation of gravity... to an extent. A good example is defining the gravitational forces affecting an object within a hollow, homogenous shell - assuming that the acceleration is simply pointing towards the center of gravity is an oversimplification that doesn't really work in all cases, and you'll need to jump through various hoops to get the correct results...and things can get hairy when resultant accelerations are considered in the more complex setups, and it's sometimes more prudent to view gravity as a force them as acceleration, if only to avoid headplosions...
