# Gravitational vs Inertial Mass

1. Dec 20, 2008

### Naty1

(I realized I have a confusing post title but can't figure how to edit it..It should read "Gravitational curvature vs gravitational force"

Under 'mass' Wikipedia makes a statement:

2. Dec 20, 2008

### A.T.

Stating that "gravitation is not a force" without mentioning that free falling objects are not considered accelerated in GR anymore, is telling just half of the story. This omission also causes the puzzling:

Since in GR a free falling object is not accelerated, it's inertial mass is irrelevant for it's trajectory, and does not have to be equal to an "active gravitational mass" causing an attractive force which doesn't exist in GR.

3. Dec 20, 2008

### daytripper

Was there a recent redefinition of gravity or something that I'm unaware of? What's all this about gravity not being a force? If it's not, then why am I being accelerated downward? If I'm not, then why do I feel a force on my back (I'm laying down)?

4. Dec 20, 2008

### Staff: Mentor

If you feel a force on your back (while laying down) then you are being accelerated upwards.

5. Dec 20, 2008

### D H

Staff Emeritus
Gravity is a pseudo-force in general relativity. Nothing new here, just something you haven't been taught. Imagine you are floating around weightlessly in a spaceship with no windows. Is there any experiment you can conduct in the confines of this spaceship that will let you determine whether you are in in some region of (nearly) flat space between galaxies or in orbit around some massive object such as a planet or a star?

The answer, according to Einstein's equivalence principle, is no. Einstein's equivalence principle is intermediate between the weak and strong forms of the equivalence principle. The strong form has been tested and now stands as one of the (if not the) most accurately verified claims in all of physics. For example, see http://physicsworld.com/cws/article/news/20870 and http://physicsworld.com/cws/article/print/21148.

Because you aren't in an inertial frame. Gravity is a pseudo-force. All pseudo-forces (e.g., centrifugal force, coriolis effect, inertial force (frame acceleration)) result from attempting to explain physical laws that strictly apply in inertial frames only in a non-inertial frame. All pseudo-forces have one thing in common: The force acting on some object is proportional to the mass of the object. Clue #1 that gravitation is a pseudo-force: The gravitational force acting on some object is proportional to the mass of the object. Clue #2 is the equivalence principle.

That is a real force; it's called the normal force.

6. Dec 20, 2008

### daytripper

Ok, I think I'm starting to understand. I didn't realize I never understood WHY space-time curvature causes me to drift toward masses. In the trampoline analogy, gravity pulls the marbles toward the bowling ball. What can accelerate me other than a force? With centrifugal force, it's my inertia moving forward as, for instance, a car turns. What causes this? Why can't I understand gravity!?! haha

7. Dec 21, 2008

### Naty1

DH posted:
THAT must be what Wikipedia was trying to say.....I still think it's an obtuse statement...

It's no wonder there are lengthy threads here where we are trying to understand each others context and choice of words....not to mention different interpretations of some mathematics...

"All pseudo-forces (e.g., centrifugal force, coriolis effect, inertial force (frame acceleration)) result from attempting to explain physical laws that strictly apply in inertial frames only in a non-inertial frame..........."
Nicely laid out!!!!!..haven't seen that description before

Thanks DH.

8. Dec 21, 2008

### George Jones

Staff Emeritus
You might also like to look at

9. Dec 22, 2008

### atyy

10. Dec 22, 2008

### A.T.

Okay, here the "active graviational mass" in fact means the energy stress tensor, causing space time curvature. That was not clear in the wiki text.

It also says that inertia is not interaction with the own gravitational field. Is there a good explanation why not, somewhere?