Negative g

[PiotrSatan]

Yeah well don't get anxious when I ask you about g, it is just second year I learn physics, anyway
can g be negative? Like g=-10 or something? And if it can, how would it all work?

 

[arkady]

Define g.

I assume you mean g-force. (which is actually not a force but an acceleration)
If so, yes you can experience a negative g-force. For example when driving fast over a small crest on a road. That tickling sensation in your stomach is actually your intestines experiencing negative g-forces forcing them upwards inside you.

Negative in relation to what?

"g" is also commonly used to represent the acceleration exacted upon a body in Earth's field of gravity. Since g is an acceleration it is considered a vector quantity, and as such it can be positive or negative depending on how you set up your coordinate system. Usually we define up (skywards) as positive, which would make g = -9.82 m/s^2. However it's perfectly reasonable to define downwards as the positive direction which would make g positive. As long as you keep it in mind when interpreting your result.


Not sure if any of this is what you are asking.

But don't forget Wikipedia is your friend. Have a look at the entry for G-forces.

 

[PiotrSatan]

Interesting, that partially answers it, but I mean like, planet has g=-5 while the Earth has g=10, is it possible that way?

 

[Shpaget]

Just to make it perfectly clear, Earth's gravitational acceleration is 1 g, also written as G (capital g), not 10 (it is roughly 10 m/s^2).

Negative gravity would mean that objects are being pushed away (repulsive force), and gravity can not do that, let alone planets (they would have never formed in the first place because particles would repel each other).
So, no planets can't have negative gs.

Also, note what arkady said. g is the expression of acceleration, and does not necessarily have to do anything with gravity.

 

[arkady]

but I mean like, planet has g=-5 while the Earth has g=10, is it possible that way?

No. Gravity exists between two objects of mass and is always attractive.

Or actually what you propose would require said planet to have a negative mass, which is somewhat contradictory as Shpaget pointed out. I'm afraid I don't know enough about the subject of anti-matter to give any relevant insight beyond this.

Take a look at the Anti-gravity article on wiki if you like. I'll warn you though, it's all very dodgy.

(seems wiki is down atm, I hope it's up and working again by the time you read this)

 

[drwayne]

Note that the motion of a negative mass object in a convnetional gravitational field may not be
what one might think, as such a beast also has the property of having an induced acceleration
that is opposite to the direction of the applied force. (Which might make for a very good anti-tank
weapon)

 

[darkmatter4brains]

Gravity can be a repulsive force when space is filled with a negative pressure, a condition thought to have existed in the very early Universe. But, in today's Universe, this just isn't gonna happen for a planet.