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SRH
2012-Sep-07, 10:20 PM
This may be a dumb question...

Have gravitational forces ever been observed between 2 massive objects, where neither object is a star/planet/moon/asteroid/comet/ etc.?

Thanks.

Rhaedas
2012-Sep-07, 10:31 PM
Gravity is very weak. It takes the whole mass of the Earth to pull you back down when you jump up. Plus, it would be difficult to measure the effect between two things in space, say a rocket and the ISS, because there are several things we need to know to very exact figures to reduce the error.

We can do experiments here on Earth, where we have a lot more precision and control.

http://www.leydenscience.org/physics/gravitation/cavend.htm

ShinAce
2012-Sep-07, 10:40 PM
That's the one! Cavendish' torsion balance.

Nereid
2012-Sep-07, 10:55 PM
This may be a dumb question...

Have gravitational forces ever been observed between 2 massive objects, where neither object is a star/planet/moon/asteroid/comet/ etc.?

Thanks.
You and I may have different ideas of what "massive" (as in "2 massive objects") means, but the overwhelming answer to your question is "Yes".

The ones I particularly like are the Eötwash experiments (http://www.npl.washington.edu/eotwash/) (experiments done at the University of Washington, in the style of Loránd EötvösWP (http://en.wikipedia.org/wiki/Lor%C3%A1nd_E%C3%B6tv%C3%B6s)), and those reported here: Measuring Gravity at Small Distances (http://www.stanford.edu/group/kgb/RGravity.html).

SRH
2012-Sep-07, 10:55 PM
Interesting experiment...thanks.

I'm curious as to how we know that the relevant force in that experiment is gravity and not the electrostatic force?

SRH
2012-Sep-07, 10:57 PM
You and I may have different ideas of what "massive" (as in "2 massive objects") means, but the overwhelming answer to your question is "Yes".

The ones I particularly like are the Eötwash experiments (http://www.npl.washington.edu/eotwash/) (experiments done at the University of Washington, in the style of Loránd EötvösWP (http://en.wikipedia.org/wiki/Lor%C3%A1nd_E%C3%B6tv%C3%B6s)), and those reported here: Measuring Gravity at Small Distances (http://www.stanford.edu/group/kgb/RGravity.html).

Thanks

HenrikOlsen
2012-Sep-14, 11:57 AM
Interesting experiment...thanks.

I'm curious as to how we know that the relevant force in that experiment is gravity and not the electrostatic force?
By keeping the objects connected through a conductor their electrical potentials can be kept equal, thus zero electrostatic force.

But good to see you're thinking of potential errors.

tusenfem
2012-Sep-14, 12:38 PM
That's the one! Cavendish' torsion balance.

Yeah been there done that, had to wait till Friday afternoon, when all other students were gone and the experiment room was quiet to make reliable measurements ... these were the best days of my life ...

cjameshuff
2012-Sep-19, 04:07 AM
By keeping the objects connected through a conductor their electrical potentials can be kept equal, thus zero electrostatic force.

Conductive barriers can be used to shield electrical fields as well. And such interference would be revealed by doing the experiment multiple times and independent reproduction...electrical charges will vary, and will sometimes be repulsive. Consistent results are an indication that you're successfully measuring what you think you are.

Jens
2012-Sep-19, 06:32 AM
Gravity is very weak. It takes the whole mass of the Earth to pull you back down when you jump up.

Something I've wondered about: Isn't it also true to say that it only requires your small mass to pull the mass of the entire earth back to you, since motion is relative?

HenrikOlsen
2012-Sep-19, 06:09 PM
Something I've wondered about: Isn't it also true to say that it only requires your small mass to pull the mass of the entire earth back to you, since motion is relative?
Correct, but that is by such a small amount that no one notices it.

Rhaedas
2012-Sep-19, 10:43 PM
You not only get pulled back to Earth when you jump, but the Earth gets pulled back up. Proportionally. It would be immeasurable on atomic scales.

grapes
2012-Sep-20, 02:03 AM
You not only get pulled back to Earth when you jump, but the Earth gets pulled back up. Proportionally. It would be immeasurable on atomic scales.And, as you jump to push yourself away from the earth, the earth is pushed away from you. :)