Exploring Gravity: Common Questions on Gravitational Fields, Forces, and Energy

[raeshun]

1.Does a gravitational field ever end for instance do we experience a gravitational force from a star light years away.

2.if there are two objects in space one 100kg and another 1kg.According to Newtons law of Universal Gravitation there will be an attractive force between these two objects. So my question is does the small object accelerate towards the bigger object or vice-versa.Or do they both accelerate at a distance between them?

3.where does gravity get the energy to move objects from?

any help will be welcome thanks.

 

[Simon Bridge]

1.Does a gravitational field ever end for instance do we experience a gravitational force from a star light years away.

No and yes.

2.if there are two objects in space one 100kg and another 1kg.According to Newtons law of Universal Gravitation there will be an attractive force between these two objects. So my question is does the small object accelerate towards the bigger object or vice-versa.Or do they both accelerate at a distance between them?

The last one - they both accelerate towards their mutual center of mass.

3.where does gravity get the energy to move objects from?

The kinetic energy comes from the gravitational potential energy. Some work had to be done on the object in order to get it to the position it "started" out in.

 

[Kevin Really]

The kinetic energy comes from the gravitational potential energy. Some work had to be done on the object in order to get it to the position it "started" out in.

If there was a big bang, would you say that gravity is the effect of objects attempting to return to the "original" singularity? Are objects falling back together?

 

[Mark M]

Welcome to PF Kevin!

If there was a big bang,

It's not a matter of 'if' the if big bang model is correct, we know it is. See here:

http://en.wikipedia.org/wiki/Big_Bang#Observational_evidence

would you say that gravity is the effect of objects attempting to return to the "original" singularity? Are objects falling back together?

You're misunderstanding what the big bang was. It was *NOT* an explosion, and a singularity is *NOT* a point. The big bang was the hot dense state the ENTIRE universe was at EVERY point. The metric of space expands from every point, so that the radiation cooled and redshifted, and the plasma cooled until it became gas. Denser regions of the gas formed, which were then driven apart by the expansion of space. These became galaxies.

See these articles:

http://www.astro.ucla.edu/~wright/cosmology_faq.html

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Finally, gravity has nothing to do with this. We understand gravity through general relativity, in which matter warps spacetime, causing objects to follow curved geodesics through spacetime. This results in an acceleration through space, gravity.

 

[Kevin Really]

Thank you, Mark M

Especially for the concise description of gravity. I'll have to give it some more thought. I couldn't get your first link to come through. I'm working on the second article, but I'll have to continue with it tommorow. I'm getting sleepy now.

And thanks for welcoming me to the board.
Kevin

 

[Mark M]

The first article was just the 'observational evidence' section if the Wikipedia page on the big bang. Just find that page with google, and then scroll down to that section.

For a good description of general relativity, Wikipedia has an introductory page:

http://en.wikipedia.org/wiki/Introduction_to_general_relativity

For a very brief explanation, see this page from the Usenet FAQ:

http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html

 

[Simon Bridge]

If there was a big bang, would you say that gravity is the effect of objects attempting to return to the "original" singularity? Are objects falling back together?

In addition to what Mark said, and at the risk of confusing things further:

My understanding is that this is another yes-and-no answer ... there is no center of mass for the Universe for all objects to gravitate towards, so: "no". OTOH: you'll have heard about how the rate of expansion is related to the amount of mass in the Universe? So: "sort of".

If you are not familiar with how the big bang model works you are well advised to become familiar before we start on about how gravity is involved. Ping us when you next get stuck ;)