Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 14/12/2016 06:23:02

Title: How quickly would planets move away from each other without gravity?
Post by: thedoc on 14/12/2016 06:23:02

John Bermingham asked the Naked Scientists:
   
Hi,

Love the show (http://www.thenakedscientists.com/HTML/podcasts/).

I have a question:

If gravity was removed, at what rate of acceleration would two adjacent objects fly apart from each other due to the expansion of space? Assuming the objects stay intact.

Thanks,
John (Dublin)

What do you think?

Title: Re: How quickly would planets move away from each other without gravity?
Post by: Colin2B on 14/12/2016 10:34:27

If gravity was removed then the objects would continue in the direction and at the speed that they were going at the time it was removed.
So an object falling towards a planet would continue to move but would not accelerate any more.
If the object was orbiting a planet it would move at a tangent to it's original path but again would not accelerate.

Title: Re: How quickly would planets move away from each other without gravity?
Post by: syhprum on 14/12/2016 10:56:05

They would of course continue to travel at their orbital speeds and in the direction they were traveling when the loss of gravity occurred.
The expansion of space is only relevant at scales much larger than the Solar system

Title: Re: How quickly would planets move away from each other without gravity?
Post by: yor_on on 14/12/2016 20:09:35

How about this then, If we assume that gravity forces 'accelerations', what is 'relative motion'?

Title: Re: How quickly would planets move away from each other without gravity?
Post by: yor_on on 15/12/2016 15:42:29

Here I'm slightly disappointed, no one that can define the difference between a 'relative motion', versus one that involves 'gravity' giving you a 'preferred path'? To me it has to do with 'energy expended', but then it also seems to say that the carousel expends 'energy', keeping itself together as it rotate.

In Newtonian terms it is 'accelerating' isn't it?