Register to reply

A question about gravity

by Deepak K Kapur
Tags: gravity
Share this thread:
D H
#19
Sep30-13, 06:14 PM
Mentor
P: 15,068
The "Hill sphere" is named after 19th century astronomer George W. Hill. Google the term "Hill sphere."
Deepak K Kapur
#20
Sep30-13, 11:15 PM
P: 23
I was reading about 'Hill Sphere', when a thought came to my mind as follows:

I can easily lift a small ball, say of 10 gm, with my hand. If I go to the top of a high tower and try to lift this ball with a super-light, super-strong thread I would also be able to do it with ease. Now suppose a tower ( or pole or whatever of some super-light and super-strong material) is made with a height of say 500 km or 50000 km. Again if I suspend this ball with a super-super-super light and super-super-super strong thread, I think I would still be able to pull the ball towards me by pulling at it slowly and slowly (ignore force due to air etc.).

If this is true (at least in principle), why do we need an escape velocity of 11.2 km/s.


conversely


I also thought of suspending a ball from a satellite right upto the surface of the earth and then pulling it up slowly.
Jimmy
#21
Sep30-13, 11:49 PM
P: 656
Quote Quote by Deepak K Kapur View Post
If this is true (at least in principle), why do we need an escape velocity of 11.2 km/s.
Escape velocity applies to ballistic trajectories.

Here's a related thread:
http://www.physicsforums.com/showthread.php?t=611415
litup
#22
Oct1-13, 02:28 AM
P: 134
Thanks for the info about the 'hill sphere', hadn't heard of that before. Had no idea Earth had a reach of 1.5 million Km. So the moon will be held in Earth's orbit long after the sun bloats out to burn them up, assuming they survive that particular ordeal.
arildno
#23
Oct1-13, 05:04 AM
Sci Advisor
HW Helper
PF Gold
P: 12,016
Quote Quote by Vanadium 50 View Post
A friend of mine lost her toolkit in orbit when a grease gun didn't shut off properly. Propelled it like a rocket. A very slow rocket, but a rocket nonetheless. It wasn't until the following year until the toolkit reentered.
QED, in a very illustrative way!
Deepak K Kapur
#24
Oct1-13, 05:52 AM
P: 23
Quote Quote by D H View Post


Another qualified yes. I'm not a fan of these kinds of questions. We don't live in a universe in which there is nothing but the Earth and the spaceship. However, the answer is yes with the assumption that the laws of physics in this nearly empty universe are the same as the laws of physics in our universe filled with lots of stuff. The spaceship needs to be moving at a ridiculously slow speed, however. Escape velocity at a million light years is 2.9◊10-4 meters per second.
Suppose the spaceship (that is a million light years away from Earth) stops firing its rockets. It will start moving towards earth.

In my view the information (happening, action) that the space ship has started moving will travel in space starting from the spaceship and then moving towards Earth.

When would Earth know (detect, come into picture) about this? Instantaneously or after a million years?
A.T.
#25
Oct1-13, 06:56 AM
P: 3,932
Quote Quote by Deepak K Kapur View Post
Suppose the spaceship (that is a million light years away from Earth) stops firing its rockets. It will start moving towards earth.
Stopping the engines doesnít necessarily make it move towards earth immediately. Especially if it was already moving faster than escape velocity at that distance.

Quote Quote by Deepak K Kapur View Post
When would Earth know (detect, come into picture) about this?
A million years.
litup
#26
Oct1-13, 07:01 PM
P: 134
Quote Quote by Vanadium 50 View Post
A friend of mine lost her toolkit in orbit when a grease gun didn't shut off properly. Propelled it like a rocket. A very slow rocket, but a rocket nonetheless. It wasn't until the following year until the toolkit reentered.
You have an astronaut friend? Awesome. The best I could do was get to hold a moon rock in my hand from the vault at Goddard when I was working Apollo, Tracking and Timing. Atomic clocks and the little transponder onboard Apollo that gave the distance to the craft within 50 feet even if it was a half million miles out, which of course it never got that far but the electronics was good for that distance. Sigh. Back when we were sending men to the moon. A distant memory now.
georgir
#27
Oct2-13, 04:49 AM
P: 131
Quote Quote by Deepak K Kapur View Post
When would Earth know (detect, come into picture) about this? Instantaneously or after a million years?
The fun part is, it doesn't matter. The Earth would be approximately equally attracted toward the spaceship and its fuel, no matter if they both fall toward the Earth together, or they split up, fuel hurling towards earth so that the ship can move further away...

And if "approximately" is not good enough for you, the answer would depend on who you ask...
Newton would say the Earth instantly knows how to react. Einstein would say the Earth instantly reacts as if it assumed the fuel/rocket fell gravitationally, and if they didn't, the information about that would spread at light speed...
Deepak K Kapur
#28
Oct3-13, 07:50 AM
P: 23
Quote Quote by georgir View Post
The fun part is, it doesn't matter. The Earth would be approximately equally attracted toward the spaceship and its fuel, no matter if they both fall toward the Earth together, or they split up, fuel hurling towards earth so that the ship can move further away...

And if "approximately" is not good enough for you, the answer would depend on who you ask...
Newton would say the Earth instantly knows how to react. Einstein would say the Earth instantly reacts as if it assumed the fuel/rocket fell gravitationally, and if they didn't, the information about that would spread at light speed...
Let us divide the space between the very-very distant satellite and Earth into small parts, say 1 light second apart, each.

Unless each part of space communicates with each other part simultaneously, the effect of gravitation cannot take place. This communication, to my mind, has to be instantaneous.
D H
#29
Oct3-13, 07:59 AM
Mentor
P: 15,068
Quote Quote by Deepak K Kapur View Post
Let us divide the space between the very-very distant satellite and Earth into small parts, say 1 light second apart, each.

Unless each part of space communicates with each other part simultaneously, the effect of gravitation cannot take place. This communication, to my mind, has to be instantaneous.
Your intuition is wrong. That's not how it works.

To understand how it works you have to learn Newtonian mechanics and simple calculus before you can even begin to dive off the deep end into general relativity and differential geometry. You apparently haven't even taken that first step.


Register to reply

Related Discussions
Does gravity affect gravity the way gravity affects time? General Physics 12
A question on Gravity Waves and Gravity Radiation Cosmology 3
Question about gravity General Physics 1
Gravity/Planetary Forces Question - High School physics question Introductory Physics Homework 5
Help with gravity Question Introductory Physics Homework 6