How does a body escape its gravitational field?

In summary, the Earth appears to be continuously moving despite its gravitational field keeping it in place. This is because, unlike the ball on a rubber sheet analogy, the Earth is not trying to escape its own gravity and therefore does not have to expend energy to move. The Earth moves as a single unit, rather than the individual particles that make it up, due to the significant impact of gravity on a large scale. Additionally, in some coordinate systems, the Earth does not move and is simply following a geodesic through space-time.
  • #1
-Job-
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Might seem like a strange question, but why does the Earth move? Shouldn't it's gravitational field keep it in place? Wouldn't the space immediatly surrounding the Earth offer considerable resistance to its movement? Things on the surface of the planet have a hard time escaping the Earth's gravitational field, and yet the whole planet, as one, does it continuously. In the traditional model of gravity consisting of ball on top of a sheet, bending the sheet, the ball wouldn't be able to just get out of its hole without considerable energy, that movement then would expend some amount of energy, due to the "resistance of space", how much energy then wouldn't we need to have the Earth moving all this time? Is this a stupid question? :uhh:
 
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  • #2
If you're using the rubber sheet analogy remember that the indentation caused by a mass is not fixed so a body cannot 'escape' its own gravity. If you roll a ball on a rubber sheet the indentation it makes does not impede its movement yet if something else were to roll close to it the indentation created by the ball would eefect that object.
 
  • #3
-Job- said:
Might seem like a strange question, but why does the Earth move? Shouldn't it's gravitational field keep it in place? Wouldn't the space immediatly surrounding the Earth offer considerable resistance to its movement?

Whoa! Since when does "space" have "considerable resistance"?

Zz.
 
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  • #4
ZapperZ said:
Whoa! Since when does "space" have "considerable resistance"?
Yes, You´re taking the rubber sheet analogy too seriously. Remember that even in GR the "speed of the Earth through space" is not defined, so contrary to a rubber sheet the indentation in spacetime does not move against spacetime. So space has no resistance against movement.
 
  • #5
I only meant "resistance" as the gravitational pull in that region in space. To any object at the surface of the planet the curvature of space prevents the object from leaving the planet easily, offering "resistance" to movement in the direction opposite to the gravitational pull.
What lead to this question was thinking about how movement of the Earth is processed. It can't be by: this portion moves, then this portion moves... etc, because each of the portions would be going against the gravitational pull of the rest of the planet. So the whole planet must move exactly at the same time, i suppose, otherwise it wouldn't be able to escape its own gravitational field, that was what i meant. But i don't know whether or not the whole planet moves at the same time, as in "after x seconds every 1/n of the planet has been displaced by exactly the same amount". I really have no idea how things move, or whether they do at all.
 
  • #6
You seem to be picturing a flat plane of space-time with the Earth plonked in the middle, stuck in its own indentation. But the plane is not flat - its sloping down towards the sun, which has made a comparitively huge indentation. So the Earth would have to be moving, like a marble spinning round in a funnel.
I would think of the Earth moving as a single unit, not the individual particles that make it up because gravity is only really significant on a large scale.
 
  • #7
-Job- said:
I only meant "resistance" as the gravitational pull in that region in space.

Then what is so special about gravitational pull if all you care about is "resistance" from being pulled away? Is it valid for me to argue that space has "resistance" since it is so difficult for me to pull an electron away from a positive charge? There's no gravity involved here.

These are nothing more than the force on an object. One requires no warping of spacetime to observe the IDENTICAL effect, i.e. the resistance of being pulled apart. So it is logically invalid to argue that "can't pull things apart" must be equal to "resistance of space".

Please note that you have a rather simplistic and naive view of GR in terms of the warping of Minkowski SPACE-TIME. It isn't just SPACE that is affected.

Zz.
 
  • #8
Might seem like a strange question, but why does the Earth move?
In some coordinate systems, it doesn't move.

And in an important sense, the Earth isn't moving: it's (more or less) blissfully following a geodesic through space-time without being accelereated.

Of course, you're not blissfully following a geodesic... you're being violently thrust away from an intertial path by that 9.8 m/s² force from the ground pushing you up.
 
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  • #9
-Job- said:
I only meant "resistance" as the gravitational pull in that region in space. To any object at the surface of the planet the curvature of space prevents the object from leaving the planet easily, offering "resistance" to movement in the direction opposite to the gravitational pull.
Right, but Earth is not leaving itself, so it isn't traveling up in its own gravitational field.

Earth doesn't just reside in that curvature in the rubber sheet, it causes the curvature in that rubber sheet. So there is no reason why it would/could resist its own movement. To itself/its gravitational field, it is always stationary.
 

1. How does a body escape its gravitational field?

To escape a gravitational field, a body must achieve enough velocity to overcome the pull of gravity. This is typically achieved by reaching what is known as escape velocity, which varies depending on the mass and radius of the body.

2. What factors affect a body's escape from a gravitational field?

The main factors that affect a body's escape from a gravitational field are the mass and radius of the body, as well as the strength of the gravitational pull at the surface of the body. Other factors such as air resistance and external forces can also play a role.

3. Can a body escape a gravitational field without any external force?

No, a body cannot escape a gravitational field without any external force. This is because the force of gravity is constantly pulling the body towards the center of the field. Without an external force, the body would continue to orbit or fall towards the center of the field.

4. What is the difference between escape velocity and orbital velocity?

Escape velocity is the minimum velocity required for a body to escape a gravitational field, while orbital velocity is the velocity required for a body to maintain a stable orbit around a larger object. Orbital velocity is typically lower than escape velocity, as it only needs to counteract the force of gravity enough to maintain a circular or elliptical orbit.

5. Can a body escape the gravitational field of a black hole?

It is not possible for a body to escape the gravitational field of a black hole once it has crossed the event horizon, as the escape velocity at this point is greater than the speed of light. However, a body can escape the gravitational pull of a black hole by achieving escape velocity before crossing the event horizon.

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