Escape velocity and gravitational freefall

[Ranku]

Is an object with escape velocity in gravitational freefall?

 

[Ibix]

Maybe. It depends if it's being accelerated by something or not.

 

[Ranku]

Maybe. It depends if it's being accelerated by something or not.

Let me re-frame the question then: Can an object attain escape velocity if it only receives an initial impulse, and not continuously accelerated, such as in a rocket ?

 

[Ibix]

Of course. You'd need a very big cannon to be able to do it on Earth, but you can jump hard enough to achieve escape velocity from smaller asteroids.

 

[Ranku]

Of course. You'd need a very big cannon to be able to do it on Earth, but you can jump hard enough to achieve escape velocity from smaller asteroids.

So such an object would be in freefall?

 

[Ibix]

If you ignore drag from the atmosphere and solar wind, and anything else like that, yes.

 

[PeroK]

Let me re-frame the question then: Can an object attain escape velocity if it only receives an initial impulse, and not continuously accelerated, such as in a rocket ?

You could attain the escape velocity of an asteroid by jumping. And, technically, if you throw an object it has attained its escape velocity from you. In the sense that if you threw a ball in space at any significant speed it would never come back on account of your gravity.

Attaining the Earth's escape velocity is more difficult, but it's not fundamentally any different.

 

[anorlunda]

If you throw a pebble while you are in space, can you imagine the pebble escaping the gravity of your mass?

If yes, you established the principle. All else is just a question of the numbers.

 

[Bandersnatch]

I think what confuses the OP is that an object can be in free >fall< while going up and never coming back.
This is a matter of different usage of the expression in physics vs the common language.
I.e. in common usage falling is generally understood to imply movement 'down, towards the ground'.
Free fall in Newtonian physics means moving solely under the influence of gravity (when it's the only accelerating force). It has nothing to do with direction or whether the object ends up on the ground or not.

 

[russ_watters]

Let me re-frame the question then: Can an object attain escape velocity if it only receives an initial impulse, and not continuously accelerated, such as in a rocket ?

You're asking a question you've asked before in a different thread. The answer is very simple (if the question is worded properly). It seems like you don't like the answer. What answer were you hoping to get?

 

[Ranku]

You're asking a question you've asked before in a different thread. The answer is very simple (if the question is worded properly). It seems like you don't like the answer. What answer were you hoping to get?

I had in mind the similarity with a universe that is expanding with or without a cosmological constant, and how to look at matter in terms of whether it is in cosmological freefall or not.

 

[Ibix]

I had in mind the similarity with a universe that is expanding with or without a cosmological constant, and how to look at matter in terms of whether it is in cosmological freefall or not.

Does said matter have any rocket engine? Is it moving through a medium? Is it charged or magnetic and in an electromagnetic field? Are there any forces you can think of at all, disregarding gravity (and other spacetime curvature effects if you are thinking of those as distinct from gravity)? No? Then it's in free fall.

 

[sophiecentaur]

Then it's in free fall.

If "free fall" is/was defined as 'without any force other than gravity acting" then it's in free fall.

The expression "free fall" was introduced before the New Cosmology arrived and there is little point in trying to make it fit in. This is yet another example of people assigning more importance to the meaning of words and classification than in try to understand what's actually going on.

 

[Ranku]

If "free fall" is/was defined as 'without any force other than gravity acting" then it's in free fall.

The expression "free fall" was introduced before the New Cosmology arrived and there is little point in trying to make it fit in. This is yet another example of people assigning more importance to the meaning of words and classification than in try to understand what's actually going on.

That's what I'm trying to do - understand, which is something to be arrived at by asking questions about that which one doesn't yet know enough about.

 

[sophiecentaur]

That's what I'm trying to do - understand, which is something to be arrived at by asking questions about that which one doesn't yet know enough about.

Ok. Fair enough. But the term “free fall” cannot really be applied to relativistic matters or to expansion. It is an ancient term so I think that ‘understanding’ doesn’t involve it.

 

[Ibix]

It's probably more precise to ask if an object is moving inertially (no forces acting on it - remember that gravity is not a force) than ask about free fall. I'd regard the two terms as equivalent, but @sophiecentaur apparently disagrees. Either way, I do kind of agree with the point that asking about escape velocity is a strange way to start asking about cosmology. Escape velocity isn't really relevant to cosmology. You'd be better served by providing a bit more context to your question - then you don't get twelve posts debating the meaning of "free fall" and its relationship to escape velocity when you are actually interested in the movement of galaxies.

 

[Ranku]

It's probably more precise to ask if an object is moving inertially (no forces acting on it - remember that gravity is not a force) than ask about free fall. I'd regard the two terms as equivalent, but @sophiecentaur apparently disagrees. Either way, I do kind of agree with the point that asking about escape velocity is a strange way to start asking about cosmology. Escape velocity isn't really relevant to cosmology. You'd be better served by providing a bit more context to your question - then you don't get twelve posts debating the meaning of "free fall" and its relationship to escape velocity when you are actually interested in the movement of galaxies.

I didn’t actually mean to bring up cosmology in this thread, but then @russ_watters accused me of not liking the answers I had already received about free fall on another thread - so then I mentioned I was trying to see how freefall might fit into the cosmological context, but first I wanted to explore all aspects about freefall; perhaps subsequently, I would have directly asked about freefall and cosmology in a separate thread.

 

[sophiecentaur]

I was trying to see how freefall might fit into the cosmological context,

Well, I think that it cannot because it is a Classical idea that was applied to the very simplest of situations. Furthermore, I would suggest that free fall refers only to gravitational forces (i.e. no fancy tricks with E fields and suitable charge distribution etc.). But this is a perfect example of the etymological tail wagging the physical dog and it can only deviate one from the path of 'deeper' understanding.