5 ms·

This is simply false. The needed escape velocity decreases with distance from the planet, so applying a change over a long period of time actually can very mate

by nilkn 9y ago

This is simply false. The needed escape velocity decreases with distance from the planet, so applying a change over a long period of time actually can very materially change the requirements. To give an example, if you provided propulsion to move an object at 1 mile per hour, you could turn off all propulsion and the object would have escaped Earth's gravity once it was 4 * 10^12 km away. This is, of course, an extraordinary waste and probably practically impossible.

larkeith 9y ago
Your "example" seems to predicate on a world without gravity. The reason escape velocity is reduced based on the distance from the surface is that you have already provided enough energy to overcome the vertical difference - you can see this reflected in the difference in gravitational potential energy.
- nilkn 9y ago
  Escape velocity is reduced based on distance because gravity itself is reduced based on distance. This is obvious from the the expression sqrt(2GM/r), which is the escape velocity at a distance r from the center of a spherically symmetric body of mass M. From this expression one can easily calculate the distance I provided. This computation is completely based on Newtonian gravity and would make no sense in a world without gravity.
  - garmaine 9y ago
    That’s not how physics works. I suggest reading up on conservation of energy and gravitational potential energy.
    - nilkn 9y ago
      With all due respect, this reply isn’t going to fly. This is a pretty pretentious copout that attempts to end the exchange while saving you from needing to provide any details or engage with the underlying material in any way. I provided a very specific computational counter-example to your statement. You’ll need to start there if you wish to compose a serious reply. I’ll restate the counter-example for your convenience. One can provide just enough continuous thrust to move an object at a fixed unchanging speed of 1mph for a long time (about 283 million years), at which point it will have reached a distance where the escape velocity is only 1mph. Once it has surpassed that distance, one can disable the thrust and the object will have completely escaped Earth’s gravity without ever having moved at more than 1mph. It will then asymptotically slow down to 0 mph over an infinite amount of time.
      - garmaine 9y ago
        Multiple people have pointed out the mistakes you are making, to no avail. I really don’t know what else can be said. It’s like you’re claiming 2+2 is 5. What else can I say? There is ample introductory material on Newtonian physics that you could learn from, only a google search away. It’s disrespectful of my time to assume I must be on the hook to handhold you through it.
        nilkn 9y ago
        You will have no trouble then pointing out the precise mistake in the calculation.
        p2t2p 9y ago
        Ok, let me ask a very straight and simple question. Imagine that I have a spacecraft with imaginable engine which is able to indefinitely provide thrust enough for this spacecraft to move straight up from the planet’s surface at the constant speed of 1 mph. Will this spacecraft leave the planet in indefinite amount of time or not? If not - what is going to stop it?
        deleted 9y ago
        [deleted]
        deleted 9y ago
        [deleted]
        zpconn 9y ago
        I found a number of sources online that disagree with you. https://www.quora.com/Escape-velocity-is-supposed-to-be-24-000-mph-but-our-rockets-never-achieve-this-speed-How-does-that-work?share=1 https://www.quora.com/Escape-velocity-is-supposed-to-be-24-0... Multiple answers explain that there isn't a fixed minimum change in velocity required for an object to escape Earth's gravity. https://www.quora.com/Does-rocket-always-need-the-escape-speed-11-2km-sec-to-escape-the-earth?share=1 https://www.quora.com/Does-rocket-always-need-the-escape-spe... The first answer here explains the same thing, using basically the same example (1 m/s instead of 1 mph). https://space.stackexchange.com/questions/4688/couldnt-i-escape-earths-gravity-traveling-only-1-mph-0-45-m-s https://space.stackexchange.com/questions/4688/couldnt-i-esc... The answers here overwhelmingly reiterate and support the same example in this thread.
marcosdumay 9y ago
You can reduce delta-v it a little by floating to the edge of the atmosphere. Beyond that, there is no method of getting higher that does not involve gaining some speed.

by nilkn 9y ago

larkeith 9y ago

Your "example" seems to predicate on a world without gravity. The reason escape velocity is reduced based on the distance from the surface is that you have already provided enough energy to overcome the vertical difference - you can see this reflected in the difference in gravitational potential energy.

nilkn 9y ago
Escape velocity is reduced based on distance because gravity itself is reduced based on distance. This is obvious from the the expression sqrt(2GM/r), which is the escape velocity at a distance r from the center of a spherically symmetric body of mass M. From this expression one can easily calculate the distance I provided. This computation is completely based on Newtonian gravity and would make no sense in a world without gravity.
- garmaine 9y ago
  That’s not how physics works. I suggest reading up on conservation of energy and gravitational potential energy.
  - nilkn 9y ago
    With all due respect, this reply isn’t going to fly. This is a pretty pretentious copout that attempts to end the exchange while saving you from needing to provide any details or engage with the underlying material in any way. I provided a very specific computational counter-example to your statement. You’ll need to start there if you wish to compose a serious reply. I’ll restate the counter-example for your convenience. One can provide just enough continuous thrust to move an object at a fixed unchanging speed of 1mph for a long time (about 283 million years), at which point it will have reached a distance where the escape velocity is only 1mph. Once it has surpassed that distance, one can disable the thrust and the object will have completely escaped Earth’s gravity without ever having moved at more than 1mph. It will then asymptotically slow down to 0 mph over an infinite amount of time.
    - garmaine 9y ago
      Multiple people have pointed out the mistakes you are making, to no avail. I really don’t know what else can be said. It’s like you’re claiming 2+2 is 5. What else can I say? There is ample introductory material on Newtonian physics that you could learn from, only a google search away. It’s disrespectful of my time to assume I must be on the hook to handhold you through it.
      - nilkn 9y ago
        You will have no trouble then pointing out the precise mistake in the calculation.
      - p2t2p 9y ago
        Ok, let me ask a very straight and simple question. Imagine that I have a spacecraft with imaginable engine which is able to indefinitely provide thrust enough for this spacecraft to move straight up from the planet’s surface at the constant speed of 1 mph. Will this spacecraft leave the planet in indefinite amount of time or not? If not - what is going to stop it?
      - deleted 9y ago
        [deleted]
      - deleted 9y ago
        [deleted]
      - zpconn 9y ago
        I found a number of sources online that disagree with you. https://www.quora.com/Escape-velocity-is-supposed-to-be-24-000-mph-but-our-rockets-never-achieve-this-speed-How-does-that-work?share=1 https://www.quora.com/Escape-velocity-is-supposed-to-be-24-0... Multiple answers explain that there isn't a fixed minimum change in velocity required for an object to escape Earth's gravity. https://www.quora.com/Does-rocket-always-need-the-escape-speed-11-2km-sec-to-escape-the-earth?share=1 https://www.quora.com/Does-rocket-always-need-the-escape-spe... The first answer here explains the same thing, using basically the same example (1 m/s instead of 1 mph). https://space.stackexchange.com/questions/4688/couldnt-i-escape-earths-gravity-traveling-only-1-mph-0-45-m-s https://space.stackexchange.com/questions/4688/couldnt-i-esc... The answers here overwhelmingly reiterate and support the same example in this thread.

marcosdumay 9y ago

You can reduce delta-v it a little by floating to the edge of the atmosphere. Beyond that, there is no method of getting higher that does not involve gaining some speed.