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I don't understand what escape velocity is. I know escape velocity from the earth is 11,186m/s. If a rocket is launched from the surface of the earth with initial speed of 11,186m/s, does it escape the influence of earth's gravitational pull? Or does it need to have that velocity at a particular distance from the surface to escape the gravitational pull?

Qmechanic
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lekarane
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  • I think your intuition is influencing your "physics" knowledge, which is that an "escape" velocity much be much greater due to friction in the atmosphere. – Jared Jan 01 '18 at 08:42

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to hoist a rocket up off the surface of the earth and send it coasting away into outer space requires the expenditure of a definite amount of energy. you can impart that amount of energy to the rocket by boosting it off the ground with a certain speed, which is what is called the escape velocity. that velocity is exactly enough to keep the rocket moving ever farther away from the earth, even though the earth's gravity never stops trying to pull it back.

And even though the rocket is being progressively slowed down by the pull of gravity, that pull gets weaker and weaker the farther away from the earth the rocket gets. In the end, the rocket loses its velocity at the same rate by which gravity fades away and it then drifts off, slower and slower and slower, becoming ever closer to being free of the earth's gravity.

If the rocket is launched slower than escape velocity, gravity will eventually win the battle and bring the rocket crashing back down to earth. If the rocket is launched faster than escape velocity, its velocity wins the battle and it sails off into space without stopping, never to return.

niels nielsen
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  • Technically it can return – QuIcKmAtHs Jan 01 '18 at 07:24
  • please explain... – niels nielsen Jan 01 '18 at 07:27
  • like you can turn around in space using thrusters... – QuIcKmAtHs Jan 01 '18 at 07:28
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    OP's question did not deal with that. what I was trying to explain is what happens when you furnish exactly enough kinetic energy to equal the depth of the gravitational potential well occupied by an object sitting on the surface of the earth: it drifts asymptotically off towards zero velocity as it climbs out of the gravity well. faster than that, and it departs the gravity well altogether with some residual velocity. slower than that and it falls back. – niels nielsen Jan 01 '18 at 07:33
  • Thank you. Answer gave me exactly what I was looking for. – lekarane Jan 01 '18 at 07:37
  • I know @nielsnielsen I was joking, that's why i said technically – QuIcKmAtHs Jan 01 '18 at 07:38
  • i get it... you were being technically funny. one never knows here though; most people are very serious about this stuff. Happy new year to you and all the best -NN – niels nielsen Jan 01 '18 at 07:40
  • Haha same to you @nielsnielsen we probably shouldn't chat here. – QuIcKmAtHs Jan 01 '18 at 07:41
  • +1 Nice answer, but maybe add that no atmosphere is assumed present, and that the speed mentioned is for the surface, it gets lower with altitude. – Orbit Jan 01 '18 at 16:02
  • I agree. in this case I tried keeping things simple but you are right on both points. – niels nielsen Jan 01 '18 at 19:07
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Simply put, any object sent from the earth's surface or anything that is moving away from the earth's surface with the escape velocity (or greater than that) will permanently escape from the earth's gravitational pull and move to infinity. The earth's gravity won't be working on that object any more from then on.

QuIcKmAtHs
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Wrichik Basu
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