# Escape velocity and acceleration from gravity

Cpt. Bob
Ive heard that the escape velocity of a body is the same as the maximum velocity that can be achieved from gravitational acceleration from the same body. Like say it requires a spacecraft to be moving at about 11 km/sec to escape earths gravity, and we isolated the earth and spacecraft, so that the only force acting on the spacecraft is earths gravity. Then if the spacecraft was placed stationary any distance from the earth, the maximum velocity it could ever achieve is 11 km/sec. Is this correct, and if so why? Would it be incorrect in the example above, but correct in the real world because of other more powerful gravitational forces, such as the suns, countering acceleration due to earths gravity till the spacecraft is very near earth, or near enough for earths gravity to become dominant? Thx in advance for any clarification.

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Tom Mattson
Staff Emeritus
Gold Member
Originally posted by Cpt. Bob
Is this correct, and if so why?
It is correct, and it is due to the conservation of energy. The escape velocity is calculated by setting the total energy equal to zero. That is, if you start at r=&infin; from rest and let go, you would return to the Earth at the escape velocity.

Would it be incorrect in the example above, but correct in the real world because of other more powerful gravitational forces, such as the suns, countering acceleration due to earths gravity till the spacecraft is very near earth, or near enough for earths gravity to become dominant? Thx in advance for any clarification.
I think you've got it backwards: It is correct in the (idealized) example above, but incorrect in the real world.

When calculating the escape velocity of the Earth, the influence of other bodies is neglected. In the real world, that effect must be taken into account to be accurate.

Originally posted by Cpt. Bob
if the spacecraft was placed stationary any distance from the earth, the maximum velocity it could ever achieve is 11 km/sec. Is this correct
Yes.
and if so why?
Because in this scenario, the ship can only gain kinetic energy from its potential energy in the earth's gravitational field. And, as the distance earth-ship approaches infinity, the potential energy approaches a certain limit.
Yet we know that meteorites can have much higher velocities than 11 km/s. Which means they carry surplus energy that does not stem from earth's gravity alone.
Would it be incorrect in the example above, but correct in the real world
I'd put it the other way round: correct in the above example, but incorrect in the real world
because of other more powerful gravitational forces, such as the suns
Yes, a celestial body that is much more massive than earth, but not much farther away (at a certain time), could (have) provide(d) the surplus energy.

Cpt. Bob
Great, thx for the help. I hadnt even thought about conservation of energy as it applies here. Last edited by a moderator:
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