Why is it necessary to travel 11km per second to escape the earth's gravity (escape velocity), when surely any speed will do, as long as you keep going up? | Notes and Queries (2024)

Why is it necessary to travel 11km per second to escape the earth's gravity (escape velocity), when surely any speed will do, as long as you keep going up?

Luke, Wellington NZ

• If you start at less than 11 km/s at ground level and don't get any further 'push' (ie you slow down as you go up, like a stone thrown into the air) you will fall to the ground again. If you start at more than 11 km/s, you will never come down.

  Phil Cohen, Sydney Australia

• In a way, you are right. if you went straight up at any speed, eventually you would get so far from the earth that the effect of the Earth's gravity was negligable. However, where the 11km per hour comes in is that it is the speed at which an object, rather than being held in orbit around the earth or falling back to it due to gravity, will be thrown outwards by sufficient centrifugal force to describe an open curve away from the planet.

  John Ramsey, Hackney UK

• Providing that you can keep your propulsion system going and the thrust is greater then gravitational pull back to Earth, yes any speed will do. Once you reach escape velocity however you are going fast enough to escape the Earth's gravitational pull without doing any more work, you can turn your engines off and you'll still keep going never to return.

  John Gresham, Waterloo Merseyside

• Escape velocity is calculated on the assumption that the only force applying is gravity, that is, only after the rocket engines (or whatever accelerated you to 11km/s) have stopped firing. Imagine yourself above the Earth’s atmosphere travelling away from the Earth at high speed. Earth’s gravitational pull is slowing you down, but as your distance from Earth increases, that pull weakens. If you were initially travelling at less than the escape velocity then eventually Earth’s pull would be enough to bring you to a stop and from then you on you would fall back to Earth. The escape velocity is the speed you need to avoid this fate and keep travelling forever, although you would need to travel at a much higher speed if you wanted to reach the stars in your lifetime.

  M Baillie, Sawston UK

• If you throw something up in the air, it will carry on rising until the force of gravity overcomes the velocity of the object and causes it to keep slowing until it stops and then starts falling back to earth. You are kind of correct in saying that you just need to "keep going up", but in order to keep going up you need a sufficient amount of velocity to overcome the force of gravity. The amount of velocity needed to escape our planets gravity is approx 11km per second, which is known as escape velocity.

  Steve, London UK

• The term “escape velocity” refers to the initial velocity of an object (at the surface of the Earth). The assumption is that no further force is applied to the object once it has set off. If the object starts at less than 11 km/sec, eventually the gravitational pull of the planet will pull it back to Earth before it escapes gravity.Of course any velocity will do if continued force is applied to the object, just as a car being driven uphill will keep going if enough power from the engine is applied via the accelerator.

  Jeremy Marchant, Stroud UK

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• It isn't necessary. As you say, any speed will do. Escape velocity is simply the speed that theoretically would be sufficient to remove something from Earth's gravitational influence with no further impulse required.

  Michael Fisher, Brisbane Australia

• Escape velocity is the speed required to escape gravity in the absence of any force being applied. To keep going up requires the continual application of force. So yes, of course it is possible to keep going upwards and further from the earth, but without reaching escape velocity you would eventually be pulled back to earth when the fuel runs out.

  Clive Gordon, Ruislip UK

• Escape velocity is literally the velocity which an object would need to be projected in order to leave the surface of a planet or other body in space. If the object were subjected to a steady upward force just greater than its weight,instead of an explosion,it would eventually leave, but much more slowly. It is interesting to note that the speed of air molecules is too low to allow them to escape the Earth, but greater than the Moon's escape velocity. That's why we have an atmosphere and the Moon hasn't. Is this rocket science?

  Alan Shepherd, Huddersfield UK

• You're right, but escape velocity is the speed a projectile (i.e. unpowered lump) would require to escape the Earth's gravity field. If you have enough power you can trundle up at your leisure. This point was ably covered in the old Peter Sellers movie "mouse on the Moon" which had a steam kettle spaceship. Rockets usually just have to get high enough to fall into orbit. However, you do need to get a rocket up to a high velocity before its fuel runs out if you want to send a probe to another planet.

  Craig Napier, Eagle Heights Australia

• It is not necessary to travel at 11km/s to escape the earth's gravity, any speed will do, as long as you keep going up. 11km/s is known as escape velocity because a body travelling up at this speed at the earth's surface will keep going up without any further force being applied; the reverse argument (that you must reach escape velocity to keep going up) does not follow, but anything travelling more slowly will need a bit of a push on the way up.

  Phil Barker, Edinburgh Scotland

• The escape velocity refers to an the speed an object needs to achieve to move from its current point in a gravitational field to infinity(as the gravitational field has no distinct end point), it is equal to the speed that the object would be traveling at if it was pulled by the same gravitational field from infinity to that same point. However this only applies for an unpowered mass that would be constantly decelerating due to the gravitational pull of the plant and you are quite right in saying a powered mass would be able to escape no matter what its speed.

  Steven, Glasgow

• You'd have to go up a very long way indeed to escape the earths gravity. Remember that this is what holds the moon in its orbit. An orbiting object has not escaped gravity, it is in constant freefall towards the earth. As long as it goes fast enough, it never reaches the ground because the earth curves away beneath it.

  Howard, Haywards Heath West Sussex

• In theory, you're right, and if only Everest was taller, you could climb it all the way up to orbit as slowly as you damn pleased. Let's assume there's 2 ways into space, the slow way and the quick way. The slow way is to have some sort of propulsion engine, at least big enough to lift the vehicle's weight (e.g. a modern jet fighter can easily climb vertically, at speeds of 500MPH). The problem is that the jet engine can't get the oxygen it needs after about 5 miles up. The answer is to use a rocket motor, which carries all the fuel & oxygen it needs. But that turns out to be a lot of fuel (a HUGE lot of fuel) that you would be carrying, all the way up to 25,000 miles high or so where gravity becomes negligible and where, ideally, the last drop of fuel runs out. That brings us to the quick way: use that fuel in one quick burst, accelerate to 11km per second (25,000 MPH) as fast as possible, then coast the rest of the way with empty tanks and a dead engine. That way, you're carrying spare fuel for the shortest possible time.

  Paul Reilly, London UK

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• You are right, but in order to "keep going up" on sustained power you'd have to carry immense reserves of fuel to push you up. When Isaac Newton first described the concept of escape velocity he was thinking of a ball shot from a cannon. And modern rockets are much like that, their fuel tanks can only push them up for a few minutes and from then on they have rely on their own inertia. If they don't reach escape velocity by then, they fall back or remain in orbit.

  Alejandro Pareja, Madrid Spain

• True, in theory any speed would do. But in practice the difficulty comes from the trade-off between the rate at which you escape the Earth's gravity and the rate at which you consume the fuel needed to do so - and hence the fuel needed at take-off. If you ascended at 5mph, but had the typical fuel capacity of the shuttle and would be largely used up in 10 to 15 minutes, you'd only get up a couple of miles.

  Martin, Manchester UK

• Escape Velocity is the minimal velocity required to escape earth's gravity without applying any further force. In this case, when any speed is maintained continuously a force is constantly applied. So the key point is the Force applied.

  Shravan Singh, Kolkata India

• An unpowered projectile needs escape velocity to travel arbitrarily high but a continually powered vehicle does not, of course. When the vehicle is in orbit above the atmosphere almost any force will do to push it further out - hence the ability of puny ion drives to push craft into higher orbits. It's getting into orbit that takes all the power. A grand space elevator is proposed to take the craft out to where the orbital speed is the rotation speed of the Earth's surface, so doing away with the flashy rocketry. Don't hold your breath.

  miles.felton, berkshire

• The 11 km/hr is the minimum speed necessary at the surface of the earth, to enable travelling any arbitrary distance from the earth without any means of propulsion. However, you are quite right that, if you have means of propulsion, such as a rocket ship, you can travel as slowly away from the surface as you like.

  Michael Hall, Canberra Australia

• A stone thrown upwards would need to achieve this speed, however the space shuttle coule go up as slow as required (Assuming enough fuel reserves).The distinction is whether the flight is powered or not.

  Lee, Leeds UK

• There is a big difference between an object being aimed vertically upwards and shot out of a cannon and a powered rocket. The ball shot from the canon receives energy only as it passes through the barrel, from then on it is unpowered and slows down as it climbs through the earths gravitational field. Escape velocity refers to this case, not a powered rocket.Incidentally the mass of the ball does not effect the escape velocity if there is no atmospheric friction, which means that an elephant and a mouse would both have to be given the same escape velocity if launched from the surface of the moon!

  Chris Folland, Moshi Tanzania

• The term escape velocity presupposes that the object being considered is not subject to any external acceleration/force other than gravity. As such an object travels upwards it will of course be slowed by gravity, but at the same time an object that moves upwards from the earth the effect of gravity gradually dimishes. If you begin travelling upwards too slowly gravity will bring you back down to earth. If you start out travelling fast enough, whilst gravity will slow you down it will not be sufficient to bring you back down to earth. The escape velocity is the break point between these two alternatives.
  The escape velocity is of course dependent upon the distance from the earth (or indeed any large body), diminshing as you travel away. Therefore if you started the earth's surface and travelled upwards at the escape velocity, although your velocity would diminish due to gravity it would still remain at what would be defined as the escape velocity.
  You could of course escape from earth's gravity if you could continuously move at even a very low speed. The problem is maintaining this speed against the pull of gravity. To do so you would need to introduce some other force, at which point the concept of escape velocity is no longer applicable.

  Paul Hoad, Sutton, UK

• At 11km/s you can successfully break orbit and escape the gravitational pull of the Earth. At 10km/s the Earth will eventually slow down your ascent till you begin falling back towards the ground. These values are at ground level. Once in orbit the escape velocity is lower than 11km/s. The greater the planet mass the greater the gravitational pull. To take off from the surface of MASSIVE Jupiter a rocket would need to be travelling at around 40 miles per second!! To escape the sun it is around 400 miles per second! To escape a black hole you would have to travel beyond 300,000km/s - faster than the speed of light - which is impossible. That is why light cannot even escape from the surface!

  Paul Steele, Marske, UK

• Escape velocity is an energy thing. If you're going at escape velocity you don't need any more energy to escape from the Earth, because your kinetic energy is already enough (assuming you don't lose it; for example air drag).But you can go more slowly if you either spend more energy, or if you go ballistic from higher altitude.

  Ian Woollard, UK

• So how do you explain a helium filled balloon with no thrust and no persistent velocity, travelling at minimal speeds and still being able to leave Earth gravitational pull, assuming no gas is lost and the balloon remains in tact? 'Floating' into space would not require great thrust or huge amount of fuel but requires a helium to rocket ratio of at least 1:000000000001 recurring

  Thomas, Croydon England

• If we are talking about the planet of earth, the escape velocity is the speed primary needed to start a shell at the top, in this case must be the top speed of 11 km / s to end up outside the Earth's atmosphere, and therefore we can say that it is the speed of liberalization of gravity.

  Abdulah . I . Meftah, Farasan Islands, Saudi Arabia

• You're all wrong. The technology exists and has been used to escape earth's atmosphere at a much slower speed. However, the technology is highly classified and is only available to a select few.

  Drewey Whittaker, Albany Ga USA

• As per everyone (aside from the Area 51 theorist at the end), there's no reason why you can't provided your acceleration is >9.8 m/s then you will travel away from the Earth for as long as the fuel lasts. One point nobody has made is that gravity is acceleration and to beat it you need opposite acceleration (something your chair is doing at them moment), so you would be accelerating away from earth. The closest Lagrange point (areas of flat space-time) of the moon would be the obvious target as you wouldn't then be pulled back to earth - because otherwise Earth's gravity will just take too long to reduce sufficiently - and anyway, you'd be stuck in deep space. As the fuel weighs a considerable amount, and needs sizable containers, there are efficiency reasons for burning it all as quickly as possible while dumping spent containers en route - standard rocket style. That said, a slow rocket would need to provide constant >10m/s thrust for the 300,000km or so it takes to get to your target L1. If your acceleration is a measly 12m/s (2.2m/s net acceleration), then even though L1 is 90% of the distance to the moon, by the halfway point you probably would have sufficient momentum to coast to L1. Even at 100km distant, gravity is only reduced by about 3% so it's not a major consideration. However, your rocket wouldn't have to endure such huge tolerances, and I guess the structure could therefore be much lighter. Ok, now I might be making Mr Area 51 sound sensible but I'm beginning to see traction in a variant of the idea... Perhaps someone could correct me, but if a rocket accelerated directly up (not orbital acceleration as usual - I mean it's the same Newton requirement after all), burnt fuel reasonably quickly but never reaching anything like 25K, only enough to drift to a near stop at say L4, then couldn't we slingshot it on L4 (is it possible to slingshot a convex Lagrange? Surely it is.), slingshot the moon and then Earth, could escape velocity be reached more efficiently that way? Or say if you're going to the moon, why provide more energy than the journey requires by insisting on reaching EV with a orbital route? Am I missing something here?

  David, Peterborough, United Kingdom

• This is a big question - Can a Space vehicle take say three weeks to get to the Moon but travelling at a far slower speed. What is the fuel required to do that and would it be possible ? The high velocity used at the moment is get into orbit around the earth however if you wanted to travel to the moon couldn't you do it slower but over a longer period of time ? Obviously if you didnt go into orbit around the moon you would have to be able to slow down so that you wouldn't crash into it. It does seem that if you had more time you could do things at slower pace

  Roy, Cape Town South Africa