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It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. Well, as far as I understand, you could easily escape Earth's gravity even at 1 mph 0. Is it because the object has to gain a certain speed once it reaches orbit in order to maintain that altitude? Or is it because practically an object can't carry infinite amount of fuel, and so it has to reach a certain speed to maintain its orbit before all fuel is gone?
The force of gravity decreases with distance. It follows an inverse-square relationship The fact that gravity decreases with distance means that at some distance, it will be negligible; an object sufficiently distant from Earth may be considered to have "escaped" Earth's gravity.
In reality, the force of gravity has no distance limit; two objects would have to be at infinite distance from each other to have no gravitational interaction, but for practical purposes, one can think of finite distances where gravitational forces become small enough to ignore. Consider an object some large distance from Earth Some tiny movement toward Earth will increase the gravitational attraction, accelerating the object toward Earth. The process will escalate with the object's velocity and acceleration increasing.
If we ignore the effects of Earth's atmosphere, the object will continue its acceleration until it strikes the Earth's surface at some velocity. Now, let's reverse everything. The object magically launches up from Earth's surface at exactly the same speed as our falling object had at the instant of impact. As it rises up, gravity tugs on it and it slows down.
As it gets further away, gravity diminishes so it decelerates more slowly. Eventually, it gets to some distance where it has come to a stop, but Earth's gravity no longer has any effect on it. The velocity our object had at Earth's surface is Earth's escape velocity. In precise terms, a body's escape velocity is the velocity an object in "free fall" must have in order to escape the gravitational influence of that body - no more and no less.
Technically, escape velocity can be specified for any distance from the center of a body, and the value will decrease with distance, but when a planet's escape velocity is stated, it is usually for the planet's surface. Mathematically, it is calculated as an integral of the body's gravitational acceleration from some specified distance to infinity.
An object does not have to travel at escape velocity to escape a planet's gravity, but the same amount of energy needed to accelerate an object to escape velocity must be applied to an object giving it potential energy to lift it out of the planet's gravitational sphere of influence.
The difference is that at escape velocity, the object needs no external influence to escape; at anything less than escape velocity, some external force must be applied. Escape velocity reduces as you get further away from the Earth. If you proceed upwards at a constant speed of 1 mph which as noted will require continuous thrust to counteract gravity , you will eventually reach a distance where the escape velocity is equal to 1 mph.
Then, you will have reached escape velocity and are no longer gravitationally bound to the Earth. In practice, third-body effects moon, sun, other planets will dominate when you get beyond 10 5 km away from the Earth. That is, if you are AU from Earth, you don't need any more fuel to counteract Earth's gravity, you just float away. However, when at Earth's surface, you will need additional acceleration to sustain the 1mph velocity - otherwise you just fall back down like the tossed ball.
You are confusing velocity and acceleration. If you have a high enough velocity, the effect of de acceleration can not slow you down before you get far enough away from the gravitational source. So if you could keep a constant velocity of 1 mph, you would defiantly be able to escape the earth. The problem is that would require constant thrust. The escape velocity is only for objects thrown projected into space , with the initial velocity and they are not powered.
Escape velocity is the speed at which you'll leave the Earth and not return if you don't continue to propel your craft. Below that speed, gravity will pull you back down. XKCD's got one of the more accessible explanations. The key difference is that "escape velocity" is how fast you would have to throw a stone straight up from the Earth's surface ignoring air drag , for it to escape from Earth's gravitational influence.
It would be coasting the whole way, always losing speed due to Earth's gravitational pull. If, on the other hand, you have a rocket engine with sufficient fuel, you can just keep rising slowly 1 mph , which is almost a hover, until you've gotten way out into space and Earth's gravity is overwhelmed by the Sun, Jupiter, etc. You could keep throttling back to maintain the same upward speed gravity decreases with distance, and the rocket carries less fuel if you wanted to, or let the rocket speed up.
Unless you are very far away from Earth, if you are only moving away at 1 mph the gravity of Earth will pull you back to Earth assuming you do not have an infinite fuel supply to maintain a 1mph thrust.
So you are correct when you say. Is it because the object has to gain a certain speed once it reaches orbit in order to maintain that altitude. Think of a ball tossed in the air, it starts by moving quickly, but as it rises higher it goes slower, than stops and falls back down.
At some point it is moving away from Earth at 1mph, but gravity overcomes that momentum. Air Resistance has some impact on the ball, but you can throw horizontally much farther than you can up. Gravity works pretty much them same on the surface of the Earth as it does a miles up. When you throw something horizontally it falls towards the earth in an arc, attracted by the gravity of the Earth. The weight of the fuel would make the spaceship so heavy it would be hard to blast it off of Earth!
What is gravity? How do we put a spacecraft into orbit? Once a ship is in orbit, do we have to do anything to keep it there? How did DS1 get into space? Could NASA use ion propulsion to put a ship into space? Don't hold your breath. 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!
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. These values are at ground level. The greater the planet mass the greater the gravitational pull.
To escape the sun it is around miles per second! That is why light cannot even escape from the surface! 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?
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. 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. 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. 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.
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