Can someone explain this to me?

I wonder why it is impossible for something which goes up at a steady speed of say 1kmph to break away from earth's gravity. Im sure it is impossible but why?

Why would it be impossible? A steady speed of 1km/h or even 1cm/h would be able to eventually leave Earth. The problem lies in the amount of thrust and fuel neccesary to maintain that speed steadily until Earth's gavitational pull was broken. At that speed, you'd either need an incredibly plentiful, but light fuel source, because it would have to last for a period of days or weeks. The reason our current spacefaring vehicles go faster is because both fuel and craft are so heavy that you need a fuel that burns hot enough and fast enough to get it up quickly, before you run out of fuel.

Hi tomcat,

You can make a vertical launch escape from earth if you can achieve and maintain a velocity of 40,248 km/hr. This is the escape velocity for earth. But this is the velocity you must MAINTAIN...which means you must be accelerating (because gravity is decreasing your velocity). So you must be accelerating at a rate that maintains a vertical speed of 40,248 km/hr (approximately Mach 25).

Otherwise you'll never get out.

Your kinetic energy must be equal to or exceed your potential energy.

Yup - what Valhall said.

Escape velocity is needed for us mortals to "break" away from our dirtball....

Actually if you want to have fun with this, there is a freebie simulation/game on the net called Orbiter that lets you experience this stuff in simulated crafts with all the orbital stuff in there as well.... You can even do Hohman transfer orbit stuff to move between bodies in the sloar system - it's actually quite nifty....

[edit on 30-6-2005 by UofCinLA]

So you cannot get away with a steady engine at 1 kmph because the gravity pulls stronger with increasing hight? I thought it would decrease?

That explanation is bogus. Escape velocity is the initial velocity (directed upward) needed to escape the earth's gravitational pull if no additional force is applied (i.e. no acceleration from rockets).

If I'm only moving up at 1 millimeter/YEAR, I would still escape the earth's gravity eventually, but I would use up a whole lot of energy accelerating against the gravitational force to maintain this velocity. That's basically why we use huge rockets (to minimize the time at which the stronger gravitational force at lower elevations must be countered by additional acceleration).

Originally posted by tomcat ha
So you cannot get away with a steady engine at 1 kmph because the gravity pulls stronger with increasing hight? I thought it would decrease?

It does decrease, but it's still there. But let's just take the first second.

If you take off at a constant velocity of 1 km/hr (which is .278 m/s), and you have a gravitational deceleration of 9.8 m/s^2, in 1 second your velocity is going to be a -9.5 m/s. Pretending you took off from a platform that allowed you to go negative for 1 second longer than you were able to go up, in 2 seconds your velocity would be -19.3 m/s....and so forth and so on. And you've got to travel out quite a distance before gravity has decreased all that much. For instance, if you manage to move 1 Earth radius away from the surface of the Earth (that is get to an altitude of 6378 km above the surface of the Earth) you still have 1/4 of the gravitational force pulling you back. So you would still be fighting a deceleration of 2.45 m/s^2.

Originally posted by utrex
That explanation is bogus. Escape velocity is the initial velocity (directed upward) needed to escape the earth's gravitational pull if no additional force is applied (i.e. no acceleration from rockets).

If I'm only moving up at 1 millimeter/YEAR, I would still escape the earth's gravity eventually, but I would use up a whole lot of energy accelerating against the gravitational force to maintain this velocity. That's basically why we use huge rockets (to minimize the time at which the stronger gravitational force at lower elevations must be countered by additional acceleration).

That's not what he asked, and that's not what I said.

I wonder why it is impossible for something which goes up at a steady speed of say 1kmph to break away from earth's gravity. Im sure it is impossible but why?

Anything moving away from the earth at any positive constant velocity will eventually be greater than any distance away from the earth (ignore possible bounds due to the size of the universe).

I think I'm going to have to go with utrex on this one. While Val's explanation does make sense to a degree, in at least my interpretation it's under the assumption that an object accellerates to a speed of 1km/hr and then all the force being applied to that object is removed. Basically, if I jump up and my velocity peaks at 1 km/hr, then yes, I will fall back down along the scenario that Valhall sets forth.

It seems to me--and correct me if I'm wrong--the confusion in that particular scenario comes in at assuming that your 1km/hr is initially outside of the forces of gravity. That's how it appears to me anyways, and in that situation also the scenario is valid. But if one is already under the force of gravity, as in an object launched from the earth, it must overcome that initial -9 m/s^2 deceleration in order to hit it's initial velocity, and it needs to be constantly accelerating to maintain its target velocity. Trying to break out of the earth's gravitational pull at 1 km/hr is definitely not a reasonable task to think of--the energy needed for it with modern rockets would be unthinkable--but theoretically it is possible.

As everyone else has said: if you're moving at a constant 1km/hr, then you will eventually leave the earth's gravitational field. It's just maintaining that constant velocity that's the hard part.

People, "thelibra" post is correct 100%

I swear by all the thousands of hours I spent studying classical mechanics.

Originally posted by utrex
That explanation is bogus. Escape velocity is the initial velocity (directed upward) needed to escape the earth's gravitational pull if no additional force is applied (i.e. no acceleration from rockets).

You are correct. This is where you were gigging me. You are correct. Escape velocity is the initial velocity from the surface of the (fill in the planet name). So, you would not need to maintain the velocity (i.e. you would not need to accelerate) but initiate at this speed from the surface to overcome the gravity throughout the distance to break free of the gravity.

It's known as escape velocity. Google that up and ye shall find the answers.

Hi,

I am an Aerospace Engineer and work for BAE SYSTEMS (I know it's in caps, I'm not shouting, it's just our marketing department spent millions changing the logo to caps so I am bloomin determined to use it!) so I figured this is one for me!

It is totally possible to escape earths gravity by ascending at ANY speed. Utrex is correct in his definition of escape velocity, it is the velocity that must be acheived initially to escape earths field without additional thrust.

The reason that we use high speed rockets is (as utrex said) the optimal combination of speed, weight, and fuel considerations.

Think about space elevators, or maybe even building a giant staircase all the way to space, you keep walking and you get there! Maybe dead, but you'd get there! :-)

Utrex, you are correct!

Originally posted by robbie414

Think about space elevators, or maybe even building a giant staircase all the way to space, you keep walking and you get there! Maybe dead, but you'd get there! :-)

Utrex, you are correct!

But this is totally different. In this manner you retain the potential energy you have gained as you elevate yourself further. Sure, we can all start climbing our ladders at whatever rate we want...but that's not the same as attempting to increase your altitude with no means other than propulsion that must not only combat the effects of gravity, but continue to propel you upward.

I dare you to retain your potential energy in mid-air without your big engine. I also want pictures.

Your kinetic energy must be greater than your potential energy in order to escape the earth's gravity.