I don't understand the concept of "escape velocity".?

Question

I do undertsand it in relation to fuel and stuff, but if fuel isn't an issue, why couldn't a spacecraft inch its way off earth at like 20 m.p.h, all the way up to the moon? Sure, it would take a long time, but if it had an infinite fuel source, why would it need to reach a certain velocity? If it just kept going up at 20 m.p.h. eventually it would be in space, right? WHy would it need to reach some "escape velocity"? I don't undertsand this at all.

Answer 1

Oh but Jason, it does apply to 20 mph. It's just a matter of where.;)
Lisa, imagine you shoot a bullet straight up. Disregard atmospheric effects. The earth's gravity will slow it down. But the farther away it gets from the earth, the less the earth's gravity pulls on it. If it is going slower than about 7miles per second (mps) then it will eventually stop and fall back. If it is going faster than 7 mps, it will slow down, theoretically forever, but never enough to stop and fall back. That's what an escape velocity is. Escape velocity from Jupiter is about 37 mps. The moon's escape velocity is about 1.5 mps.

Note, it is usually considered from the surface you are taking off from, but it can be determined at any distance. For example, the earth's escape velocity from the sun is greater than Saturn's because the sun's gravity is stronger at earth's orbit than it is at Saturn's.

Now what about your 20 mph space ship? Disregard concerns about the fuel. That is irrelevant to your hypothetical space ship. Yes, it will keep going. If it maintains 20 mph indefinitely, it will keep going indefinitely. There will actually be some point, very far away from the earth, where you could turn off the engines and continue to drift forever, never to fall back to the earth. That would be the distance at which your escape velocity from the earth has gone all the way down to 20 mph.

Answer 2

You are right, a space ship could get to the Moon slowly. You are also right that it would need infinite fuel. Escape velocity is the speed at which something has to be thrown so that WITHOUT ANY MORE POWER being used, it will never fall back again. Rockets only have enough fuel to burn the main engines for a few minutes, total. So during the 3 day trip the Moon, they are just coasting with no power at all most of the time. Space flight isn't really flight in the same sense that an airplane flight is. Space flight is more like space throwing. The space craft is thrown off the Earth by a 10 minutes blast from a rocket. The moment the rocket engine stops burning, the space craft starts slowing down because Earth's gravity is still pulling it down. But gravity gets weaker with distance. If you are thrown fast enough, then you escape to a greater distance where gravity isn't slowing you down so much before you have slowed down too much. You need calculus to calculate the speed at any given distance, because you are slowing down all the time, but at each second you are slowing by a little less than the second before because you are farther away where gravity is weaker. A rocket that leaves Earth at 25,000 MPH is only going about 5,000 MPH by the time it gets near the Moon. It is like throwing a ball to a person on the roof of a house. If you throw hard enough, the ball leaves your hand very fast, slows down all the time it is in flight, and is going very slow when the guy on the roof catches it. If it is a 2 story house, you have to throw faster to make it go higher. If you throw it really fast, at escape velocity, then it would go higher and higher, and slower and slower, forever. At very great distances, it is still slowing down, but just barely. It might take a year to slow down by 1 MPH if you were far enough away, but you would still be slowing down all the time. But if you left with enough speed in the first place, the slowing slows fast enough that you never stop. Sounds strange, forever slowing but never slowing all the way to a stop. This is the concept of converging to a limit in calculus.

Answer 3

What Gene said is correct. It would take an enormous amount of fuel. Since there is no such thing as an infinite fuel source (yet), at some point your engine would shut down from lack of fuel. If at that time you've attained about 17,500 MPH, you'll be in orbit. But if not, which is the case if you're only going 20 MPH, you will fall back to Earth because you haven't attained sufficient velocity to counter the pull of Earth's gravity. And then everyone will laugh at you.

Answer 4

The ampunt of fuel required to go up at a constant 20 mph velocity would be unbelievable. You'd probably have to burn for quite a few hours. Escape velocity only comes in to play when you shut down the engine after a quick burn and then coast

Answer 5

Simply put, escape velocity refers to the speed needed for a set period of time to achieve a speed needed where an object can travel without propultion and maintain such a course that the object overcomes the gravitational field of the object it is leaving.

Now a bit longer if you wish to read on.

V = s/t Escape velocity(V) takes into account time (t) and speed (s).

Unlimited time suggests unlimited fuel. If that is the case why not one inch per day? All that is described here is speed though s = D/t distance per frame of time. If we had unlimited resources why not indeed, although it would be horribly in efficient, especially with the payloads we bring up there and the price of gas today... it is far better to get something going fast enough for a short period of time to eventually "coast" the rest of the way... kind of like putting your bike into a low power gear on level ground and giving a few good pumps versus putting it in a high gear and peddling 8 times to spin the wheel once with hardly enough power to keep you upright and balanced let alone coasting.

Velocity refers to Newtonian Physics specifically your question refers to the first two laws,

1) An object at rest will remain at rest unless acted upon by an external and unbalanced force. An object in motion will remain in motion unless acted upon by an external and unbalanced force.

2)The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction.

I might be getting more detailed than you need at this point. I don't want to blabber too much so I will leave it at that. you can e-mail me if you want me to go on.

Answer 6

Let me put it to you simply. You've heard the old expression "What goes up must come down" right? Well, thats not really true. The escape velocity is how fast you need to throw something up so that it will never come back down. It will escape to infinity. In the case of a rocket, once it reaches escape velocity, it can stop burning fuel and just coast into space.

Answer 7

You are correct in that escape velocity as a concept does not apply to your example of an object travelling at a constant 20mph. However, the simple fact is that there is no way to achieve a constant 20mph against the gravitational pull of Earth in reality because fuel IS an issue, so deep space probes must be hurled out of Earth's gravitational field by giving them huge acceleration to start with so they can coast the rest of the way.

Answer 8

Because our atmosphere and gravity would stop us if going too slow. Escape velocity is the speed something needs to obtain in order to leave the planet

Answer 9

It's because the Earth is turning. To maintain 20mph straight up you have to overcome being whizzed around in orbit at up to 25,000mph.

Answer 10

Don't you worry your pretty little head over it. Just get back in the kitchen and make me a quesadilla and get me another beer.