Physics students: I am confused by escape velocity.

[FunkyTown]

As the title says, I am confused by escape velocity and want to understand it.

What would happen with the following scenario:

1) Bob the Skinny Giant weighs 2 pounds on earth, but is strong enough and tall enough to lift a rocket to the moon. He lifts at a speed of 10 feet/minute, so it will take Bob some time to lift it.

What happens when Bob tries? Why doesn't that work?

If it does work, how is that different if we developed a propulsion technique that applied even pressure throughout the journey rather than a slingshot effect?

[NeuroTypical]

I have to admit, I didn't exactly wake up this morning expecting to think about this. But now that I am, is someone telling me the cool space elevator I keep hearing about won't work for some reason?

[slamjet]

As my father always told me "word problems are useless." But I'm sure we can help with escape velocity when it comes to running away from in-laws.

[MrShorty]

I'm not a physics student, but here's one possible way to look at it.

We talk about escape velocity with a rocket because the rocket engine doesn't burn all the way to the moon. The goal with a rocket engine is to provide enough velocity in the initial burn(s) so that inertia will carry the rocket away from earth. With a rocket engine, you don't usually apply a constant force for the entire journey to the moon.

In the case of a giant, the giant can apply a constant force all the way to the moon. In this case, he applies a continous force to the object for the entire journey.

[Guest]

As the title says, I am confused by escape velocity and want to understand it.

What would happen with the following scenario:

1) Bob the Skinny Giant weighs 2 pounds on earth, but is strong enough and tall enough to lift a rocket to the moon. He lifts at a speed of 10 feet/minute, so it will take Bob some time to lift it.

What happens when Bob tries? Why doesn't that work?

If it does work, how is that different if we developed a propulsion technique that applied even pressure throughout the journey rather than a slingshot effect?

The thing about Bob lifting the rocket is that 10 feet/minute may mean different kinetic energy required to maintain that velocity within the distance between the earth and the moon. Because the earth and the moon's gravity act on that rocket throughout the distance. But, if he can apply pressure to counteract gravity to maintain that speed all throughout the distance, then there's no reason why it wouldn't work.

Why NASA doesn't do that to a rocket is because NASA wants to use the LEAST kinetic energy to launch the rockets to a target. The reason why we don't all fly out to space is because of gravity so that it requires kinetic energy to counter gravitational force so that we can fly off (ok, it's a lot more complicated than that of course - there's friction, etc.)

Hence, escape velocity - stated plainly as the velocity at which kinetic and gravitational energy acting on an object is zero. So that, once an object is pointed at a specific direction and it achieves that velocity, it will maintain that speed in that direction the entire way without the need to apply more energy (which requires fuel) until it hits a different gravitational force.

[mordorbund]

I'm not a physics student, but here's one possible way to look at it.

We talk about escape velocity with a rocket because the rocket engine doesn't burn all the way to the moon. The goal with a rocket engine is to provide enough velocity in the initial burn(s) so that inertia will carry the rocket away from earth. With a rocket engine, you don't usually apply a constant force for the entire journey to the moon.

In the case of a giant, the giant can apply a constant force all the way to the moon. In this case, he applies a continous force to the object for the entire journey.

Having the giant apply a constant force is different from the problem presented. The giant is delivering variable force that exactly counteracts the gravitational force.

F = GMm/R^2

Note that everything here is constant except R, so the giant's force will decrease as the rocket moves away from the earth (maintaining a constant velocity).

[NeuroTypical]

So the studly space elevator will work then. Whew!

[bcguy]

There was the space elevator competition in sorcoro nm this past summer. Very fascinating seeing a object walk up a tether to a hovering helicopter at some height.

[CopenKagan]

So the studly space elevator will work then. Whew!

In theory. But when you take into account high speed wind, possible meteor strikes into the cable, etc. That's when things get dicey.

The one that I can't get around for space and physics is orbit velocity of the Earth (or any object floating around in space really). Everything has it's own unique orbit speed based on elevation. For example, the shuttle orbits the Earth at ~17,600mph. The easiest way to think of an orbit is an object in perpetual free fall. HOWEVER, since there is virtually no air resistance while in orbit, in theory, the object orbiting the Earth technically doesn't have a terminal velocity as there is nothing pushing back against it. Going by my logic, the orbiter would have to travel faster and faster to compensate for the ever increasing fall speed of the orbiter.

That being said, wouldn't the Earth continue pulling on it at a faster and faster rate, since it technically doesn't have a terminal velocity in its free fall? There is obviously something I'm overlooking here, but it has been bugging me for the past few weeks.

[slamjet]

In theory. But when you take into account high speed wind, possible meteor strikes into the cable, etc. That's when things get dicey.

The one that I can't get around for space and physics is orbit velocity of the Earth (or any object floating around in space really). Everything has it's own unique orbit speed based on elevation. For example, the shuttle orbits the Earth at ~17,600mph. The easiest way to think of an orbit is an object in perpetual free fall. HOWEVER, since there is virtually no air resistance while in orbit, in theory, the object orbiting the Earth technically doesn't have a terminal velocity as there is nothing pushing back against it. Going by my logic, the orbiter would have to travel faster and faster to compensate for the ever increasing fall speed of the orbiter.

That being said, wouldn't the Earth continue pulling on it at a faster and faster rate, since it technically doesn't have a terminal velocity in its free fall? There is obviously something I'm overlooking here, but it has been bugging me for the past few weeks.

My brain just went BOOM.

[Saintmichaeldefendthem1]

In theory. But when you take into account high speed wind, possible meteor strikes into the cable, etc. That's when things get dicey.

The one that I can't get around for space and physics is orbit velocity of the Earth (or any object floating around in space really). Everything has it's own unique orbit speed based on elevation. For example, the shuttle orbits the Earth at ~17,600mph. The easiest way to think of an orbit is an object in perpetual free fall. HOWEVER, since there is virtually no air resistance while in orbit, in theory, the object orbiting the Earth technically doesn't have a terminal velocity as there is nothing pushing back against it. Going by my logic, the orbiter would have to travel faster and faster to compensate for the ever increasing fall speed of the orbiter.

That being said, wouldn't the Earth continue pulling on it at a faster and faster rate, since it technically doesn't have a terminal velocity in its free fall? There is obviously something I'm overlooking here, but it has been bugging me for the past few weeks.

The best way to get a visual on this is to picture a gravity well. The gravity wells at science and exploration facilities allow you to toss a coin or a ball that rotates around the hole in the center in ever tightening circles until it can no longer escape the downward grade and it sinks into the hole. You're right that satellites cannot maintain the same orbit and when scientists talk about orbital decay, they are referring to satellites that have circled the earth getting closer and closer to it until it succumbs to gravity and falls into the atmosphere. Certain satellites that have to maintain a set distance, like geostationary satellites are often equipped with thrusters that fire periodically to maintain their distance.

Now this doesn't apply to objects that are further out in their orbit. The moon, for instance, has an increasing orbit and eventually, the moon will spin free of earth and become an independent planet, albeit orbiting the Sun. This is what scientists believe happened to Pluto, a former moon of Neptune.

Another way to understand escape velocity is to factor in the limitations on fuel. How fast does a rocket have to travel to achieve orbit without running out of fuel? That's escape velocity....roughly 2 times the speed of a bullet by the time it reaches the upper atmosphere.

[Guest]

In theory. But when you take into account high speed wind, possible meteor strikes into the cable, etc. That's when things get dicey.

The one that I can't get around for space and physics is orbit velocity of the Earth (or any object floating around in space really). Everything has it's own unique orbit speed based on elevation. For example, the shuttle orbits the Earth at ~17,600mph. The easiest way to think of an orbit is an object in perpetual free fall. HOWEVER, since there is virtually no air resistance while in orbit, in theory, the object orbiting the Earth technically doesn't have a terminal velocity as there is nothing pushing back against it. Going by my logic, the orbiter would have to travel faster and faster to compensate for the ever increasing fall speed of the orbiter.

That being said, wouldn't the Earth continue pulling on it at a faster and faster rate, since it technically doesn't have a terminal velocity in its free fall? There is obviously something I'm overlooking here, but it has been bugging me for the past few weeks.

Gravitational potential energy is not constant across outer space because each body has gravitational force. Besides the moon, there's also the sun. The larger the body, the larger the force, therefore, as you get farther from earth the closer/farther you get from the sun's gravitational pull. These bodies are in perpetual motion. So, an object achieving escape velocity at a specific point in outer space does not escape at a straight line... It goes on a parabolic orbit according to it's position relative to the center of gravity of 2 bodies.

[rameumptom]

One issue that must be considered is that as Skinny stretches further out into space, he is impacting the gravity between himself, the moon and earth. He essentially becomes a fulcrum in lifting the cargo to the moon. The question would be is the continually increasing pressure that he places on the earth to overcome the gravitational effects, also greater than the mass of the earth? If so, then the earth will in effect shift away from the moon, preventing him from ever being able to deliver the rocket to the moon.

Meanwhile, the earth is pushed a bit closer to the Sun, potentially changing its orbit sufficiently to cause true global warming, and searing the earth and killing all inhabitants, including Skinny.

Alternatively, he pushes the earth away from the Sun, and we get global cooling, setting us into a major Ice Age, and wiping out most plant and animal life on earth.

So Funky, thanks to your word problem, you have now set in place our earth's destruction. Thanks. Thanks alot!

[Traveler]

If a giant is lifting an object he is acting on it with a force. As the giant sets the object on the moon the force of the moon is added to the force applied by the giant. To get the object to the moon the giant had to apply so much force (work) to get it there.

If we add up all the force needed to get the object to the moon - the force is the same if we fired the object from a gun with the same force. If firing the object from a gun the object would reach a speed leaving the barrel of the gun at the speed of what is called escape velocity.

If an object does not have enough force behind it to overcome gravity it will eventually reach a peak altitude and return to the earth.

The Traveler

[CopenKagan]

This thread is relevant to my interests.

[Guest leodown]

This research shows the sense of new york asian escorts euphoria and the endorphin rush women get when they achieve their goal of fitting back into that one special new york asian escort pair of jeans is even better or on a par with some of the other great pleasures in life

[Blackmarch]

As the title says, I am confused by escape velocity and want to understand it.

What would happen with the following scenario:

1) Bob the Skinny Giant weighs 2 pounds on earth, but is strong enough and tall enough to lift a rocket to the moon. He lifts at a speed of 10 feet/minute, so it will take Bob some time to lift it.

What happens when Bob tries? Why doesn't that work?

If it does work, how is that different if we developed a propulsion technique that applied even pressure throughout the journey rather than a slingshot effect?

because you are leaving an important part of the equation out-

you have to be able to lift (or force) a certain amount of weight at distance/time... and currently we dont have any technology in use or available that can provide a constant thrust (or force) that is reasonably economical for getting from the earth to the moon in one go for a given weight and volume. (but we are trying to find one trust me, plenty of researcheres that are working on that). IE the stuff that we have that has the ability to push the weights we want to space and beyond do not have the life span to do 10ft/min in our gravity and atmosphere, so to use what have effectively we try to get them to a spot that counteracts earths gravity, as well as avoid the atmosphere as fast as possible.

Now once you don't have to fight to overcome earths gravity, we do have a "constant" thrust technology- ion engines... however their output is very weak so the acceleration is very very low... so low infact that they arent economocal to get to the moon, but if you wanted to get to somewhere like pluto quicker than your normal gravity assist routes, then ion engines become much more appealing.

Research is also going into solar sailing, I believe japan either just recently or in the near future will be launching a satellite to test some materials for solar sail purposes.

as for escape velocity, it can basically be thought like this: how fast you need to throw something straight up so that it won't come back down (not counting the atmosphere)

Edited January 19, 2011 by Blackmarch

[CopenKagan]

because you are leaving an important part of the equation out-

you have to be able to lift (or force) a certain amount of weight at distance/time... and currently we dont have any technology in use or available that can provide a constant thrust (or force) that is reasonably economical for getting from the earth to the moon in one go for a given weight and volume. (but we are trying to find one trust me, plenty of researcheres that are working on that). IE the stuff that we have that has the ability to push the weights we want to space and beyond do not have the life span to do 10ft/min in our gravity and atmosphere, so to use what have effectively we try to get them to a spot that counteracts earths gravity, as well as avoid the atmosphere as fast as possible.

Now once you don't have to fight to overcome earths gravity, we do have a "constant" thrust technology- ion engines... however their output is very weak so the acceleration is very very low... so low infact that they arent economocal to get to the moon, but if you wanted to get to somewhere like pluto quicker than your normal gravity assist routes, then ion engines become much more appealing.

Research is also going into solar sailing, I believe japan either just recently or in the near future will be launching a satellite to test some materials for solar sail purposes.

as for escape velocity, it can basically be thought like this: how fast you need to throw something straight up so that it won't come back down (not counting the atmosphere)

There are also plasma and nuclear thruster designs. In theory, an ion thruster could get you just shy of the speed of light if you gave it enough time to accelerate.

Scientists think that they will be able to nail down nuclear fusion in the next few decades. Once we achieve that, space travel will be much more economic. Even beyond that, there is antimatter propulsion, which has the highest known amount of energy. Pound for pound, antimatter is about 10,000,000,000 times as powerful as conventional oxygen/hydrogen chemical boosters that we use today, and 300 times as powerful as a fusion thruster. I think that we will be planning a flight to Alpha Centauri A and B as well as Proxima Centauri within the next 50 years.

This thread has been officially hi-jacked.

Edited January 20, 2011 by CopenKagan

[Jamie123]

LOL - this problem was addressed quite correctly in the 1963 comedy movie Mouse on the Moon, sequel to the famous Sellers movie The Mouse that Roared about the tiny wine-producing nation of "Grand Fenwick" that declares war on the USA. In Mouse on the Moon (which sadly does not star Peter Sellers) Grand Fenwick intends to beat the USA and Russia to the moon using a space vehicle powered by Grand Fenwick wine. Before the launch, skeptical visiting scientists inspect the space-ship:

Visiting Scientist: (Pointing to what appear to be shower-heads attached to the side of the rocket) Surely these will tear off long before you reach escape velocity.

Grand Fenwick Scientist: We're not going to reach escape velocity.

Visiting Scientist: (Perplexed) Well if you don't reach escape velocity you won't escape the Earth's gravity!

Grand Fenwick Scientist: Not correct! If you have enough energy to escape you can travel at any speed you like!

[Blackmarch]

There are also plasma and nuclear thruster designs. In theory, an ion thruster could get you just shy of the speed of light if you gave it enough time to accelerate.

Scientists think that they will be able to nail down nuclear fusion in the next few decades. Once we achieve that, space travel will be much more economic. Even beyond that, there is antimatter propulsion, which has the highest known amount of energy. Pound for pound, antimatter is about 10,000,000,000 times as powerful as conventional oxygen/hydrogen chemical boosters that we use today, and 300 times as powerful as a fusion thruster. I think that we will be planning a flight to Alpha Centauri A and B as well as Proxima Centauri within the next 50 years.

This thread has been officially hi-jacked.

these are all huge potentials...

which is why i mentioned economically, and current tech-

rigth now it's not econonomical to use nukes for propulsion, and we don' yet have the tech to create a continious fusion reaction.

At one point the US Military did have a plan underway to create a nuclear powered rocket... while it was cancelled long before anything got into production al the research that went into making nukes more efficient so they could be more economical so they can be carried as fuel jumped our military nuke capability ahead quite a bit.

The big reaons for not using nukes is 1) all the treaties we have with other nations that regulate what you put into space.... and 2) do you really want to be detonating nukes near earth?

Antimatter would be super, but currently our hurdles with anti-matter is 1) producing it in quantity, and 2) containing it... right now the only things that make it are the particle colliders, and its only a few particles at a time, and its not exactly cheap to run those things and maintain them.

Another form of propulsion that's being researched is laser assisted propulsion, where you use a laser direct energy to the object that is to be lifted, that then has a special reflective surface that focuses the light at a point so that it superheats the atmosphere just below it and it gets pushed by the expanding gas.

Edited January 20, 2011 by Blackmarch