"Escape Velocity"

[danbaron]

As time passes I become more and more convinced that many people who communicate for their livelihoods do not know the meanings of a lot of the words and phrases which they use. They act as though even kindergarteners know these meanings, and they never explain them - the reason being, in my opinion, because they are pretending, because they themselves do not know.

I think that one of these phrases is, "escape velocity". I hear it used quite often when communicators are talking about science. I don't know for sure what it means. I have never looked at the definition, but, I have an idea, which I bet most communicators are ignorant of, but will never admit. (Of course I could be wrong about both of my speculations, that most media people who use the phrase publicly don't know what it means, and that I do know.)

Anyway, here is my idea of its meaning.

Say you were standing on the surface of a planet isolated in outer space. Say that the planet had no atmosphere. Say that you had a baseball and threw it vertically (perpendicular to the surface) into the "sky". As soon as it left your hand, it would begin to decelerate, because, the only force acting upon it would be the planet's gravitational force. The ball would continue decelerating, until, most likely, it instantaneously became motionless above the planet, and then fell back to the surface.

The faster it was going when it left your hand, the higher it would rise, before falling back to the planet. It turns out that for the planet's particular gravitational force, there is a (minimum) velocity which if you could throw the ball with, then, the ball would never return to the planet's surface. That velocity is, in my opinion, the planet's escape velocity.

So, I think that "escape velocity" means, the minimal velocity at which an object shot ("shot", meaning that once in flight the object has no propulsive force) from an "attractor's" (planet's, moon's, asteroids's, rock's, etc.) surface, does not fall back to the surface. Another example would be shooting a projectile vertically from a cannon - once the projectile leaves the muzzle, it is no longer being propelled upward.

And, I think there are some technicalities associated with the phrase. One is that the escape velocity only considers gravity as the retarding force. That is why above I used the example of a planet with no atmosphere. I think that, for instance, when the escape velocity from Earth is calculated, it does not include the fact that wind resistance (which is proportional to the square of the object's velocity) will further decelerate the object, meaning that the actual escape velocity will be somewhat larger than the calculated value. The second technicality is that when an escape velocity is calculated, the assumption is made that the object's velocity is perpendicular to, and directed away from the attractor's surface. If an object is fired parallel to Earth's surface, it's escape velocity is much greater. And, if you think about it, in the worst case, if an object is fired perpendicular to Earth's surface, but towards it, the escape velocity effectively becomes infinite.

One more thing. I think the idea of escape velocity is also ambiguous. For instance, I can imagine a way that the "escape velocity" from Earth could be 1 mph. Imagine that a ladder is built from Earth's surface, far far into outer space. If you climb that ladder "high" enough, say, at 1 mph, then, sooner or later you will be far enough from Earth so that if you let go of the ladder, Earth's gravity will never pull you back. (Admittedly, in this case you would be adding propulsive energy during each step, but, you would still be escaping.)

Finally, since if an object's velocity is less than the escape velocity, it falls back to the attractor, and if it is more than the escape velocity, it recedes forever from the attractor at ever decreasing speeds, there should be a "balance velocity", i.e., a velocity at which the object decelerates to zero, and then remains a fixed distance from the attractor, forever. Practically, though, I think it is almost impossible to attain such a balance velocity.

[kryton9]

Dan, escape velocity is something I was thinking of about 10 days ago, so it is interesting you brought it up. Here is the strange thing... the shuttle for instance goes into space, but it is not out of the reach of Earth's gravity. It instead is constantly falling around Earth and moving at a high rate of speed. That is why some satellites eventually come back down to Earth as they lose speed and gravity finally gets the better of them again.

So is escape velocity the speed at which you can keep spinning around Earth without falling back into it? It is still confusing to me.

[danbaron]

(This is my speculation, I am no expert on this subject.)

No, the speed at which you remain in a circular orbit around the Earth, depends on the radius of the orbit (how far you are from the Earth's surface). The bigger the radius, the slower is the equilibrium velocity. Escape velocity is constant, and depends only on the radius of the planet, and its mass.

Escape velocity is the minimum initial velocity of an object, fired ("fired", meaning that the object has no capacity for self-propulsion, it is released with an initial velocity, and then the only force acting on it is the Earth's gravity - the Earth pulls it back, but it has no pushing force resisting the Earth's pull) perpendicularly from the Earth's surface, such that it never falls back to Earth. Instead, it recedes forever from Earth at an ever decreasing velocity, which never reaches zero.

An object launched perpendicularly from the Earth's surface, and then not steered, can never go into orbit, it will remain on its straight line path, it can only go up or come down.

Rockets are launched perpendicularly, but, they are self-propelled, and they are turned into a non-vertical path, which results in orbit.

And again, remember that, if I am correct, only gravity is included when calculating escape velocity, wind resistance is neglected.

You could look at escape velocity like this. Say that somehow, we create a vacuum between the Earth's surface and outer space (so that there is no wind resistance). You stand in the vacuum on Earth's surface and hit a baseball exactly vertically into the sky. The harder you hit it, the longer it will take before it falls back to Earth. It so happens that if you hit it hard enough, it will never fall back to Earth. The escape velocity is the minimum speed at which the ball leaves the bat, such that it never falls back to Earth. (We are neglecting the fact that if you actually hit the ball that hard, you would crush both it and the bat, we are assuming that the collision is elastic.)

[kryton9]

I guess escape velocity is a good name then. I think we just over think what it is because it feels like we need to. From wikipedia:

Escape velocity is sometimes misunderstood to be the speed a powered vehicle, such as a rocket, must reach to leave orbit and travel through outer space. The quoted escape velocity is commonly the escape velocity at a planet's surface, but it actually decreases with altitude. It is the speed above which an object will depart on a ballistic trajectory, i.e. in free-fall, and never fall back to the surface nor assume a closed orbit. Such an object is said to "escape" the gravity of the planet.
A vehicle with a propulsion system can continue to gain energy and travel away from the planet, in any direction, at a speed lower than escape velocity so long as it is under power. If the vehicle's speed is below the escape velocity and the propulsion is removed, the vehicle will assume a closed orbit or fall back to the surface. If its speed is at or above the escape velocity, it has enough mechanical energy to "escape" and will neither orbit nor fall back to the surface.

http://en.wikipedia.org/wiki/Escape_velocity

[danbaron]

That Wikipedia article is really good.

According to it, Earth's escape velocity is approximately 6.96 miles/second, or, 25056 mph.

You could look at it like this.

If you blast off in a rocket going directly away from Earth, then, when you reach 25056 mph, you can turn off the engines if you want to.

In that case, you (the rocket) will never fall back to Earth.

If you turn off the engines before you reach 25056 mph, you will fall back to Earth.

Of course, if you turn off the engines when you are going 25055 mph, it will take a long time before you fall back and hit Earth's surface, but, sooner or later, you will.

(Actually, 6.96 miles/second, is the escape velocity at Earth's surface. It decreases with distance above the surface. So, 25056 mph, is a slightly exaggerated estimate of how fast you need to be going in order to escape from Earth forever.)

[kryton9]

Thanks Dan, I think I will use this info for a test project to take the latest thinbasic for a spin.

[REDEBOLT]

I remember working a problem in physics which asked about the escape velocity of an hydogen atom from earth. It showed that the hydrogen atom does not have sufficient mass to attain 25000 mph.

Then the problem asked the same about Mars. Mars, being about half the size of earth, does not have sufficient gravity to prevent the escape of the hydrogen atom from its surface.

This explains why, even though Mars had water on its surface when it was young, there is no longer water on its surface. So the hydrogen escaped the planet leaving the oxygen behind. The oxygen united with iron on the surface and causing rust to form. Thus the "red planet."

Regards,
Bob

[LanceGary]

I remember working a problem in physics which asked about the escape velocity of an hydogen atom from earth. It showed that the hydrogen atom does not have sufficient mass to attain 25000 mph.

Then the problem asked the same about Mars. Mars, being about half the size of earth, does not have sufficient gravity to prevent the escape of the hydrogen atom from its surface.

This explains why, even though Mars had water on its surface when it was young, there is no longer water on its surface. So the hydrogen escaped the planet leaving the oxygen behind. The oxygen united with iron on the surface and causing rust to form. Thus the "red planet."

Regards,
Bob

I am a great ignoramus but surely the first part of your post must be relative to the amount of force applied to the hydrogen atom. After all some very tiny particles (such as cosmic rays) have such high velocities that they can only be detected in very deep mines, and some pass right through the earth. Of course forces capable of propeeling hydrogen to such enormous velocoties are unlikely to be found on earth...

Lance

[danbaron]

I know nothing about this stuff, either.

I imagine that the speed a hydrogen atom attains is due to banging into other atoms in the atmosphere.

I think Mar's gravity is approximately 1/3rd of Earth's.

And, I have heard that is the reason that its atmosphere is approximately 1% the density of Earth's - almost everything escapes.

On the other hand, I think Jupiter is all atmosphere - almost nothing escapes.

Neutrinos have a very small mass, are electrically neutral, and travel at almost light speed. They seemingly pass right through Earth with no problems.

http://en.wikipedia.org/wiki/Neutrino

89% of cosmic rays are protons.

I know that cosmic rays are blamed for unaccountable computer bit-flips.

http://en.wikipedia.org/wiki/Cosmic_ray

(If on Mars, copper had been substituted for iron, then it would be called, the "green planet".)

(And, if on the Moon, Cheddar was substituted for Swiss, it would be orange instead of yellow.)