danbaron
24-07-2011, 23:36
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.
:evil: :twisted:
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.
:evil: :twisted: