Comment by TangoTrotFox | Hacker News Reader

Comment by TangoTrotFox | Hacker News Reader

Parent Full thread

Pretty common misconception, but what you're implying is not how/why you have 0G in space. So for instance the space station still experiences about 90% of Earth's gravity. Orbit is about going really fast, horizontally. If you flew straight up to the ISS, you'd fall right back down. This is why when you see a rocket take off their course looks extremely bent - it's not an optical illusion.

So why horizontally? Imagine there was no air resistance on Earth. If you shot a bullet that bullet would keep going until the force of gravity pulled it down and it hit the Earth. But now imagine that you shoot it fast enough that the vertical distance gravity is pulling it down is less than the vertical distance it gains due to the curvature of the Earth. That equilibrium is exactly what orbit is. It also leads to the highly counter intuitive fact that the height of a given circular orbit is determined exclusively by how fast an object is moving relative to the body it's orbiting. Mass doesn't matter.

Okay so back to the plane. If it's not intuitive yet imagine throwing a ball. It works exactly the same as our bullet, but we can visualize one important part easier. The ball's trajectory will be a parabola. And at the highest point of that parabola the net vertical force on the ball is zero. It's where the force you exerted on it to send it up, and the force of gravity pulling it down eventually reach an equilibrium. Something inside of that ball would experience 0g at the moment when it was at its parabolic peak. And that's exactly what these planes do. They simply 'throw' the planes into a parabolic path, and the passengers experience near 0 g while traveling through the parabolic peak.

> at the highest point of that parabola the net vertical force on the ball is zero

This is not correct. If we count gravity as a force, then it is pulling on the ball just as much at the peak of the parabola as anywhere else, and once the ball leaves your hand gravity is the only force on the ball (leaving out air resistance); your hand doesn't magically exert force on the ball once it's thrown.

If we do not count gravity as a force (which is the approach taken in General Relativity), then there is no force on the ball at all (leaving out air resistance) once it leaves your hand.

Let's talk about pedantry for a minute. Did you notice the comment I was responding to? It was an individual who thought orbit had something to do with escape velocity. Like many people he probably thought that orbit was about 'escaping' Earth's gravity and then just floating in 0g. In other words he knows nothing about orbital mechanics and, most likely, next to nothing about physics in general.

There is no vernacular in my post. When I use the word force, I am stating it in a purely colloquial sense. And in this regard everything is completely cogent and clear description of the forces (har har) in play. By contrast look at the top post. It provides a couple of sentences along with a link to Wiki for further elaboration that immediately jumps into orbital mechanics, assuming an understanding of delta v, etc. There's nothing wrong with the comment in and of itself, but it's an absolutely awful comment in regards to the audience it's talking to.

And I think this pedantry a big part of the reason that so many individuals are completely scientifically illiterate. Most of all science is relatively simple, but one of the biggest issues is vernacular. And indeed within a field there is extremely good reason for this vernacular. It is not only vastly more concise than trying to obtusely explain every single concept from the ground up, but it is also more precise. Do I mean force? Do I mean momentum? Speed? Velocity? Every concept is entirely different, but in the world outside outside of the field -- none of this matters. Theories are just ideas, speed and velocity are same thing, and so on.

The point of this is, do you think my post would be clear and accurate in what it is understood to mean from the demographic that the message was directed at? I think the answer is absolutely yes. And the casual use of terms that have more precise meanings within a vernacular is in no way going to mislead them as to the meaning of what is said. Far from it, in my opinion - using more appropriate terminology is likely to lead to a less elucidating post!

> Did you notice the comment I was responding to? It was an individual who thought orbit had something to do with escape velocity.

Yes, and did you notice that I didn't object at all to the part of your post that corrected the "escaping Earth's gravity" misconception? That's because there was nothing wrong with it. I only objected to the part of your post that was incorrect.

> he knows nothing about orbital mechanics and, most likely, next to nothing about physics in general.

In which case the last thing you should want to do is to tell him things about physics that are wrong. Which is why I corrected the wrong thing you told him.

> There is no vernacular in my post.

My objection had nothing whatever to do with your choice of words. You made a factually incorrect statement and I corrected it. That's all there is to it.

I don’t think it was an attack on your use of language. Even if most probably don’t agree that you are helping illiteracy by alternative use of well defined concepts.

It’s factually wrong to say that the net force is zero at the peak, when what is zero, is the vertical speed.

What was questioned was the feeling of weightlessness. You imply that weightlessness is only felt at the peak of the parabolic path. When the ball leaves the throwers hand, the only force, disregarding air resistance, acting on the ball is the force of gravity, until it hits the ground. Someone inside the ball will feel weightlessness all the way from leaving the throwers hand to hitting the ground. There is no special feeling at or around the peak.

You're completely correct. I unintentionally implied weightlessness only at the parabolic peak. And I think what you've said is how you should correct things. Words convey ideas. What matters is not the words, but the ideas. Correct the idea and things are clear, focus on the words and things tend to muddle.

> in this regard everything is completely cogent and clear description of the forces (har har) in play.

Not everything you said, no. That's why I objected. If you want more detail, you said:

> The ball's trajectory will be a parabola. And at the highest point of that parabola the net vertical force on the ball is zero. It's where the force you exerted on it to send it up, and the force of gravity pulling it down eventually reach an equilibrium. Something inside of that ball would experience 0g at the moment when it was at its parabolic peak.

Actually, the net force on the ball (disregarding air resistance) is the same throughout the entire trajectory once it leaves your hand. (Here I'm taking the Newtonian view that considers gravity to be a force.) That's why the ball continuously accelerates downward by the same amount throughout the entire trajectory once it leaves your hand--which it has to in order for the trajectory to be a parabola. The force you exert on it to throw it upward stops as soon as it leaves your hand, so the only force thereafter is gravity. And since the force of gravity is not felt, the ball is in free fall, feeling 0 g, for the entire parabola.

So the part of your post that I quoted was not a "completely cogent and clear description of the forces"; it was a wrong description of the forces. That's why I corrected it.

...throwing a ball... disregarding air resistance it will be in free fall all the way back to the ground. There is nothing special about the peak.

Orbiting a planet is basically free fall with translational velocity. If the earth were flat we would fall down and horizontally and eventually hit the ground.

However the earth is a sphere and the direction of the free-fall constantly adjusts itself to point towards the center of the sphere creating an orbit.

It is literally throwing oneself at the ground and missing.

I have no idea what's up with HN lately. Your comment is completely correct... and downvoted? This site is starting to become more like Reddit everyday.

Reply on news.ycombinator.com