[TangoTrotFox]

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.

[pdonis]

> 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.

[TangoTrotFox]

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!

[jonsen]

...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.

[crimsonalucard]

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.

[TangoTrotFox]

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.