ELI5- Can someone the physics behind Zero-G flights (Parabolic flight) ?
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Gravity makes you fall downwards.
Plane carries you high up.
Then plane stops flying and starts falling.
You and the plane falls at the same speed.
Since you are no longer being pulled down into the plane, you will seemingly float.
You will, relative to the plane, be in zero-gravity.
A lesson on inertia: Inertia is an objects tendency to stay at rest or stay in motion within it’s current trajectory unless acted upon by another force (Newtons first law). Every object has inertia, even objects within objects. What we experience in 0-G is simply falling.
Picture yourself on a roller coaster. The direction and inertia of the train is guided by the track. Your direction and inertia is guided by the train with some wiggle room from the restraints. When you go up a hill at speed, you feel the sensation of being pressed into your seat. These are positive Gs, and it’s caused by your direction being altered. Your body naturally wants to continue forward and down, but the train pushes up against you, causing those g forces. When you’re at the mid point of the incline, things will feel normal as your inertia is now matching (or at least close to) that of the train: up and forward. Until you crest the hill, your inertia is still directing you up and forward, but the train is going down. This creates negative G’s and gives the sensation of flying out of your seat – which you would if you weren’t restrained. That’s essentially how a parabolic flight works: 0G is achieved by the plane falling. Unrestrained in the plane, you fall as well and this creates the sensation of weightlessness. Because you’re not restrained in the plane, you can float around a bit until the plane needs to level out.
The plane simply falls at the same speed as you.
Imagine you’re in a room. If you are floating in the middle of the room, you will accelerate downward at 9.8 m/s^2 , and if the room is sitting on the ground, the room isn’t accelerating at all, thus you accelerate relative to the room and you will hit the floor.
Now lets put the room in deep space. You float in the middle, not accelerating, and the room isn’t accelerating, and you’re free to float around the room.
Now lets put a rocket engine on the room to accelerate the room at 9.8 m/s^2. You’re floating and not accelerating, and the room accelerates up into you. Relative to the room, you accelerate downwards at 9.8 m/s^2 , so it seems like you are being affected by gravity.
Parabolic flight is just the opposite of that. The room is falling, accelerating at 9.8 m/s^2 , and you are inside that room accelerating at 9.8 m/s^2 . Relative to the room, you aren’t accelerating, and you’re free to float around the room.
Zero-G is weightlessness that happens when you feel no weight. This is what happens for example ISS that obit earth. It is not that there is no gravity there, it is around 90% of surface gravity. It is the opposite is just gravity so everything accelerates the same way because of it and you are weightless.
When you sit on a chair you are not feeling gravity pulling you down but what is called the normal force from the chair that stops you from falling down.
Consider what would happen if you throw a ball on the moon where it is no atmosphere. It will fly in a parabola and the only force on it is gravity, it is weightless. On earth, we have an atmosphere so if you throw a ball there will be air resistance so gravity is not the only force.
A parabolic flight follows the path that the airplane would have moved if it was not for air resistance, just like a thrown ball on the moon. Because air resistance exists the pilot uses the engine and control surfaces to compensate for it and the airplane will move very close to the parabolic path it would have followed if the atmosphere was not there.
You could do the same by just diving with the airplane the problem is airplane have a max speed and the acceleration down need to be 9.8m/s^2. So just diving results in a short weightless time compared to if you start when you move up.
This is comparable different to that a ball you throw in an arch is longer in the air than the one you just drop.
Apparent zero G happens when you and everything around you fall at the same rate so a climb in an aeroplane followed by a steep dive can create an apparent zero G experience. https://youtu.be/Zu-Sp3I0c1Q
I can’t explain parabolas without algebra, so this will be closer to eli 15, but I’ll try and limit it as much as possible. Basically, there are only 2 things about algebra that you need to understand. I will use letters a, b, c, and t to represent numbers that I don’t know. The important one is t. The number that t can represent is allowed to change, so we have to pay more attention to it. a, b, and c all stay the same, so we can *almost* ignore them.
A parabola is just another term for a degree-2 polynomial. Polynomials can come in many different degrees. Poly means multiple. This means there can be multiple terms in the equation. Doing as little math as possible for eli5, there’s a variable multiplied by itself some number of times. We call that the power. Whatever the highest power among the multiple terms is determines the degree. t²+t+1 is degree-2 because the highest power is 2 (on t²)… If there was an t³, it would be degree 3. So when you read “parabolic” it just means it looks like at²+bt+c and once you enter free fall, the only force acting on you is gravity, in that case, a, b, and c are already set and will not change until the end of free-fall.
Why exactly is free-fall parabolic? Because gravity determines your acceleration. Acceleration is the change in velocity and velocity is the change in position. If you had no acceleration and no velocity, the only thing you would have is position. That’s the place you are right now… And since you have no velocity or acceleration, you will continue to be there forever. This is called a “constant” and just like “parabola,” “constant” is another word to represent a polynomial. This time, a polynomial of degree-0. In my earlier framework, I did t²+t+1 notice there’s a t² that’s power 2. There’s a t (if no small number exists, it’s implied to be t¹) that’s power 1. And there’s a term with no t so that’s power 0. And as I mentioned a degree-0 polynomial means 0 is the highest power. So there is no t anywhere in the equation. The general form of a degree-2 polynomial is at²+bt+c but for a degree-0, it’s just c. There is no term including t. This is actually a bit fortuitous because c stands for constant.
Before I go further, I’d like to point out the parallels to physics that this entails. Notice there’s no t² term or t term. Similarly, to get there, I said there is no acceleration or velocity. That’s not a coincidence that there are 2 terms missing, both tied to t (the thing that can change) and also 2 things that change position also missing. Those are perfectly symmetrical. If I add back in velocity, that defines the change in my position. This makes a degree-1 polynomial. That’s going to look like bt+c. Again, it’s actually kinda nice because the letter b sounds a lot like the letter v for velocity. (This is not important to the explanation, but degree-1 polynomials also have a special name called linear. So degrees 0, 1, and 2 are called constant, linear, and parabolic respectively)
Lastly, if I add back in acceleration, that defines the change in velocity and velocity defines the change in position. So acceleration has double the effect which means we’re going up to a degree-2 polynomial. Which looks like at²+bt+c and once more we have a perfect way to help remember that because a stands for acceleration (due to some calculus, it happens to be half the acceleration, but it’s at least related to acceleration). And notice, a does have twice the effect because it’s part of the t² term. Velocity only has a single effect and b is part of the t term (which is just a shorthand for t¹ don’t forget the 1 is implied).
Ok, so to bring it back, as soon as you enter free-fall, a, b, and c are set and do not change. a is determined by you acceleration due to gravity… The only force acting on you in free-fall. b is whatever your initial velocity happened to be the moment you enter free-fall. And c is whatever your position was whenever you entered free-fall. So what is t? I did say it represents a number that can change, so your first guess might be position because I did say with acceleration and velocity, your position will change, but it turns out the way the position changes is perfectly described by at²+bt+c if we let t be some other number that changes… Time. In fact, it doesn’t matter if there’s acceleration or velocity or not. Time still changes with or without us. It marches on without our consent. It seems like a nice, stable thing to base our function on (oh, and it starts with t).
So let’s say if your position in time is perfectly defined by at²+bt+c (which is a parabola), then what happens if you force a plane to follow the exact same position predicted by that equation? If you were inside that plane and got closer to the walls, that would mean your position is changing relative to the plane, so then you aren’t following the same equation. If you were following the exact same equation, you would not move relative to the plane at all. But that means you wouldn’t get closer to any of the walls, ceiling, or floor. That’s what being in zero-G is like. You float around without falling towards the floor (or any direction).
Let’s be physics students for a minute and ignore air friction😊 Imagine a thrown baseball (on earth). It flies in a parabolic arc under the influence only of its own inertia plus the force of gravity. From the pov of a bug on the ball, the ball and bug are both in an identical free fall situation. This feels just like it would if there was no gravity (or lots more gravity – the “free” fall is about not resisting gravity.)
Have you seen videos of astronaut trainees inside the Vomit Comet? They are in the same situation as the bug on the ball.
In practice, since we’re doing this in the air, it takes a pilot and engines and wings to control the arc of the plane to match the arc you’d get if there were no air.
As it happens, my father flew many such missions as pilot. It’s hard to control the flight arc precisely, so they needed a very sensitive instrument to tell if they were flying above or below the correct arc. He designed it: a golf ball, suspended on a long rubber band stretching horizontally across the cockpit. The slightest deviation from the right arc moved the ball up or down in a very easy way to see. He basically “flew to the ball”. (These were not vomit comet flights. They were done in a NASA Learjet.)
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