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People in the ISS feel weightless because they are in perfect orbit around the Earth, and only gravity is pulling on them. By that logic, why aren’t people in airplanes weightless? A plane stays at a relatively constant altitude, meaning that it technically orbits around the Earth. What’s the difference between people in an airplane and people in the ISS?

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    $\begingroup$ Search term: "vomit comet" (yes, really). $\endgroup$
    – rob
    Nov 11, 2019 at 14:25
  • $\begingroup$ People in the ISS keep falling. People in an airplane don't. Heck, given your argument, you should expect a person sitting on the floor to experience weightlessness, since he's at "constant altitude", while a person in an elliptical orbit wouldn't experience weightlessnes, since he's not in constant altitude. $\endgroup$
    – Luaan
    Nov 18, 2019 at 15:12

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Staying at the same altitude is not the same as orbiting. You can imagine a helicopter or rocket hovering in the same place above the ground: they are certainly not orbiting.

In order to orbit you need to move so fast that you fall and miss the ground. This is the idea behind Newton's famous diagram with the cannon throwing balls faster and faster: Newton's cannonball. From Wikimedia commons.

As they move faster they go further, until at C and D the balls always miss the ground: they are in orbit (at least until they hit the back of the cannon).

The key concept here is that something that orbits like this is in free fall: there are no forces except gravity that acts on it, and this is why inside the an orbiting craft you will not experience any gravity.

The airplane is not in free fall. It is moving much slower than the ISS and were the engines to cut out it would fall to the ground even if there (suddenly) had not been any air resistance.

How big is the difference? The orbital velocity at distance $r$ from the Earth's center is $v=\sqrt{GM/r}$. For the ISS at 360 km altitude this is 7703 m/s. At 10 km altitude it is 7899 m/s - almost the same, a tiny bit higher. But airplanes usually fly at velocities lower than the speed of sound, 333 m/s. So they are far from orbital velocity.

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The people on the ISS are in freefall- gravity is keeping them in orbit, its value being just enough to overcome the inertial tendency of the ISS to fly off in a straight line.

People in a plane are not in orbit, and are not free-falling

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The International Space Station is in freefall: there are no forces acting on it besides the force of gravity. This causes the occupants to have the feeling of weightlessness, since there's nothing pushing them against the walls of the spacecraft.

An airplane in flight, however, is (usually) not in freefall. Instead, an airplane is held up by its wings; the air is pushing up on the airplane with about the same amount of force that gravity is pulling down on it.

This upward force from the air is transmitted through the seats to the passengers: the seats are constantly pushing up on the passengers. This force is what gives the passengers the feeling of weight.

The pilots of an airplane are capable of adjusting the amount of upward force on the airplane: if they pull back on the yoke, this will raise the nose, which causes the angle of attack to increase, which causes the amount of lift to increase (unless the aircraft stalls), which causes the passengers to feel heavier, as though the force of gravity had been increased. Likewise, if they push forward, this lowers the nose, decreases the angle of attack, decreases the amount of lift, and makes the passengers feel lighter.

In the case of a zero-gravity flight, the pilots reduce the amount of lift to zero for 20-30 seconds, causing the passengers to feel weightless for that period of time.

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A plane stays at a relatively constant altitude, meaning that it technically orbits around the Earth

That's not what an orbit is. An orbit is essentially a constant state of falling (commonly called freefall); this is why you experience weightlessness. To have an orbit, you need to be moving fast enough tangentially to your circular path, so that the gravitational force keeps the orbit approximately circular.

For airplanes to be in orbit, they would need to be going much faster. A plane pushes against the air around it to stay at the same altitude. An orbit satellite can remain in orbit indefinitely without pushing against anything, if the orbit were perfectly stable.

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