# Astronauts weightlessness: free fall or centrifugal force?

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When we see the astronauts clips in the ISS or the old Shuttle, we think that they are floating because there is no gravity. In factthe gravity in those orbits is almost the same as here on earth. So what keeps them floating is a never ending freefall. That's what I was reading, but I am having problem to understand it well. As I see it the ISS or the Shuttle, is not freefalling, but is just moving damn fast, so fast that it can escape the gravity pull. A freefall example to me would imply that you are falling towards the centre of the earth, and not east or west. When we are dropped from a tower, sitting on a chair, or inside an elevator, we would experience a weghtless sensation, the same as the astronauts, but this is indeed a freefall, because there is a downwards direction. In the case of the ISS, to me it's not a freefall, but the weightlessness is given by a constant balance between centrifugal force, speed of the ship, and gravity. What do you think?

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Free fall means there is no force preventing your fall under the influence of gravity. A sideways motion doesn't do that; for example, a ball thrown horizontally will fall to the ground at the same time as one that is just dropped.

Newton came up with the cannonball analogy to explain orbits as an example of free fall: https://en.wikipedia.org/wiki/Newton's_cannonball

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Free fall means there is no force preventing your fall under the influence of gravity. A sideways motion doesn't do that; for example, a ball thrown horizontally will fall to the ground at the same time as one that is just dropped.

Newton came up with the cannonball analogy to explain orbits as an example of free fall: https://en.wikipedia.org/wiki/Newton's_cannonball

Sorry, double post, how can I delete it?

Yes, in fact to me the ISS is not freefalling, as I read often. Don't you think the word freefall has nothing to do with the orbits cases of most satellites and ISS?

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No, I think they are all in freefall. There is nothing stopping or slowing their fall.

Edited by Strange
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No, I think they are all in freefall. There is nothing stopping or slowing their fall.

Our fastest aircrafts travel at about 800 km/h, which is nothing compared to the ISS (27.600 km/h). But if you could travel at that speed with an aircraft would you experience the same weightlessness?

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The ISS is traveling close to 20,000 m/h (32,000 km/h). If the Earth's gravity didn't pull down, it would fly in a straight line. The effect of the two, gravity and forward speed, means it falls towards the Earth, but flies forward fast enough that it cannot fall into the Earth. Thus, it is in free fall (always falling) and moving forward fast.

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The ISS is traveling close to 20,000 m/h (32,000 km/h). If the Earth's gravity didn't pull down, it would fly in a straight line. The effect of the two, gravity and forward speed, means it falls towards the Earth, but flies forward fast enough that it cannot fall into the Earth. Thus, it is in free fall (always falling) and moving forward fast.

and that's what I don't understand, how can it be considered a freefall if it never go towards the centre of the earth?

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It is defined that way. You are describing a special case that does not result in an orbit.

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and that's what I don't understand, how can it be considered a freefall if it never go towards the centre of the earth?

It is being pulled toward the center of the Earth the same as anything else in free fall. It's just going so fast that it keeps missing the ground so it never stops falling.

It's not the speed that causes the weightlessness. It's being in free fall that does that. What the speed does is allow the spacecraft to remain in free fall for long periods of time without hitting the ground. Things can move very quickly without being in free fall, and things can be in free fall without moving very quickly. In the first case, you don't get weightlessness, while in the latter case it tends to not last very long.

It's as if the spacecraft is falling straight down to Earth, but by the time that the craft reaches where the surface of the Earth was, the Earth has moved out of the way, and the spacecraft is now being pulled in a new direction.

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There's not much more can be said about free fall. Just memorize this post, #2, #4, #6, #8 and #9 and you are an expert.

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Our fastest aircrafts travel at about 800 km/h, which is nothing compared to the ISS (27.600 km/h). But if you could travel at that speed with an aircraft would you experience the same weightlessness?

No, because an aircraft is not falling. The wings lift it.

and that's what I don't understand, how can it be considered a freefall if it never go towards the centre of the earth?

Did you look at the Newton Cannonball?

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Strange question.

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When we see the astronauts clips in the ISS or the old Shuttle, we think that they are floating because there is no gravity.

Who is "we"?

A freefall example to me would imply that you are falling towards the centre of the earth, and not east or west.

And that is precisely the direction of their acceleration.

When we are dropped from a tower, sitting on a chair, or inside an elevator, we would experience a weghtless sensation, the same as the astronauts, but this is indeed a freefall, because there is a downwards direction. In the case of the ISS, to me it's not a freefall, but the weightlessness is given by a constant balance between centrifugal force, speed of the ship, and gravity. What do you think?

Centrifugal force is a made up force to allow us to use Newton's laws in a non-inertial frame. So lets's not use a non-inertial frame. It will only confuse matters.

and that's what I don't understand, how can it be considered a freefall if it never go towards the centre of the earth?

That's the direction of the acceleration. The bodies change direction — they're moving in a circle. They have to accelerate to do this. The direction of that acceleration is toward the center of the earth. They're just moving to the side fast enough that they continually miss it, as Delta1212 has said.

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Who is "we"?

We is me...or common mortals like me. Not everyone has your expertise swansont, relax.

And that is precisely the direction of their acceleration.

Centrifugal force is a made up force to allow us to use Newton's laws in a non-inertial frame. So lets's not use a non-inertial frame. It will only confuse matters.

That's the direction of the acceleration. The bodies change direction — they're moving in a circle. They have to accelerate to do this. The direction of that acceleration is toward the center of the earth. They're just moving to the side fast enough that they continually miss it, as Delta1212 has said.

I am talking about the centrifugal force, because it is rarely mentioned as the cause of weightlessness in the ISS astronauts case. The case of free falling is when there is only gravity acting, but in the case of a satellite, shuttle or ISS, you need rockets to move that fast, that's why I would say that they are spinning, they are not free falling. Example, if you are dropped inside an elevator, you are free falling vertically (not spinning around the earth), at about 300 km/h, and the weightless sensation, in this case, is given by the free fall, your feet don't stay on the floor, and if you try to drop an orange, it won't go to the floor, but will float in front of you. Second example, you are inside an aircraft travelling at a constant 300 km/h, just like in the free falling elevator, but I guess you won't feel any weightlessness sensation, you stay seated, and if you try to drop an orange it will fall down on the floor. Third example, you are on a roller coaster, going up fast, at the top of the track, for a moment you will feel weightless, lifted out of the seat, the orange will stay afloat. Which of these example can be associated to the astronauts weightlessness? To me only the third example, for the ISS men it will feel like and endless "top of the roller coaster hill".

No, because an aircraft is not falling. The wings lift it.

Did you look at the Newton Cannonball?

Yes, the cannonball example is useful to understand why things can spin around the earth, but it doesn't make you understand the weightlessness sensation inside a ship.

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We is me...or common mortals like me. Not everyone has your expertise swansont, relax.

I am talking about the centrifugal force, because it is rarely mentioned as the cause of weightlessness in the ISS astronauts case. The case of free falling is when there is only gravity acting, but in the case of a satellite, shuttle or ISS, you need rockets to move that fast, that's why I would say that they are spinning, they are not free falling. Example, if you are dropped inside an elevator, you are free falling vertically (not spinning around the earth), at about 300 km/h, and the weightless sensation, in this case, is given by the free fall, your feet don't stay on the floor, and if you try to drop an orange, it won't go to the floor, but will float in front of you. Second example, you are inside an aircraft travelling at a constant 300 km/h, just like in the free falling elevator, but I guess you won't feel any weightlessness sensation, you stay seated, and if you try to drop an orange it will fall down on the floor. Third example, you are on a roller coaster, going up fast, at the top of the track, for a moment you will feel weightless, lifted out of the seat, the orange will stay afloat. Which of these example can be associated to the astronauts weightlessness? To me only the third example, for the ISS men it will feel like and endless "top of the roller coaster hill".

The centrifugal force never enters the analysis if you look at the mtion from an inertial frame. It doesn't exist.

You elevator example works...for a few seconds. But what if you took that same exact example and gave it a sideways velocity. Is that still freefall?

Because that's all the ISS is doing.

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The centrifugal force never enters the analysis if you look at the mtion from an inertial frame. It doesn't exist.

You elevator example works...for a few seconds. But what if you took that same exact example and gave it a sideways velocity. Is that still freefall?

Because that's all the ISS is doing.

You mean that if you are falling inside the elevator moving sideways, the orange will stay in front of you instead of touching the floor? An aircraft can move as fast as a free falling elevator, wouldn't be the same situation?

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You mean that if you are falling inside the elevator moving sideways, the orange will stay in front of you instead of touching the floor? An aircraft can move as fast as a free falling elevator, wouldn't be the same situation?

Ain airplane is not accelerating downward. It flies level. If it dove, then you would feel weightlessness. That what they do on the "vomit comet"

Moving sideways in the elevator will be completely unnoticeable to you.

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Ain airplane is not accelerating downward. It flies level. If it dove, then you would feel weightlessness. That what they do on the "vomit comet"

Moving sideways in the elevator will be completely unnoticeable to you.

ah ok, I will read more about the vomit comet, maybe it's a good way to understand the subject.

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Second example, you are inside an aircraft travelling at a constant 300 km/h, just like in the free falling elevator, but I guess you won't feel any weightlessness sensation, you stay seated, and if you try to drop an orange it will fall down on the floor.

That is because you are not falling. You are constantly being pushed up by the wings providing lift.

Third example, you are on a roller coaster, going up fast, at the top of the track, for a moment you will feel weightless, lifted out of the seat, the orange will stay afloat. Which of these example can be associated to the astronauts weightlessness? To me only the third example, for the ISS men it will feel like and endless "top of the roller coaster hill".

That is pretty close. Now imagine the roller coaster is going so fast it comes off the tracks at the top and goes flying off, falling to Earth. But luckily, because the Earth is round, the ground falls away beneath it and so you never crash, You just keep falling....

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That is pretty close. Now imagine the roller coaster is going so fast it comes off the tracks at the top and goes flying off, falling to Earth. But luckily, because the Earth is round, the ground falls away beneath it and so you never crash, You just keep falling....

Alternately, it stays on the tracks that go around the world, and it goes so fast that the same thing happens.

We can look at what happens as it speeds up:

At rest, there is no net force, so mg + N = 0 (i.e. gravity, or the weight, and the normal force cancel). In scalar form, we can write this as mg - N = 0 (down is positive in this coordinate system)

When it's moving with some speed v, there must be a net force in order for it to move in a circle (from Newton's first law — if the motion is not straight-line, there must be a force). That is the centripetal force, Fc = mv2/r

So now mg - N = Fc

mg doesn't change. What happens is the normal force decreases. Which means you can go fast enough that N goes to zero. That's the orbital speed. The earth's surface (via the track and car) no longer pushes up at you. Gravity alone pulls you in a circular path.

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and that's what I don't understand, how can it be considered a freefall if it never go towards the centre of the earth?

Think about where it would be if it didn't fall. It would go straight ahead and would be nearly out of the Solar System by now.

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Think about where it would be if it didn't fall. It would go straight ahead and would be nearly out of the Solar System by now.

Depends on the orbit, it might end up in the Sun or elsewhere in the solar system.

Edited by EdEarl
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Think about where it would be if it didn't fall. It would go straight ahead and would be nearly out of the Solar System by now.

Depends on the orbit, it might end up in the Sun or elsewhere in the solar system.

Now...you guys are going off on a tangent

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Now...you guys are going off on a tangent

The thought of going off on the tangent makes the idea of free falling not so bad. It was how I got it into my head but I'm always flying off.

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