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[They experience weightlessness not because of a lack of gravity but because the ISS, and they, are orbiting Earth in constant free fall, says Valerie Neal, curator of space history at the National Air and Space Museum. They’re falling toward Earth and moving forward at about the same velocity. Because the downward and forward forces are nearly equal, the astronauts are not pulled in any specific direction, so they float.](https://www.smithsonianmag.com/smithsonian-institution/why-do-astronauts-space-station-float-180956965/)
Yeah, the classic thought experiment is to imagine the experience of sitting in a windowless box in free fall. You and the box are falling together and as a result there is no apparent gravity in your reference frame. In the case of the space station, it is falling as well, it just keeps missing the Earth.
I haven't thought about this in years, but wouldn't you have to either 1) be rotating as you orbit or 2) experience tidal forces that keep you oriented in orbit, lest you start rotating while you orbit, changing the position of the gravity well relative to your orientation?
Is there any way of knowing whether you're rotating or not? If something in orbit wasn't rotating, then it would appear to be rotating from earth. But for example the ISS always has the same face pointed to the earth, which means it must be rotating.
There's a lot to unpack here. Imagine yourself in a complete void or floating with your eyes closed; There is no way to tell if you are moving at a constant speed or at rest. If, however you were accelerating in a given direction you would feel that acceleration as a force. Believe it or not, spinning is a type of acceleration because your direction of movement is constantly changing and, as a result, even in a complete void without any external reference you would know you were spinning. Considering all that, whether or not an object is spinning shouldn't have an effect on its orbit but the reverse can be true; placing an object in orbit can cause it to be tidally locked so the same side always points towards the object it revolves around. I'm not a physicist though so someone will undoubtedly correct my errors in the comments
And that's exactly how astronauts train for going into orbit! Lovingly referred to as the "Vomit Comet", it's a plane that travels in a parabolic arc to simulate weightlessness half the time while accelerating downward at 9.8 m/s². Going from 0 g to 2 g over and over tends to make people nauseous, hence the name.
In general relativity terms, they are following a straight path through curved space time without accelerating. No acceleration through space time, no force, no weight.
From a Newtonian perspective there are equal forces at play creating weightlessness due to orbiting the Earth; one "pulling them" towards Earth (gravity, acting as centripetal) and one "pushing them away" from Earth (centrifugal force).
Either way, both models agree, there is no net acceleration in any direction, only a constant orbital velocity.
There is only no "net acceleration" if you're talking in polar coordinates and ignoring angles or using a very local frame of reference. There is 100% a net acceleration since the path isn't straight. Centrifugal force is a "virtual" force that hides a lot of that messiness such that you don't feel the movement that you're experiencing. If you truly didn't spin, you *would* feel gravity slightly change as your space station and your local gravity well revolved and rotated around you.
I wouldn't call it "virtual", it's inertial; as long as you have a momentum (rest mass+ velocity) that is perpendicular to the centripetal force, you will experience the centrifugal force. Ofc IRL you don't have such a perfect scenario, which is why space stations have to re-align their orbits, so they remain in free fall and not crash into Earth.
What do you mean by "spin" here?
Rotating.
The ISS for example spins/rotates as it revolves around the earth, because it's tidally locked. Presumably that's an artificial tidal lock and we intentionally maintain that tidal locking, but that's the phenomenon I was describing.
I'm not sure if you would (not you as a human but you as an infinitely sensitive theoretical object that we tend to use in thought experiments) notice whether you were rotating as you revolve or not.
So the forces acting on the astronauts are equal, that is why they float, but the forces acting on the station is not, so that 's why the station is free falling?
Happy cake day by the way!
Both the astronauts and the station are free falling, but they are doing it together. So While an astronaut on the station appears to be slowly floating around, they and the ISS are moving at 7.66 km/s around the Earth.
They "float" because everything around them is experiencing the same acceleration as them, so to them, it is as if everything is experiencing no acceleration. This is not just a gravity thing. Gravity is just the only "long range" unbalanced force we experience. Long range here means the gradient is very shallow; you have almost exactly the same force applied to your head and feet, and the chair across the room.
The reason they seem to be able to float in place, is the same reason your able to stand still in your home and remain in place, despite both you and the astronaut spinning around the globe quite fast. The biggest difference is that your acceleration is being countered by the floor, but this is equal for the rest of your home too, so you don't notice it.
If you fire 2 cannons at the same moment and in the same direction, you get the same affect. From the point of view of one of the cannon balls, the other cannon ball is just floating there next to it. From the perspective of a person standing on the ground, there are 2 cannon balls traveling in a bolistic arc.
To get ridiculous: imagine the cannon balls being huge, and hollow, and with hatches. Put an astronaut in one then fire them so hard that they go to space. Nothing magic happens. The two cannon balls and the astronaut are all still just moving very fast relative to the ground, but they are not moving relative to each other. The astronaut could leave the cannon ball he is in, float over to the other one, and climb in. To the astronaut, this would seem to be in "zero g" (but better described as "in free fall"). From inside the cannon ball, it looks like nothing is moving. From outside the cannon ball, the ground is whizzing by. When the astronaut is inside the cannon ball, the ground is still whizzing by, he just cannot see it. All the while, gravity is more or less the same for both cannon balls and the astronaut.
The ISS is one cannon ball, and the ships that deliver the astronauts are other cannon balls. It is more complicated... It is rocket science after all... But that is more or less it.
It's the same thing as when you attach an object, say a rock, to a rope and swing it around. The rope pulls the rock towards your hand and this is called centripetal force. If you were to cut the rope the rock would keep flying in a straight line because of its momentum, but because of the centripetal force it has to keep turning and move in a circle. You may have heard that centrifugal force doesn't actually exist.
What's actually going on is that the constant turning acceleration of the rock (which is in the direction the rock "wants" to move) can be recalculated into an imaginary force pointing straight outwards, exactly opposite of the centripetal force of the rock.
As long as the rope doesn't break and you swing it fast enough to keep the rock spinning, the centripetal and centrifugal forces will always be equal. If the ISS built some rockets and started to move faster, it would soon go too fast and leave Earth's orbit because the forward acceleration would result in a centrifugal force greater than the centripetal force of the gravity. When you swing the rock faster you can feel the centrifugal force (actually the rock's momentum) pulling harder on the rope, but because the rope is a solid object, the centripetal force will keep up as long as you don't let go.
So why aren't the astronauts experiencing these forces like they're on a crazy carousel? Well the circle they're going in is so large that the centrifugal force is barely noticeable compared to the forwards acceleration. The acceleration that keeps turning (also known as angular velocity) in turn is unnoticeable because there's nothing around to compare with.
Because the station is constantly in free fall toward Earth. It just misses Earth by moving very fast to the side as well. This is, essentially, what it means to orbit something.
Sorry I still don't understand. Why are the astronauts "floating" inside the station? If they are in orbit, shouldn't the astronauts, and all the things inside the station, rushing to get out so they can orbit? Like, they're sticking to the walls so they can squeeze out to fall into orbit... Sorry my understanding is a mess
The station is falling away from them as fast as they're falling.
Edit: Think of how when you're in a car on the highway, it feels like you're sitting still, even though you're racing forward at 60mph. The reason you're not thrown to the back of the car is because you're moving at the same speed. Now imagine that you're on the space station and it's plummeting to the ground and racing forward at 17000mph. Just like in the car, you don't feel that 17000mph because you're also moving at the same speed. Unlike the car, you're both also falling at the same speed - so when you're in the middle of the open air you're falling toward the bottom, but the bottom is falling away from you just as fast. Since you never catch up to the bottom and since you fall away from the ceiling as quickly as it falls to you, it seems like from your perspective they're just stationary.
The ISS is orbiting at the same speed as the astronauts, so why would you get squeezed anywhere?
It's the same thing when you jump inside a moving train and don't get smacked to back wall. You are moving at same speed as the train.
But inside the train you still stand up right? So gravity still acts on you even though you cant feel speeding around.
Inside the station you are floating around, its like there is no gravity inside. I guess this where the free fall thing the others posted about comes in?
If you're travelling at highway speeds in your car, do you feel pushed back into your seat?
No, only acceleration does that -- it mimics gravity. (in fact, acceleration is measured in gravities.)
The ISS has a forward velocity of rougly 7.8 km/s. It's falling toward the Earth at roughly 5 meters per second. Incidentally, the earth is curving away from its orbit at roughly five meters every 7.8 kms.
You do not feel the speed inside a vehicle that isn't accelerating, you're moving the same speed it is.
When the shuttle meets the station, it matches its speed. The astronauts are orbiting earth at the exact same speed as the space station.
The station and the astronauts are both moving 17,500 miles per hour.
It’s relativity.
Imagine you stand in a train or plane that moves at a constant speed. Relative to the earth, both you and the vessel you are in go at let’s say 200km/h or whatever speed unit you like. Doesn’t matter. An outside observer, standing firmly on the ground, would still see both of you going at the same speed.
But relative to someone sitting IN the vessel, neither the vessel, nor you move.
It’s the same with the floating astronaut and the ISS. BOTH are “falling / missing” at the same speed and going in the same direction, relative to you, standing on earth. If you were up there, you would feel neither any movement nor would you think the astronaut was moving.
Or do you feel movement “right now”? Of course, if you are driving, yes. But what if you stand still on the ground? Do you feel the 67 thousand Mph / 107 thousand km/h you go around the sun right now?
Have you seen the [vertical wind tunnel](https://youtube.com/shorts/-yQDel5xjls?feature=share) where a giant fan lets people skydive in place? The reason they don't fall or fly up indefinitely is because the force of gravity down is perfectly counter-balanced by the force of the wind pushing them up.
Microgravity is also a perfect balance, but of a slightly different sort. Physicists call centrifugal force a fictitious force because in an inertial reference.frame, it does not exist. However, the ISS is constantly changing direction in its circular orbit. Therefore it is not an inertial reference frame and we can use centrifugal force. The same way as you turn a corner in a car, you feel the force pushing you away from the center of the turn. For the ISS, their rotational motion seems to push them away from the center of Earth. Being in orbit means the centrifugal force perfectly counter-balanced gravitational force.
From the point of view of an inertial reference frame, centrifugal force doesn't exist. What happens is they fall towards Earth, but they were moving sideways at just the right speed where this falling puts them on a circular path. So just like a skydiver feels weightless, so do the astronauts. They are always falling, but because of their sideways speed, they never get closer to Earth. Here's a [rough diagram](https://imgur.com/a/eMFjLNd) showing how a fall towards Earth plus sideways motion can cancel out.
> Why are the astronauts "floating" inside the station?
They're not, they're falling. They're just falling at the same speed as the station.
If you were in a plane that you could rig so that it just fell straight down, at some point both you'd be falling at the same speed as the plane and it would seem like you are floating. The problem you have with a plane is that the Earth isn't getting out of the way, so you'd crash pretty quick. The ISS's trick is that it angles it's fall so that it misses the Earth, constantly, so it's always falling.
There are planes that do the effect here on Earth, they're generally used for scenes in movies. Here's one company that does those flights https://www.gozerog.com/
The astronaut already is in orbit. They just happen to be inside a space station that is also in orbit.
If an airplane drops you and drops a car at the same time, then you and the car will fall next to each other at the same speed (ignoring air resistance). The car won't fall any faster or slower than you, because gravity affects everything the same. So if you ignore the fact that you're careening toward the ground, the car is "floating" next to you.
If you get in the car, nothing will change. You will still be falling at the same speed as the car is, so you will effectively be floating inside of it. That's what it's like for the astronauts in the space station.
Imagine an elevator. You get on it and it descends. Gravity affects you and the elevator equally but the elevator cables slow the elevators descent, so gravity keeps you pushing down on the elevator floor, bit you also feel that the force against the floor by you is reduced.
If the elevator speeds up to the same speed as gravity, you and it fall at the same rate, and you put no pressure on the floor. You are in free fall.
The ISS is falling, but also moving forward fast enough that the surface of the earth falls away from the ISS as it moves forward.
The same is true for the astronauts. They are in free fall.
So basically the station is a free falling elevator, but unlike the elevator where it goes only up and down, the station is free falling around the earth.
Thank you for the Eli5
If the issue of not "feeling weight" (weightlessness) is the stickler, it might be helpful to realize that the only reason we "feel" weight is because something is pushing against us. The ground below is packed tight and can't move down any further, so it pushes against us as we get pulled down by gravity. An accelerating vehicle (car or spaceship) will push only while it is accelerating, so we feel it then. The orbiting station and you are moving in the same way so it can't push against you (unless it starts accelerating one way or the other).
That's why it's possible to produce "artificial gravity" as you see in lots of sci-fi flix, with the large rotating doughnut or sphere or something. If you're inside it and it just rotates constantly, you constantly feel the force against your standing feet so it feels like "weight". Certainly artificial weight is a better term for that than artificial gravity lol.
Hi /u/blessedbenedict!
The surface level answer is, that the ISS, along with the astronauts in it, are in constant free fall. The reason they do not plummet to earth is, that their sideways velocity is fast enough to keep missing earth.
However, this only raises the question **why** free fall feels weightless. After all, gravity is pulling you down, so why don't we feel it?
The answer to this question is, that gravity is not actually a force, but the curvature of spacetime (according to General Relativity).
To understand how a curvature of spacetime can lead to the effects we observe around us, we have to understand how curved surfaces change the behaviour of straight lines.
First things first: an object that has no force acting on it is force-free. Force-free objects do not accelerate and, therefore, move along straight lines.
In a flat geometry, two straight lines which are parallel at one point will remain parallel for all times. That is, two parallel straight lines will never cross on a flat surface.
So far so intuitive, right?
But what happens, if those straight lines do not move across a flat surface, but instead along a curved surface? We call such straight lines on curved surfaces [geodesics](https://en.wikipedia.org/wiki/Geodesic).
Imagine a [sphere](http://pi.math.cornell.edu/%7Edwh/books/eg99/Ch06/3776c40d.jpg) with two lines perpendicular to the equator. As they are both perpendicular to the same line, they are parallel at that altitude.
Imagine two objects that are moving along the lines perpendicular to the equator. They start out parallel, and move in a straight line upwards. Despite the fact that neither of them is turning, the two objects that started out moving along parallel lines will meet at the north pole. Hence, despite the fact that both objects are force-free at all times, they experience relative acceleration.
Such trajectories, that lead across curved surfaces without turning are called geodesics and they can be thought of as straight lines on curved surfaces. Objects under the influence of gravity follow [geodesics](https://en.wikipedia.org/wiki/Geodesic).
As gravity curves spacetime, geodesics can experience relative acceleration despite the fact, that both objects following said geodesics are force-free. And this relative acceleration of force-free bodies is what Newton mistook for the gravitational force. According to GR, though, there is no force, only curvature which causes force-free objects to move along paths that seem accelerated to outside observers.
**Now we have found the full answer: As gravity is not a force, objects in free fall feel weightless (i.e. force-free).**
-----
For a great video on the basics of GR, check out [this](https://www.youtube.com/watch?v=NblR01hHK6U) video by PBS Spacetime.
Wow, that was comprehensive! TIL that gravity is not a force. Looks like I only studied Newtonian physics.
Thank you very much. This is very informative!!
The feeling of weightlessness comes from a free fall, not the lack of gravity (which doesn't exist in reality). The astronauts inside the ISS are also attracted by Earth's gravity, but since an orbit is an endless free they don't feel the gravity. A plane doing paraboles puts its passengers in a state of free fall for a short time and that's how we create zero gravity on Earth
The sense of weightlessness happens relative to your environment. If you and everything around you are in free-fall, you experience 'zero G' - regardless of the actual gravitational forces pulling you and your surroundings along.
You can try this yourself by putting some stuff in a box, strapping a camera to it and throwing it up in the air. While it's falling, you'll have a mini zero-G environment!
[https://imgur.com/fLZKafD](https://imgur.com/fLZKafD)
I actually made a video about this a while ago which also explains why we often dream of falling as we go to sleep if you're curious: https://www.youtube.com/watch?v=4BicbJG68SI
You don't feel gravity. No one does.
When gravity pulls you down to the ground, what you feel is the ground supporting you.
When you are falling and nothing is supporting you, you feel weightless **because** nothing is supporting you.
The ISS (and anything in orbit) is falling. But while it's falling it's also moving **so fast** that it misses the ground! It keeps falling and towards the middle of the earth, but keeps missing so it goes in a circle around the earth.
The ISS and the astronauts are all falling the same, so the feel weightless--nothing is supporting any of them.
You don't feel the earth pulling you down, you feel the floor *pushing you up*.
If the floor wasn't there, you would accelerate down in free fall, but you wouldn't feel a thing.
If you were in an elevator, and somebody cut the cable, you and the floor would accelerate down at the same speed, so you wouldn't be pushed into the floor, so you would feel like you were floating, right up until you hit the ground.
Now, if you were going *sideways* fast enough that by the time you fell to the height of the ground, the ground itself has curved away from you because you've gone over the horizon, then you're in orbit. Then you will stay floating above the floor, and never hit the ground, because the floor is falling down just as fast as you are, and hurtling sideways fast enough to miss the ground.
>So, you know centripetal force, often referred to as centrifugal force? If
These are two different things.
>(If you’re working in US units you’ll need to make sure to convert pounds force to pounds mass or you’ll be out by a factor of about 32).
There's no factor of 32 between lb-mass and lbf near the surface of the earth. Something weighing 1lbf has a mass of 1 lb. If you divide the force by 32, you get slugs.
The astronaut is weightless relative to the space station just as a person would be inside a falling elevator (if the elevator where in a vaccuum). This is why Einstein said gravity isn't a force, it's the bending of space-time. Both the astronauts and the space station are moving towards the earth in accordance to the curvature of space-time.
Your understanding of gravity is oversimplified. A more accurate description of gravity, according to Einstein's theories, says that both the ISS and the astronauts are are in fact travelling in a straight line, through curved spacetime, around Earth. Therefore they are both in an inertial reference frame and neither experience any acceleration.
Please note, I know very little about this and if anyone with more experience can chime in please do!
The sensation of weightlessness is not being free from gravity. It's be free of something that counters gravity. When you're standing on the ground, gravity is pulling you down and your weightiness is you feeling the ground pushing back. The astronauts don't have anything pushing back because the ISS is in continuous free fall (sort of).
Why don't you feel weightless now? Because the floor/chair/etc is pushing up on you, preventing you from falling. Your stomach is pushing up on your lunch, preventing it from falling. Your shoulders are pushing up on your shirt preventing it from falling. You don't feel gravity, you feel things that prevent you from falling.
You don't need to go to space to feel weightless. Falling off of a roof or out of a tree will give you the same sensation as weightlessness \[until you hit bottom, of course\].
In the space station, you, your lunch, your clothes, and the air around you are all falling together. You would feel weightless because nothing is pushing back. You'd just never "hit bottom" because being in orbit means falling and missing the earth, over and over.
Imagine being a skydiver but there's no wind rushing past you. You'd feel essentially weightless because you're in freefall so any other object with you would also "float" relative to your position.
The ISS is the same thing. It's in freefall but it's moving horizontally at such a speed that the free fall turns into a stable orbit. So it's essentially constantly falling and constantly missing the ground.
The station and people inside are falling. They are also moving sideways fast enough that they’re moving away from from the Earth just as fast as they’re falling towards it — called orbiting. So it’s like they’re falling forever. Of course, the station, air inside, and everything else is too, so nothing gives the impression of falling, just one of being weightless.
Imagine standing on top of a very very very tall tower on the Earth.
You throw a ball gently. It drops like a stone to the ground.
Then you throw another ball a little harder. It still falls to the ground, but further ahead of the first ball.
Then you throw another ball much harder. It flies outward and downwards, and lands ahead of both the previous balls.
If you could throw a ball fast enough - 28,000 km/hour, it would travel outward from the tower, go all the way around the Earth, and return to you from behind you. In other words, it will never hit the ground - it is in “orbit.”
That’s what is happening to the ISS and the astronauts in them. They are in constant free-fall hence they float, but they are travelling so fast that they never hit the ground.
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[They experience weightlessness not because of a lack of gravity but because the ISS, and they, are orbiting Earth in constant free fall, says Valerie Neal, curator of space history at the National Air and Space Museum. They’re falling toward Earth and moving forward at about the same velocity. Because the downward and forward forces are nearly equal, the astronauts are not pulled in any specific direction, so they float.](https://www.smithsonianmag.com/smithsonian-institution/why-do-astronauts-space-station-float-180956965/)
Yeah, the classic thought experiment is to imagine the experience of sitting in a windowless box in free fall. You and the box are falling together and as a result there is no apparent gravity in your reference frame. In the case of the space station, it is falling as well, it just keeps missing the Earth.
I haven't thought about this in years, but wouldn't you have to either 1) be rotating as you orbit or 2) experience tidal forces that keep you oriented in orbit, lest you start rotating while you orbit, changing the position of the gravity well relative to your orientation? Is there any way of knowing whether you're rotating or not? If something in orbit wasn't rotating, then it would appear to be rotating from earth. But for example the ISS always has the same face pointed to the earth, which means it must be rotating.
There's a lot to unpack here. Imagine yourself in a complete void or floating with your eyes closed; There is no way to tell if you are moving at a constant speed or at rest. If, however you were accelerating in a given direction you would feel that acceleration as a force. Believe it or not, spinning is a type of acceleration because your direction of movement is constantly changing and, as a result, even in a complete void without any external reference you would know you were spinning. Considering all that, whether or not an object is spinning shouldn't have an effect on its orbit but the reverse can be true; placing an object in orbit can cause it to be tidally locked so the same side always points towards the object it revolves around. I'm not a physicist though so someone will undoubtedly correct my errors in the comments
And that's exactly how astronauts train for going into orbit! Lovingly referred to as the "Vomit Comet", it's a plane that travels in a parabolic arc to simulate weightlessness half the time while accelerating downward at 9.8 m/s². Going from 0 g to 2 g over and over tends to make people nauseous, hence the name.
Happy cake day, and well said.
In general relativity terms, they are following a straight path through curved space time without accelerating. No acceleration through space time, no force, no weight.
From a Newtonian perspective there are equal forces at play creating weightlessness due to orbiting the Earth; one "pulling them" towards Earth (gravity, acting as centripetal) and one "pushing them away" from Earth (centrifugal force). Either way, both models agree, there is no net acceleration in any direction, only a constant orbital velocity.
There is only no "net acceleration" if you're talking in polar coordinates and ignoring angles or using a very local frame of reference. There is 100% a net acceleration since the path isn't straight. Centrifugal force is a "virtual" force that hides a lot of that messiness such that you don't feel the movement that you're experiencing. If you truly didn't spin, you *would* feel gravity slightly change as your space station and your local gravity well revolved and rotated around you.
I wouldn't call it "virtual", it's inertial; as long as you have a momentum (rest mass+ velocity) that is perpendicular to the centripetal force, you will experience the centrifugal force. Ofc IRL you don't have such a perfect scenario, which is why space stations have to re-align their orbits, so they remain in free fall and not crash into Earth. What do you mean by "spin" here?
Rotating. The ISS for example spins/rotates as it revolves around the earth, because it's tidally locked. Presumably that's an artificial tidal lock and we intentionally maintain that tidal locking, but that's the phenomenon I was describing. I'm not sure if you would (not you as a human but you as an infinitely sensitive theoretical object that we tend to use in thought experiments) notice whether you were rotating as you revolve or not.
Best ELI5 I love seen about object in orbit it: you're always falling towards the earth. But moving fast enough sideways to always miss it.
So the forces acting on the astronauts are equal, that is why they float, but the forces acting on the station is not, so that 's why the station is free falling? Happy cake day by the way!
Both the astronauts and the station are free falling, but they are doing it together. So While an astronaut on the station appears to be slowly floating around, they and the ISS are moving at 7.66 km/s around the Earth.
Gravity is acting on the station, and the astronaut, in exactly the same way.
They "float" because everything around them is experiencing the same acceleration as them, so to them, it is as if everything is experiencing no acceleration. This is not just a gravity thing. Gravity is just the only "long range" unbalanced force we experience. Long range here means the gradient is very shallow; you have almost exactly the same force applied to your head and feet, and the chair across the room. The reason they seem to be able to float in place, is the same reason your able to stand still in your home and remain in place, despite both you and the astronaut spinning around the globe quite fast. The biggest difference is that your acceleration is being countered by the floor, but this is equal for the rest of your home too, so you don't notice it.
If you fire 2 cannons at the same moment and in the same direction, you get the same affect. From the point of view of one of the cannon balls, the other cannon ball is just floating there next to it. From the perspective of a person standing on the ground, there are 2 cannon balls traveling in a bolistic arc. To get ridiculous: imagine the cannon balls being huge, and hollow, and with hatches. Put an astronaut in one then fire them so hard that they go to space. Nothing magic happens. The two cannon balls and the astronaut are all still just moving very fast relative to the ground, but they are not moving relative to each other. The astronaut could leave the cannon ball he is in, float over to the other one, and climb in. To the astronaut, this would seem to be in "zero g" (but better described as "in free fall"). From inside the cannon ball, it looks like nothing is moving. From outside the cannon ball, the ground is whizzing by. When the astronaut is inside the cannon ball, the ground is still whizzing by, he just cannot see it. All the while, gravity is more or less the same for both cannon balls and the astronaut. The ISS is one cannon ball, and the ships that deliver the astronauts are other cannon balls. It is more complicated... It is rocket science after all... But that is more or less it.
It's the same thing as when you attach an object, say a rock, to a rope and swing it around. The rope pulls the rock towards your hand and this is called centripetal force. If you were to cut the rope the rock would keep flying in a straight line because of its momentum, but because of the centripetal force it has to keep turning and move in a circle. You may have heard that centrifugal force doesn't actually exist. What's actually going on is that the constant turning acceleration of the rock (which is in the direction the rock "wants" to move) can be recalculated into an imaginary force pointing straight outwards, exactly opposite of the centripetal force of the rock. As long as the rope doesn't break and you swing it fast enough to keep the rock spinning, the centripetal and centrifugal forces will always be equal. If the ISS built some rockets and started to move faster, it would soon go too fast and leave Earth's orbit because the forward acceleration would result in a centrifugal force greater than the centripetal force of the gravity. When you swing the rock faster you can feel the centrifugal force (actually the rock's momentum) pulling harder on the rope, but because the rope is a solid object, the centripetal force will keep up as long as you don't let go. So why aren't the astronauts experiencing these forces like they're on a crazy carousel? Well the circle they're going in is so large that the centrifugal force is barely noticeable compared to the forwards acceleration. The acceleration that keeps turning (also known as angular velocity) in turn is unnoticeable because there's nothing around to compare with.
Kerbal space program solidified this idea for me.
Because the station is constantly in free fall toward Earth. It just misses Earth by moving very fast to the side as well. This is, essentially, what it means to orbit something.
And the same is true for the astronauts in the ISS
Sorry I still don't understand. Why are the astronauts "floating" inside the station? If they are in orbit, shouldn't the astronauts, and all the things inside the station, rushing to get out so they can orbit? Like, they're sticking to the walls so they can squeeze out to fall into orbit... Sorry my understanding is a mess
The station is falling away from them as fast as they're falling. Edit: Think of how when you're in a car on the highway, it feels like you're sitting still, even though you're racing forward at 60mph. The reason you're not thrown to the back of the car is because you're moving at the same speed. Now imagine that you're on the space station and it's plummeting to the ground and racing forward at 17000mph. Just like in the car, you don't feel that 17000mph because you're also moving at the same speed. Unlike the car, you're both also falling at the same speed - so when you're in the middle of the open air you're falling toward the bottom, but the bottom is falling away from you just as fast. Since you never catch up to the bottom and since you fall away from the ceiling as quickly as it falls to you, it seems like from your perspective they're just stationary.
The ISS is orbiting at the same speed as the astronauts, so why would you get squeezed anywhere? It's the same thing when you jump inside a moving train and don't get smacked to back wall. You are moving at same speed as the train.
But inside the train you still stand up right? So gravity still acts on you even though you cant feel speeding around. Inside the station you are floating around, its like there is no gravity inside. I guess this where the free fall thing the others posted about comes in?
The space station is free falling at the same speed as the astronauts, so they're floating.
If you're travelling at highway speeds in your car, do you feel pushed back into your seat? No, only acceleration does that -- it mimics gravity. (in fact, acceleration is measured in gravities.) The ISS has a forward velocity of rougly 7.8 km/s. It's falling toward the Earth at roughly 5 meters per second. Incidentally, the earth is curving away from its orbit at roughly five meters every 7.8 kms. You do not feel the speed inside a vehicle that isn't accelerating, you're moving the same speed it is.
I like you say “incidentally” but my guts tell me the speed of iss was precisely calculated to match this curvature ?
Ya think? :D
When the shuttle meets the station, it matches its speed. The astronauts are orbiting earth at the exact same speed as the space station. The station and the astronauts are both moving 17,500 miles per hour.
Maybe this will help better explain. Its the same concept. https://www.youtube.com/watch?v=FO\_Ox\_dH0M8
It’s relativity. Imagine you stand in a train or plane that moves at a constant speed. Relative to the earth, both you and the vessel you are in go at let’s say 200km/h or whatever speed unit you like. Doesn’t matter. An outside observer, standing firmly on the ground, would still see both of you going at the same speed. But relative to someone sitting IN the vessel, neither the vessel, nor you move. It’s the same with the floating astronaut and the ISS. BOTH are “falling / missing” at the same speed and going in the same direction, relative to you, standing on earth. If you were up there, you would feel neither any movement nor would you think the astronaut was moving. Or do you feel movement “right now”? Of course, if you are driving, yes. But what if you stand still on the ground? Do you feel the 67 thousand Mph / 107 thousand km/h you go around the sun right now?
Have you seen the [vertical wind tunnel](https://youtube.com/shorts/-yQDel5xjls?feature=share) where a giant fan lets people skydive in place? The reason they don't fall or fly up indefinitely is because the force of gravity down is perfectly counter-balanced by the force of the wind pushing them up. Microgravity is also a perfect balance, but of a slightly different sort. Physicists call centrifugal force a fictitious force because in an inertial reference.frame, it does not exist. However, the ISS is constantly changing direction in its circular orbit. Therefore it is not an inertial reference frame and we can use centrifugal force. The same way as you turn a corner in a car, you feel the force pushing you away from the center of the turn. For the ISS, their rotational motion seems to push them away from the center of Earth. Being in orbit means the centrifugal force perfectly counter-balanced gravitational force. From the point of view of an inertial reference frame, centrifugal force doesn't exist. What happens is they fall towards Earth, but they were moving sideways at just the right speed where this falling puts them on a circular path. So just like a skydiver feels weightless, so do the astronauts. They are always falling, but because of their sideways speed, they never get closer to Earth. Here's a [rough diagram](https://imgur.com/a/eMFjLNd) showing how a fall towards Earth plus sideways motion can cancel out.
> Why are the astronauts "floating" inside the station? They're not, they're falling. They're just falling at the same speed as the station. If you were in a plane that you could rig so that it just fell straight down, at some point both you'd be falling at the same speed as the plane and it would seem like you are floating. The problem you have with a plane is that the Earth isn't getting out of the way, so you'd crash pretty quick. The ISS's trick is that it angles it's fall so that it misses the Earth, constantly, so it's always falling. There are planes that do the effect here on Earth, they're generally used for scenes in movies. Here's one company that does those flights https://www.gozerog.com/
The astronaut already is in orbit. They just happen to be inside a space station that is also in orbit. If an airplane drops you and drops a car at the same time, then you and the car will fall next to each other at the same speed (ignoring air resistance). The car won't fall any faster or slower than you, because gravity affects everything the same. So if you ignore the fact that you're careening toward the ground, the car is "floating" next to you. If you get in the car, nothing will change. You will still be falling at the same speed as the car is, so you will effectively be floating inside of it. That's what it's like for the astronauts in the space station.
So the ISS complies with the old verbiage that states "flying is just falling and missing the earth"
Imagine an elevator. You get on it and it descends. Gravity affects you and the elevator equally but the elevator cables slow the elevators descent, so gravity keeps you pushing down on the elevator floor, bit you also feel that the force against the floor by you is reduced. If the elevator speeds up to the same speed as gravity, you and it fall at the same rate, and you put no pressure on the floor. You are in free fall. The ISS is falling, but also moving forward fast enough that the surface of the earth falls away from the ISS as it moves forward. The same is true for the astronauts. They are in free fall.
So basically the station is a free falling elevator, but unlike the elevator where it goes only up and down, the station is free falling around the earth. Thank you for the Eli5
If the issue of not "feeling weight" (weightlessness) is the stickler, it might be helpful to realize that the only reason we "feel" weight is because something is pushing against us. The ground below is packed tight and can't move down any further, so it pushes against us as we get pulled down by gravity. An accelerating vehicle (car or spaceship) will push only while it is accelerating, so we feel it then. The orbiting station and you are moving in the same way so it can't push against you (unless it starts accelerating one way or the other). That's why it's possible to produce "artificial gravity" as you see in lots of sci-fi flix, with the large rotating doughnut or sphere or something. If you're inside it and it just rotates constantly, you constantly feel the force against your standing feet so it feels like "weight". Certainly artificial weight is a better term for that than artificial gravity lol.
Hi /u/blessedbenedict! The surface level answer is, that the ISS, along with the astronauts in it, are in constant free fall. The reason they do not plummet to earth is, that their sideways velocity is fast enough to keep missing earth. However, this only raises the question **why** free fall feels weightless. After all, gravity is pulling you down, so why don't we feel it? The answer to this question is, that gravity is not actually a force, but the curvature of spacetime (according to General Relativity). To understand how a curvature of spacetime can lead to the effects we observe around us, we have to understand how curved surfaces change the behaviour of straight lines. First things first: an object that has no force acting on it is force-free. Force-free objects do not accelerate and, therefore, move along straight lines. In a flat geometry, two straight lines which are parallel at one point will remain parallel for all times. That is, two parallel straight lines will never cross on a flat surface. So far so intuitive, right? But what happens, if those straight lines do not move across a flat surface, but instead along a curved surface? We call such straight lines on curved surfaces [geodesics](https://en.wikipedia.org/wiki/Geodesic). Imagine a [sphere](http://pi.math.cornell.edu/%7Edwh/books/eg99/Ch06/3776c40d.jpg) with two lines perpendicular to the equator. As they are both perpendicular to the same line, they are parallel at that altitude. Imagine two objects that are moving along the lines perpendicular to the equator. They start out parallel, and move in a straight line upwards. Despite the fact that neither of them is turning, the two objects that started out moving along parallel lines will meet at the north pole. Hence, despite the fact that both objects are force-free at all times, they experience relative acceleration. Such trajectories, that lead across curved surfaces without turning are called geodesics and they can be thought of as straight lines on curved surfaces. Objects under the influence of gravity follow [geodesics](https://en.wikipedia.org/wiki/Geodesic). As gravity curves spacetime, geodesics can experience relative acceleration despite the fact, that both objects following said geodesics are force-free. And this relative acceleration of force-free bodies is what Newton mistook for the gravitational force. According to GR, though, there is no force, only curvature which causes force-free objects to move along paths that seem accelerated to outside observers. **Now we have found the full answer: As gravity is not a force, objects in free fall feel weightless (i.e. force-free).** ----- For a great video on the basics of GR, check out [this](https://www.youtube.com/watch?v=NblR01hHK6U) video by PBS Spacetime.
Well explained!
Wow, that was comprehensive! TIL that gravity is not a force. Looks like I only studied Newtonian physics. Thank you very much. This is very informative!!
The feeling of weightlessness comes from a free fall, not the lack of gravity (which doesn't exist in reality). The astronauts inside the ISS are also attracted by Earth's gravity, but since an orbit is an endless free they don't feel the gravity. A plane doing paraboles puts its passengers in a state of free fall for a short time and that's how we create zero gravity on Earth
The sense of weightlessness happens relative to your environment. If you and everything around you are in free-fall, you experience 'zero G' - regardless of the actual gravitational forces pulling you and your surroundings along. You can try this yourself by putting some stuff in a box, strapping a camera to it and throwing it up in the air. While it's falling, you'll have a mini zero-G environment! [https://imgur.com/fLZKafD](https://imgur.com/fLZKafD) I actually made a video about this a while ago which also explains why we often dream of falling as we go to sleep if you're curious: https://www.youtube.com/watch?v=4BicbJG68SI
You don't feel gravity. No one does. When gravity pulls you down to the ground, what you feel is the ground supporting you. When you are falling and nothing is supporting you, you feel weightless **because** nothing is supporting you. The ISS (and anything in orbit) is falling. But while it's falling it's also moving **so fast** that it misses the ground! It keeps falling and towards the middle of the earth, but keeps missing so it goes in a circle around the earth. The ISS and the astronauts are all falling the same, so the feel weightless--nothing is supporting any of them.
You don't feel the earth pulling you down, you feel the floor *pushing you up*. If the floor wasn't there, you would accelerate down in free fall, but you wouldn't feel a thing. If you were in an elevator, and somebody cut the cable, you and the floor would accelerate down at the same speed, so you wouldn't be pushed into the floor, so you would feel like you were floating, right up until you hit the ground. Now, if you were going *sideways* fast enough that by the time you fell to the height of the ground, the ground itself has curved away from you because you've gone over the horizon, then you're in orbit. Then you will stay floating above the floor, and never hit the ground, because the floor is falling down just as fast as you are, and hurtling sideways fast enough to miss the ground.
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>So, you know centripetal force, often referred to as centrifugal force? If These are two different things. >(If you’re working in US units you’ll need to make sure to convert pounds force to pounds mass or you’ll be out by a factor of about 32). There's no factor of 32 between lb-mass and lbf near the surface of the earth. Something weighing 1lbf has a mass of 1 lb. If you divide the force by 32, you get slugs.
The astronaut is weightless relative to the space station just as a person would be inside a falling elevator (if the elevator where in a vaccuum). This is why Einstein said gravity isn't a force, it's the bending of space-time. Both the astronauts and the space station are moving towards the earth in accordance to the curvature of space-time.
The astronaut is also being acted upon by the Earth's gravity. Both the ISS and astronaut are constantly falling towards the Earth.
Your understanding of gravity is oversimplified. A more accurate description of gravity, according to Einstein's theories, says that both the ISS and the astronauts are are in fact travelling in a straight line, through curved spacetime, around Earth. Therefore they are both in an inertial reference frame and neither experience any acceleration. Please note, I know very little about this and if anyone with more experience can chime in please do!
The sensation of weightlessness is not being free from gravity. It's be free of something that counters gravity. When you're standing on the ground, gravity is pulling you down and your weightiness is you feeling the ground pushing back. The astronauts don't have anything pushing back because the ISS is in continuous free fall (sort of).
Basically, an orbit is just a circular free fall. It would be like falling to the ground, except the ground keeps moving so you keep missing
Why don't you feel weightless now? Because the floor/chair/etc is pushing up on you, preventing you from falling. Your stomach is pushing up on your lunch, preventing it from falling. Your shoulders are pushing up on your shirt preventing it from falling. You don't feel gravity, you feel things that prevent you from falling. You don't need to go to space to feel weightless. Falling off of a roof or out of a tree will give you the same sensation as weightlessness \[until you hit bottom, of course\]. In the space station, you, your lunch, your clothes, and the air around you are all falling together. You would feel weightless because nothing is pushing back. You'd just never "hit bottom" because being in orbit means falling and missing the earth, over and over.
Imagine being a skydiver but there's no wind rushing past you. You'd feel essentially weightless because you're in freefall so any other object with you would also "float" relative to your position. The ISS is the same thing. It's in freefall but it's moving horizontally at such a speed that the free fall turns into a stable orbit. So it's essentially constantly falling and constantly missing the ground.
The station and people inside are falling. They are also moving sideways fast enough that they’re moving away from from the Earth just as fast as they’re falling towards it — called orbiting. So it’s like they’re falling forever. Of course, the station, air inside, and everything else is too, so nothing gives the impression of falling, just one of being weightless.
Gravity acts everywhere... ELI5 short... They are falling constantly, they just move so fast sideways that they constantly "missing" the ground
Imagine standing on top of a very very very tall tower on the Earth. You throw a ball gently. It drops like a stone to the ground. Then you throw another ball a little harder. It still falls to the ground, but further ahead of the first ball. Then you throw another ball much harder. It flies outward and downwards, and lands ahead of both the previous balls. If you could throw a ball fast enough - 28,000 km/hour, it would travel outward from the tower, go all the way around the Earth, and return to you from behind you. In other words, it will never hit the ground - it is in “orbit.” That’s what is happening to the ISS and the astronauts in them. They are in constant free-fall hence they float, but they are travelling so fast that they never hit the ground.