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Gravity for astronauts in orbit

Why do astronauts appear weightless despite being near the Earth? Created by Sal Khan.

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  • marcimus pink style avatar for user emo_girl93
    ok.so if the space shuttle is moving super fast than how come those astronauts don't feel any thing when they go out of the ship for what ever reason they show it like they are motionless you know like floating away why don't they move downwards to the earth?
    (108 votes)
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    • blobby green style avatar for user aiabgold
      When they step (or float) out of the shuttle, the astronauts are still moving at the same speed as the shuttle. Since there's nothing to slow them down, they continue to move at the same speed and direction as the shuttle. If someone were to jump out of a moving vehicle (don't try this at home!), the person would, just for an instant, be moving at the same exact speed as the vehicle. However, friction from the air around you—and the asphalt below—would quickly slow a person down. Because there's nothing to slow the astronauts down, they continue to move at the same speed as the shuttle.
      (229 votes)
  • male robot donald style avatar for user Samir
    Sorry still didn't get it. Sal says, "The astronauts are going so fast, they keep missing the earth".
    What is that supposed to mean? How can they "miss" the earth? And if they do miss it at a point, why do they keep orbiting?
    Also, is this centripetal force? Then why doesn't the shuttle fly towards the centre of the earth?
    I guess I can also ask the same for comets, planets etc.
    (19 votes)
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    • aqualine ultimate style avatar for user DeadshotJoeZ
      Try to imagine it using the centripetal force diagram. For example, tie a stone to a rope and spin it. Here your hand (assuming you are holding one end of the rope and spinning it) is the Earth, the stone is ISS and the Tension of the rope is gravity. I know this might be a bit difficult but hope it helps.
      (1 vote)
  • leaf green style avatar for user Vamshireddy Mk
    My mass is what the stuff im having in my body. if i get into space station i become free from gravity. now there, if i wish to measure my mass or weight, what is the method of doing it.
    (7 votes)
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    • blobby green style avatar for user Mark G
      Your mass is a measure of how difficult you are to accelerate given a certain amount of force (it can be described as the proportionality constant that relates these two vectors). Like this second definition implies, your mass is constant. Your weight, on the other hand, is the amount of force exerted upon you due to acceleration by a gravitational field. As gravitational acceleration varies depending on the strength of the field (you weight less on the moon or at the Earth's equator), this value is not constant. In space, assuming no gravitational field--or one that could be approximated as zero--you would be weightless. You would still have mass however that can be measured in space using what is called an "inertial balance." These devices use the oscillations of a spring with a known spring constant to determine the mass of an attached object and work in microgravity scenarios.
      (20 votes)
  • blobby green style avatar for user yoni.dimitrova
    Why don't the astronauts fall while spacewalking? I'm guessing it's beacause they are attached to the space station which is moving so they move too. But if they were not attached to it, would they just fall on the surface of the Earth?
    (4 votes)
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    • leaf green style avatar for user knutover
      They do fall. In fact, both the astronauts and the space station and all other things in orbit are constantly falling in exactly the same way as a rock falls to the ground when you drop it. We say that things in orbit are in a free fall. The reason why they don't eventually hit the ground is that while they are falling downwards, they are also moving very quickly forwards.

      The Earth is round. This means that the ground is curving away from you in every direction. The most visible effect this has is the horizon, which is the distance where the ground has curved so much away from you that you can't see it anymore. The fact that the Earth is curved also means that if you were to move forwards in a perfectly straight line, the ground would drop away from your feet. The faster you move, the faster the ground would drop. At a certain speed, the ground would be dropping away as fast as you would fall towards it. This means that if you move at this speed, even though you are constantly falling towards the ground, you never hit it, since the ground is curving away from you at the same rate. You are falling down, but by the time you have fallen to where the ground was, you have moved so far that the ground is still just as far below you. This is what we call orbiting.

      Since you are in a free fall when you are in orbit (and you are also generally above the atmosphere, since that would slow you down enough to hit the ground pretty quickly) there is no force that is pushing on you like the ground pushes on you on the surface. This makes it seem like there is no gravity, but in fact the force of gravity is almost as strong at the space station as it is on the ground. It is just that instead of being canceled out by the force from your shoes or your chair, it is being canceled out by the centrifugal force of your orbit around the Earth. Since both the astronauts and the space station are moving with the same (really fast) velocity, they are at rest relative to each other, and the astronauts can float around the station on their space walks. Constantly falling, but never hitting the ground.
      (21 votes)
  • spunky sam blue style avatar for user Talha
    If they have so high velocity so that they go around the earth that means that we cannot reach the moon.Because when ever we are going little far from earth (let we go little far than setalites) our shuttle would be in free fall and just moving round and round the earth.
    (7 votes)
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  • duskpin ultimate style avatar for user Abdul Rehman
    i cant understand Why do astronauts appear weightless despite being near the Earth?
    (5 votes)
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    • leafers ultimate style avatar for user Diogo Ribeiro
      Imagine there is no atmosphere in the Earth. You're superman and are throwing rocks horizontally. When you throw the rock really really fast, it is possible for it to fall at the same rate the Earth is curving, which means it will stay rotating around the Earth forever.
      This is the same principle that allows satellites and astronauts to stay in orbit. They are falling all the time, but missing the Earth every time. It looks like there is no gravity for them because everything around them is also falling with them.
      (7 votes)
  • blobby green style avatar for user Can Acar
    Is there any specifically measured distance between the earth and space that any spacecraft or body do not feel gravity?
    (4 votes)
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    • starky ultimate style avatar for user Shashwat Mishra
      the gravitational field of the Earth extends infinitely throughout space but its intensity is so less that it doesn't effects bodies outside a certain limit. just like Andrew said, the strength of the gravitational field declines with the square of the distance which is known as the inverse square law;
      (7 votes)
  • blobby green style avatar for user sonika
    If the astronauts are near the earth and are experiencing gravity to keep them in orbit, why would they float, they should just be sitting on their seats and going around the earth, why is the gravity affecting the space shuttle- keeping it in orbit but not affecting the astronauts inside it and keeping them down in their seats?
    (3 votes)
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    • male robot hal style avatar for user Satwik Pasani
      They are indeed going around the earth. As a part of the shuttle-astronaut system, they are acted on by gravity and they go around the earth.
      What you are perhaps confusing about, is the force necessary to keep them in their seats. This must be a force that pulls them towards the seat while the seat does not concomitantly move downwards (otherwise they would not be able to sit in the seat). This is sort of a force acting on the astronaut relative (in a fuzzy way) to the seat, which gravity cannot provide since it acts on both the shuttle and the astronaut.
      (3 votes)
  • aqualine sapling style avatar for user Jason Koo
    Does time affect gravity? If it does, is time traveling through space time possible?
    (2 votes)
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  • female robot ada style avatar for user The Midnight Rose
    What creates mass and gravitational pull? As in, if we pulled a bunch of space junk together and created a planet-sized hunk of waste, would that have gravitational pull, and, if not, why wouldn’t it?
    (2 votes)
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    • aqualine ultimate style avatar for user Inspiron13
      From a Newtonian perspective, mass creates gravity. From Einstien's perspective, mass, energy, and momentum create a manifestation of curved spacetime, which we call gravity.

      If we pulled a bunck of space junk and created an artificial planet, it woudl still have that gravitional our pull. This is because even you, your computer, etc. has gravity...but too miniscule to make a noticible difference to us.

      If we took a million of computers and put them together, the small gravity of those computers would add up to become a huge gravitional field.

      So the answer to your question is yes, it would have gravitional pull.
      (3 votes)

Video transcript

When you look at footage or photographs of astronauts in space, it doesn't look like there's any gravity at work here. Everything is not falling down in one direction. In fact, it's not even clear what up or down is. Everything just floats around. If I were to push off of this wall, I would just float in that direction. So it doesn't look like there's this overarching influence like gravity that's trying to pull everything down. But the question is that these astronauts are still not too far away from a supermassive body. In fact, the space shuttle gets up only a couple of hundred miles above the surface of the Earth. So the space shuttle, if I were to draw it to scale, would probably be right about there. And we know that the force of gravity between two objects is equal to big G, the gravitational constant, times the mass of the first object, times the mass of the second object over the distance between the two objects squared. And if the space shuttle is right here, only a few hundred miles above the surface of the Earth, this r isn't that different. It's a little bit further than if you were at the surface of the Earth. Remember that r is measured from wherever you are to the center, from the center of the Earth, or really the center of the object to the center of the Earth. The center of the Earth represents most of the distance here. So if I'm at the surface of the Earth or if I'm just a few hundred miles above the surface of the Earth, it's not going to change r that dramatically, especially in terms of percentage. So when you look at it this way, it seems pretty clear that the force of gravity for someone who is in space only a few hundred miles above the Earth should not be that different than the force of gravity for someone who is on the surface of the Earth. So my question to you is, what gives? If there should be gravity in space, how can we see all of these pictures of people floating around like this? And the answer is that there is gravity in space, and that these people actually are falling. They're just moving fast enough relative to the Earth that they keep missing it. And let me show you what I'm talking about there. Let's say I'm sitting here in Africa, and I were to shoot something, if maybe I have a really good sling shot, and I were to sling something super fast and maybe at a 45 degree angle, it might take off a little bit and eventually hit another point. And this would actually already be a super duper slingshot. I just made it travel a couple of thousand miles or at least over 1,000 miles. If I make it go a little bit faster, if I put a little bit more force on, if I just propelled the projectile a little bit faster, it might go a little bit further, but it will eventually fall back to the Earth. Let's try to propel it a little bit faster than that. Then it'll still fall to the Earth. Let's propel it even faster than that. Well then, it's still eventually going to fall to the Earth. I think you might see where this is going. Let's go even faster than that. So if we go even faster than that, eventually it'll fall to the Earth. Even faster than that, so if you were to throw an object even faster than that, it would then go really far and then fall to the Earth. I think you see what's happening. Every time you go faster and faster, you throw this projectile faster and faster, it gets further and further, up to some velocity that you release this projectile, and whenever it's trying to fall to the Earth, it's going so fast that it keeps missing the Earth. So it'll keep going around and around and around the Earth, and a projectile like that would essentially be in orbit. So what's happening is if there was no gravity for that projectile, if there was no gravity, the projectile would just go straight away into space. But because there's gravity, it's constantly pulling it towards the center of the Earth, or the center of that projectile and the center of the Earth are being pulled towards each other, I guess is a better way to think about it. The force of gravity is doing that. And so it's curving its path. And if it's going fast enough, if the projectile or whatever object we're talking about is going fast enough, it'll just keep going round and round the Earth. And since there is almost, pretty much, for most purposes no air if you go high enough, especially the altitude that the space shuttle is, no noticeable drag, this thing can just keep on going for a substantial amount of time. Although there is just a little bit of drag, and that's why over time you do have satellites slow down, because there is just a little bit of air resistance. So the answer to this conundrum is that there actually is gravity. It's not a gravity-free environment. It's just that the astronauts and the space shuttle and everything else that's in the space shuttle, it's all falling, but it's moving fast enough that it never hits the Earth. It keeps missing the Earth. It keeps going round and round and round, but it is completely under the influence of gravity. If they were to just slow themselves down, if they were to just brake relative to the Earth, and if they were to just put their brakes on right over there, they would all just plummet to the Earth. So there's nothing special about going 300 or 400 miles up into space, that all of a sudden gravity disappears. The influence of gravity, actually on some level, it just keeps going. You can't, it might become unnoticeably small at some point, but definitely for only a couple of hundred miles up in the air, there is definitely gravity there. It's just they're in orbit, they're going fast enough. So if they just keep falling, they're never going to hit the Earth. And if you want to simulate gravity, and this is actually how NASA does simulate gravity, is that they will put people in a plane, and they call it the vomit rocket because it's known to make people sick, and they'll make them go in a projectile motion. So if this is the ground, in a projectile path or in a parabolic path I should say, so the plane will take off, and it will do a path exactly the same as something in free fall or in a parabolic path. And so anyone who's sitting in that plane will experience free fall. So if you've ever been in, if you've ever right when you jump off of a or if you've ever bungee jumped or skydived or even the feeling when a roller coaster is going right over the top, and it's pulling you down, and your stomach feels a little ill, that feeling of free fall, that's the exact same feeling that these astronauts feel because they're in a constant state of free fall. But that is an indistinguishable feeling from, if you were just in deep space and you weren't anywhere close any noticeable mass, that is an identical feeling that you would feel to having no gravitational force around you. So hopefully that clarifies things a little bit. To someone who's just sitting in the space shuttle, and if they had no windows, there's no way of them knowing whether they are close to a massive object and they're just in free fall around it, they're in orbit, or whether they're just completely far away from any massive object, and they really are in a state of or in a place where there's very little gravity.