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Why are astronauts weightless?

  1. Apr 8, 2013 #1
    If they're orbiting Earth, then they're in uniform circular motion. And if they're in uniform circular motion, then they experience a centripetal force (and thus centripetal acceleration) towards the center of Earth. Therefore, they are constantly accelerating (direction changes, magnitude does not change) and you can feel acceleration but it appears that they do not feel any force.
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  3. Apr 8, 2013 #2


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    You don't have to go into orbit to be weightless. If you were to go to the "Burj Al Arab" in Dubai, the tallest building on earth, and jump of the top, you would be weightless- until you hit the ground! The force of gravity would still be acting on you but you would not feel any weight because there is nothing pusing up on you.

    When astronauts are in orbit, the force of gravity is acting on them and their ship equally. Astronauts and ship are all falling toward the center of the earth. There is no "floor" or "ground" pushing up on them so they feel no weight.
  4. Apr 8, 2013 #3


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    Or for a safer experience, arrange a ride on the "Vomit Comet". :smile:
  5. Apr 8, 2013 #4


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    What you usually feel as a force, is the deformation of your body caused by it. The special thing about the force of gravity is that is is proportional to mass, so it is accelerating small objects almost uniformly, without causing significant deformations. There fore you don't feel it, if it the only force acting on you.
  6. Apr 8, 2013 #5
    So relative to the frame of reference of the ship, the astronauts appear to be weightless, but they are not actually weightless. However, do the astronauts not feel the pull of the force? If you're in an zero-acceleration frame of reference, you cannot tell what velocity you are traveling at. However if you're in an accelerating frame of reference, can't you feel that you are accelerating?

    So because the force is proportional to mass, the acceleration is constant, right? Doesn't gravity acceleration both small and large objects almost uniformly?
  7. Apr 8, 2013 #6


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    If free falling objects are small enough, so gravity gradient is negligible, they are accelerated uniformly along their volume. You cannot "feel" such acceleration, because it doesn't create deformation.
  8. Apr 8, 2013 #7


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    If all parts of your body are accelerated uniformly, as it happens in the gravitational field(caveat: far away from the source), then no, you can't.

    Human beings perceive linear acceleration mostly via the otholitic organ in the inner ear. In it, suspended in viscous fluid are small crystals. Since the crystals are not rigidly connected to the rest of the body, whenever your head experiences acceleration they lag behind the bulk of your tissues due to inertia. Their displacement bends the tiny hairs lining the walls of the organ. These hairs are connected to nerves that then transmit the signal to your brain.

    Another, similar in principle, way we feel acceleration is via kinesthesia. I.e., the relative position of our limbs, that also changes due to inertia, as long as the accelerating force is applied non-uniformly.

    If the force permeats the space in uniform fashion, all parts of your body change their velocities at the same rate, so neither the crystals in your inner ear, nor your limbs ever lag behind, and so, never produce a neural response in your brain.

    So, in a way, one could say you never really feel gravity, even sitting in your chair. It's the ground pushing the chair pushing on your bottom parts that disrupts the uniform acceleration of the gravity, providing localised force that then is transferred to the rest of your body through the tissues, with inertial lag across the board.

    Now, IF you are close to the source of gravity, the parts of your body farther from the source experience less acceleration(inverse square law) than the closer ones. These are tidal forces that you could feel as being stretched along one axis and compressed along the two other, providing your total length is significant as compared to the distance to the source and its strength(effect also known under the scientific term of spaghettification).
  9. Apr 8, 2013 #8
    forget about what people may 'feel'....that's too subjective. [I assume you are interested in the physics, not 'feelings']

    The astronauts 'appear' weightless because they ARE actually weightless. Have you ever seen pictures??...they' float' inside a space station and so does, say, a tool they release. Things maintain their relative positions inside...like in free fall because it IS freefall. Everything nearby floats because there are no net forces. An accelerometer shows no acceleration.

    [In the context of General Relativity gravitation is space-time curvature and a body in free fall has no force acting on it as it moves along a geodesic.....a particular type curve in spacetime.]

    Some interesting Newtonian [not GR] comments here:

  10. Apr 8, 2013 #9
    The astronauts 'appear' weightless because they ARE actually weightless. Have you ever seen pictures??...they' float' inside a space station and so does, say, a tool they release. Things maintain their relative positions inside...like in free fall because it IS freefall. Everything nearby floats because there are no net forces. An accelerometer shows no acceleration.

    It depends on what you mean by 'weightless'
    If 'weight' is the force due to gravity then they are NOT weightless. There is a resultant force on the astronaut and the spacecraft....centripetal force.

    How do you 'experience'/'feel' (this is not subjective) weight?....It is the upwards force on your feet/backside as indicated on a weighing machine. The astronaut experiences no force between himself/herself and any weighing machine.
    They EXPERIENCE weightlessness

    A fish 'floats' in water....is it weightless....does a fish experience the force due to gravity?...have you seen pictures of fish floating....there ARE NO NET forces on a fish !!!!
    What is the difference between a fish and an astronaut??

    Draw a freebody diagram of forces on an astronaut and forces on a fish...use this diagram to explain 'weightlessness' (do not refer to GR)
    Last edited: Apr 8, 2013
  11. Apr 8, 2013 #10


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    The astronaut is undergoing centripetal acceleration, so there obviously is a net force on him. Bringing GR into it will probably just confuse the OP.
  12. Apr 8, 2013 #11
    These are ridiculous statements, completely contrary to conventional teaching and text book explanations.
    This is not physics !!!!
    Someone with some authority should check posts like this.
  13. Apr 8, 2013 #12


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    This is a forum, not a scientific journal. We don't really appeal to an authority. If you feel someones post is incorrect, then correct them. If you are right, then you're right.
  14. Apr 8, 2013 #13
    There is a difference between mass and weight I think that's what he is missing.
  15. Apr 8, 2013 #14
    The rules of these forums dictate that posts should be in line with current/text book explanations.
    What is stated here is plane wrong........ in my opinion.
    As far as I understand it these are not 'opinion' forums
    Anything else should be backed up by some sort of references that can be checked.
    The 'report' option is only for spam, advertising, etc.
  16. Apr 8, 2013 #15
    It's not really wrong, they are [just about] weightless in microgravity. Their mass doesn't change but their weight changes with gravity.
  17. Apr 8, 2013 #16
    Don't astronauts feel weightless due to the equivalence principle? Technically, above earth, in space, the gravity is around 9/10 as strong as on the ground. The reason that causes weightlessness is because the station astronauts are in, is travelling so fast that the earth curves away under the ship. So they are not weightless, they are constantly falling, but due to the speed of their orbit, the earth curves away below them just as fast that they never seem to fall relative to the ship: thus, causing what they feel as weightlessness.
  18. Apr 8, 2013 #17
    What is 'microgravity'...cant find it in the index of my text book !!!!
    How does weight change with gravity (I think I have the answer to this but I am interested in your view)
  19. Apr 8, 2013 #18
    juzzagon....you are not wrong
  20. Apr 8, 2013 #19


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    That's just a another way to say the same thing:

    force proportional to mass -> force can be seen as an inertial force -> equivalence principle
  21. Apr 8, 2013 #20


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    It's not really wrong, in the context of GR. That should be made clear, otherwise it will confuse.

    The general explanation (Newton or GR): Gravity is a force (real or inertial) that accelerates everything by the same amount. So it doesn't cause deformation of the body and cannot be felt.
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