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Space Station Floating Objects

  1. May 25, 2014 #1
    I am confused about why astronauts and objects around them float in The Space Station [SS].

    In space a long way from massive bodies, objects float.
    In free fall, objects behave as if in a Gravity free region; they also float.

    But the SS has 90% of the Gravity that exists on Earth.
    The SS is not accelerating in orbit, so its velocity is constant.

    So shouldn't objects sink toward the Earth
    move toward the rear of the SS, opposite the direction of motion
    [just as a rock in a wagon appears to move to the back of the wagon].

    Which assumption or reasoning is flawed here ?
  2. jcsd
  3. May 25, 2014 #2


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    This doesn't have anything to due with GR at the moment, it's purely classical physics. Your arguments are purely Newtonian.

    If the space station was not accelerating, it would be moving in a straight line. But it's not moving in a straight line, it's moving in a circular orbit.

    Thus, the space station is accelerating, because it's not moving in a straight line. The type of acceleration associated with moving in a circle is known as "centriptal acceleration". The centriptal acceleration of the space station is equal to the Earth's gravity at its height.
  4. May 25, 2014 #3
    Yes the Centripetal Acceleration is created by Gravity.
    So the SS is in free fall vertically.
    That would explain the floating objects vertically.

    But the SS must also fire engines perpendicular to the fall to stay
    in orbit. If it does this on a continuous basis, then objects would
    appear to move to the back of the SS opposite the direction of
    horizontal acceleration.
    If they fire only intermittently to maintain orbit height then
    the motion when not accelerating is inertial and again objects float.

    BTW, this Q came about because Einstein's thought process
    about GR began with his Equivalence Principle of Gravity and Acceleration.
    Free fall is part of the reasoning that led him to GR and geodesics.
  5. May 25, 2014 #4


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    While they fire the engines, the station is not in free fall anymore, so the free falling astronauts accelerate relative to the station.

    I don't see what our question has to do with Newtonian gravity vs. GR. Both models explain the relative acceleration between station and astronauts.
  6. May 25, 2014 #5
    The ISS is in free fall. It doesn't "fire engines perpendicular to the fall to stay
    in orbit". It only does that occasionally to prevent orbit decay due to air drag and solar wind.
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