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Simple Question on Gravity to Earth

  1. Aug 9, 2014 #1
    Hi Folks,

    Why does a parachutist experience 0g (assuming falling in a vacuum, no drag) when falling towards the earth?
    How can he not experience gravity when he is in a gravitational field which is pulling him towards the earth.
    Im sure this is a classical question but I have not found a an answer online
    Can anyone enlighten me?

  2. jcsd
  3. Aug 9, 2014 #2


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    Staff: Mentor

    The "g" in "0g" is not a measure of the gravitational force you're experiencing, it's a measure of the forces that are stopping you from moving freely. When you're standing on the surface of the earth, your body wants to accelerate under the influence of gravity downwards at 9.8 meters per second per second, but the surface of the earth is pushing upwards on the soles of your shoes with sufficient force to hold you stationary. If you were standing on a spring scale (these devices measure force, not mass) it would show some non-zero force.

    However, when you're skydiving there's no force stopping you from falling freely. A scale under a skydiver's feet would read zero (in fact, we'd have to fasten it to his shoes to stop it from drifting away). If this isn't completely clear, you could imagine standing on a spring scale on top of a trapdoor - as long as the trapdoor is closed the scale reads your weight, but as when it opens and lets you and the scale fall free, the reading goes to zero as the scale is no longer being squeezed between your feet and the trapdoor.

    As an aside... When astronauts in orbit around the earth experience 0g, it's not because they aren't experiencing gravity (if that were the case, there wouldn't be anything holding them in orbit), it's because they're free-falling just like the skydiver. The only difference is that their trajectory doesn't intersect the surface of the earth while the skydiver's trajectory does (which is why he will not be happy if his parachute doesn't open to stop his free fall).
  4. Aug 9, 2014 #3
    So the chutist does not feel any load on his body during vacuum freefall? Ok, that makes sense.
  5. Aug 10, 2014 #4
    an important point to note in this answer is that to remember to measure the forces from an inertial frame otherwise the pseudo force concept will come into play. that is avoid viewing the skydiver in the frame in which he is at rest
  6. Aug 12, 2014 #5
    Right. Another way to think of it is that gravity pulls on every single piece of your body equally. Every cell and every atom in your body is experiencing the same force, and so there's no stretching or pulling or pushing between them going on. And so, you will feel NOTHING. Weightlessness.

    On earth, even though gravity is still pulling on ALL of you equally, the ground is pushing back on you only at your feet (or whatever). That force is then transferred through your bones and flesh, and you WILL feel that.
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