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Jumping, weightlessness and movement in tennis

  1. Dec 24, 2006 #1
    I have just jumped up in air and when my feet hit the ground my body is weightless for a couple of seconds isn't it?
    Is this why when in tennis that when you do a split step that you can move so fast in ether direction because you are weightless when your feet touch the ground?
  2. jcsd
  3. Dec 24, 2006 #2


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    Your body is never weightless. It always has mass, and always feels the force of gravity.

    Some people refer to free-fall as being "weightless," but this is just a poor choice of words. Your body is certainly not "weightless," even in that sense, after you hit the ground.

    - Warren
  4. Dec 24, 2006 #3
    I hear that when you body hits the ground from the split step that it streches the musles.
  5. Dec 24, 2006 #4


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    And stretched muscles somehow make you "weightless?"

    - Warren
  6. Dec 24, 2006 #5
    I ment when you hit the ground you musles strech. I just don't under stand the physics of the split step thought you guys would be able to help.
  7. Dec 25, 2006 #6
    Mass and weight are different. Weight can change (to zero or weightless) but mass will be the same.
    If you still have weight during free fall, prove it. Or rather how can one prove your weight during free fall, and remember weight is different then inertial mass.
  8. Dec 25, 2006 #7

    D H

    Staff: Mentor

    Weight is a poorly understood subject. An object's "actual weight" is often defined as the force on that object due to gravity. Scales do not measure "actual weight". They measure "apparent weight". I would call an object's "apparent weight" its "real weight" because one can build sensors to measure it (e.g., scales).

    The object's "actual weight" is actually unmeasurable. There is no device that can measure "actual weight". Why? Free-fall is indistinguishable from inertial motion per the equivalence principle. Per general relativity, free-fall is inertial motion. One cannot measure gravity (directly, that is).

    Weightlessness is a condition of lack of apparent weight. The accelerometers in a spacecraft orbiting the Earth and the vestibular systems of the astronauts in that spacecraft register near zero acceleration. When a person on the Earth jumps up or a skydiver jumps out of an airplane their apparent weight momentarily vanishes. The Earth-bound jumper's weight returns when the jumper lands. A skydiver's weight builds up as he builds up speed and reaches normal weight when the skydiver reaches terminal velocity.
  9. Dec 25, 2006 #8
    You could accelerate (to either side) faster (more suddenly) if you increased your grip on the ground. One way to do this is to increase the pressure you place on the ground, which inevitably occurs naturally as you are landing from a jump. No idea whether this is relevent to your "split step". There could also be more physiological or mental elements: eg., by deferring judgment (for the moment while your full weight isn't upon the ground) you could be less likely to twitch, hesitate further, or be fooled by a "faint" (any of which are costly in competition).
  10. Dec 25, 2006 #9


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    Well, you could measure "weight" when you add force to a object in space with 0g and no air resistance. The amount of force you need to push an object in a certain amount of time in a certain amount of length measures weight. The gravity on earth equals 9.8 N which is the force you need to add to a object in space (no gravity) to make it accelerate in 9.8m\s^2 as we accelerate on earth leaving out air resistance.

    When you jump you could say you are weightless because you are in free fall. This is only if you define eightless as zero G's. But your gravitational weight differs from where you are, what amount of force gravity pulls you in. You inertianl weight stays the same, as it is your mass.
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