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Someone Explain these Concepts to ME!

  1. Oct 12, 2005 #1

    Ok I have several concepts I've tried to undestand but still can't seem to get how it works... :confused:

    1) In sports, you see people "hang" in the air? What exactly is the mechanism behind this...All I can understand is that say a basketball player is jumping and his legs are bent vs straight. He can "stay" longer in the air because his center of mass stays at the same level?? or something like that?

    2) Also, during an airplane takeoff, you have a odd feeling due to the acceleration but when you're in the sky, eg. once the plane levels off, you don't feel anything...why the heck is that?

    3)Why is it that you get on an elevator and then when it goes up, you feel heavier? my book said something about the reaction force of the ground being less...etc, I want to know a bit more.

    If anyone can just answer one of these, that would be much appreciated! :rofl: Remember, dumb it down, I mean really dumb it, I'm not very physics apt lol.

    Thanks all.
  2. jcsd
  3. Oct 12, 2005 #2


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    1) You're likely watching video in slow motion!

    2) You only have the feeling of acceleration while you're acceleration. Once the craft has reached cruising altitude and speed it is no longer accelerating.

    3) You do not feel heavier in an elevator that is going up - except while it is accelerating. For most of the trip the elevator is moving at constant speed - i.e. it is not accelerating. At constant speed, the floor is pushing you upward with a force equal to your weight. During acceleration, it has to push you harder to balance your weight AND to accelerate you upward.
  4. Oct 12, 2005 #3
    1. The only way I can think of that you could 'hang' in the air if you jumped and quickly curled up your legs. Then when you are at your highest point you straighten your legs. By straightening your legs your centre of mass is still accelerating downwards but your upper body, briefly, remains in the same place. The tradeoff for this is that your maximum height would be reduced.
  5. Oct 12, 2005 #4


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    Most of the "hanging" effect is probably due to the slow motion video. There is a slight effect however, if the player jumps into the air with legs pulled up then at the peak of his jump stretches them, he will "appear" to be in the air longer (i.e. his hands will maintain a constant elevation for a bit). What he is essentially doing is shifting how his body is distributed around his center of gravity. While his center of gravity is undoubtedly subjected to parabolic projectile motion, he can keep upper parts of his body in high up longer by sacrificing having his legs go lower.

    Other cool effects you should notice is now an ice skater can start spinning faster when he/she pulls in his/her arms or legs.
  6. Oct 12, 2005 #5
    lol, what about ballet dancers, who jump in the air and seem to hang there. I think Kazza's explanation makes some sense.

    I still don't get why you can stay up longer by sacrificing some height or redistributing some of your body. Is that so the center of gravity is not falling as quickly as if his legs (which contain a large portion of the mass) are straight? :uhh:

    About the airplane thing, I thought it could be due to constant velocity are something at flying altitudes?

    Thanks all.
  7. Oct 13, 2005 #6


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    They aren't "hanging". It is a total illusion. Watch their heads and it should show constant acceleration down.
  8. Oct 13, 2005 #7


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    You can only feel forces. When you are finally in the air, there is no force acting on you relatively. When you are taking off, you are feeling the acceleration of you going up and you going down at accelerating rates.

    Think about how we live on Earth. We're traveling at what... 5 or 10 or 20,000 mph or something to that effect but we sure don't feel like it. That is because there is no force being applied, you're simply moving at a constant speed. This also makes me wonder about something myself that I want to ask in another thread lol.

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