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Free fall ?

  1. Sep 18, 2010 #1
    If I am in free-fall towards earth or in this case lets use a planet with no atmosphere to get rid of air drag. So I am in free-fall and then I throw a ball up above me. From my point of view the ball would appear to keep going away from me until i hit the ground. Is this correct?
     
  2. jcsd
  3. Sep 18, 2010 #2
    At that moment, of course it would.
    What's your point?
     
  4. Sep 18, 2010 #3
    Im just making sure that from my point of view , it would always look as if it were going away from me .
     
  5. Sep 19, 2010 #4

    Femme_physics

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    I think it depends if the ball weigh more than you, and then you have to take aerodynamicability into account...

    But saying basketball, tennis ball, etc... light ball objects, then yes...I'd reckon this would be the case.
     
  6. Sep 19, 2010 #5

    Danger

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    That would be good thinking under normal circumstances, but Cragar did specify that there is no atmosphere.
     
  7. Sep 19, 2010 #6

    Femme_physics

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    Yea, but there's still gravity.
     
  8. Sep 19, 2010 #7

    Danger

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    Correct. Aerodynamics, however, doesn't apply when there is no aer for the thing to be dynamic with.

    edit: I originally, for reasons based upon excessive Scotch, treated "Aerodynamics" as a plural and thus followed it with "don't apply". How wretchedly embarrassing. :redface:
    Anyhow... my friends will forgive me, 99.7% of you don't give a **** because you don't know me, and the remainder hiding in the trees can consider themselves invited to emerge and bite me.

    Okay, now... I meant the first 2 sentences of my edit, up to the ":redface:". As I mentioned to another member earlier this evening, I used to be a pro writer. Since going onto the meds for my ADD about 10 years ago, I haven't been able to write a damned thing. For some reason, a touch of creativity crept in while I was doing my edit, and it was weird. I'm leaving it up, with this explanation, rather than delete it. I want to assure everyone that there is no animosity intended toward anyone... it's just something that my fingers did when my brain wasn't paying attention.
    Cheers to all;
    Dan
     
    Last edited: Sep 19, 2010
  9. Sep 19, 2010 #8

    Cleonis

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    You write 'would appear to keep going away from me'.

    An occupational hazard of being a physicist is that the word 'apparent' is used with more gravity than in everyday language. When a physicist says 'apparent' he means that looks are deceptive.

    You write 'the ball would appear to keep going away from me', but to a physicist the word 'appear' makes the statement fuzzy. A sharper formulation is: 'the ball would keep going away from me'.

    Indeed the ball would keep going away from you.

    For the most simplified setting I take you falling straight down, no orbiting motion involved, and the height so small that tidal effects are negligable. Then acceleration and velocity are independent. If the two of you are co-accelerating then the distance between you and the ball remains the same, add a velocity of the ball relative to you and the distance between the ball and you keeps increasing.
     
  10. Sep 19, 2010 #9

    Pythagorean

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    ...and it's a constant negative acceleration so, regardless of the initial velocity conditions (as long as v_ball != v_person) they will always maintain the same velocity difference after the balls leaves hand, which will be a positive value, meaning that they're going away from each other, regardless of the planet's frame.
     
  11. Sep 19, 2010 #10

    Femme_physics

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    I imagine there has to be some gases floating around in any planet. Or we're talking simply purely gravity from a hypothetical PoV?

    Also, don't gravity take magnetism into account? So a ball made of a metallic substance would be drawn more towards the gravitational pull.
     
  12. Sep 19, 2010 #11

    Pythagorean

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    hypothetical, of course. And no, gravity and magnetism are independent phenomena.
     
  13. Sep 19, 2010 #12

    Femme_physics

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