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Homework Help: Free fall

  1. Dec 30, 2012 #1
    1. The problem statement, all variables and given/known data
    We have got a ball of radius r, which is falling from the roof of the house. How is the radius r with respect to time? We are looking at the ball directly from above and ball is at the beginning of fall x from our eyes. Neglect air resistance.

    2. Relevant equations
    I'm not sure what to write here. So I think the nub is equation of free fall: s= 1/2gt^2 radius decreases quadratically

    3. The attempt at a solution
    So, I know equation: s= 1/2gt^2
    I think the radius decreases quadratically, because t^2.
    I must attributed the x (s from eyes before free fall)
    And my equation: r(t)= r/(1/2gt^2 + x)

    It is right? Thanks for help.

    PS: Sorry for bad English, but I don't know better yet.
  2. jcsd
  3. Dec 30, 2012 #2


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    If r is the radius of the ball, I don't see any reason why it should decrease. Do you mean the visible angle for the observer?
    If the initial distance of the ball is large compared to the radius of the ball, your r(t) is an approximation for that angle.

    Hmm... the original problem statement would be interesting - with translated words maybe ;).
  4. Dec 30, 2012 #3
    No, no... How changing the radius of sight of the observer, when ball is falling free fall.

    Hmmm... You don't interesting what I think? :-( Describe better?
  5. Dec 30, 2012 #4


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    Hi Numeriprimi! :smile:
    i agree with mfb :smile:

    you seem to mean (half) the angle subtended at the eye, and your formula for r(t) confirms that (correctly, for small r) :wink:
  6. Jan 5, 2013 #5
    So... Is my formula well?
  7. Jan 5, 2013 #6


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    your formula is good :smile:
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