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Homework Help: Motion of two balls which collide

  1. Aug 30, 2011 #1
    A ball is thrown straight up from the ground with speed v0. At the same instant, a second ball is dropped from rest from a height H, directly above the point where the first ball was thrown upward. There is no air resistance. How to I find the time at which the two balls collide. A how do I find the value of H in terms of v0 and g so that at the instant when the balls colide, the first ball is at the heighest point of its motion.

    Anybody who can help?
     
  2. jcsd
  3. Aug 30, 2011 #2

    tiny-tim

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    Last edited by a moderator: Apr 26, 2017
  4. Aug 30, 2011 #3
    Is it this equation I need to use s = u*t + 1/2*a*t^2 ?
     
  5. Aug 30, 2011 #4

    tiny-tim

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  6. Aug 30, 2011 #5
    Ball 1 (Going upward with a v0):

    s1 = vo*t-0,5*g*t^2

    Ball 2 (Dropped from a height H)

    s2 = h-0,5*g*t

    And then I need to find the time t:
    s1 = s2

    I get something crazy !

    Is my method good?
     
  7. Aug 30, 2011 #6

    tiny-tim

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    Hi Faka! :smile:

    (try using the X2 and X2 icons just above the Reply box :wink:)
    you mean s2 = h - 0,5*g*t2 :wink:

    otherwise, that's ok :smile:
     
  8. Aug 30, 2011 #7
    Yes, its t^2 .
    I think that I have made a mistake. Shouldn't it be:
    Ball 1

    s1 = Vo*t+0,5*g*t^2

    Ball 2

    s2 = h+0,5*g*t

    Then
    s1 = s2

    I mean "+" instead of "-".
     
  9. Aug 30, 2011 #8
    s2 = h+0,5*g*t^2
     
  10. Aug 30, 2011 #9

    tiny-tim

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    it doesn't matter whether you use +g and g = -9.81, or -g and g = -9.81 :wink:

    show us your full calculations :smile:
     
  11. Aug 30, 2011 #10
    Ball 1 (Upward)
    x1 = 0 + v0*t + 1/2*(g)*t^2
    = vo*t + g*t^2

    Ball 2 (Dropped)
    x2 = h + 0*t + 1/2*(-g)*t^2
    = h - 1/2*g*t^2


    x1 = x2

    vo*t + g*t^2 = h - 1/2*g*t^2


    And then I solve for t, I get something that I cant write here. Its something divided by 2*g
     
  12. Aug 30, 2011 #11

    tiny-tim

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    No, you can't have g in one equation and -g in the other. :redface:
     
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