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Homework Help: Finding inital velocity

  1. Oct 29, 2004 #1
    A ball is kicked horizontally off a cliff 40m high. After 3seconds, the person that is on top of the cliff hears the thump of the ball hitting the floor. Assuming speed of sound travles 343m/s in air, what is the initial velocity that the ball was kicked at?

    Please check if I did it correct....

    Find time it takes for the ball to hit the floor:
    d=.5at^2
    40m=.5(9.8)(t^2)
    t=2.857142857 s

    Find time it takes for the sound to travel back to person on cliff:
    3-2.857142857=.1428571429

    Find the distance away from cliff:
    d=vt
    d=343*.1428571429
    d=49

    Find the distance travled in the x-direction:
    49^2=40^2+x^2
    x=28.3019434m

    Find the initial speed:
    v=d/t
    v=28.3019434m/2.857142857s
    v=9.9m/s
     
  2. jcsd
  3. Oct 29, 2004 #2
    Since the ball was kicked at an initial velocity [tex]v_1[/tex], [tex] d = v_1t + \frac {1}{2}at^2[/tex]
    You're solving for [tex]v_1[/tex] eventually. The calculation you made for v after that was the average velocity which isn't entirely right.
     
  4. Oct 30, 2004 #3
    shoot, you're right

    how should I go about solving this?
     
  5. Oct 30, 2004 #4
    I was solving for the time it took the ball to drop. It has no initial velocity.

    That's exactly what I did.
     
  6. Oct 30, 2004 #5

    Doc Al

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    Staff: Mentor

    Looks good to me.
     
  7. Oct 30, 2004 #6
    I got 9.9 also.
     
  8. Oct 30, 2004 #7

    Galileo

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    Science Advisor
    Homework Helper

    The initial velocity was in the x-direction, while the acceleration is in the y-direction. So this isn't the way to get the distance d.

    UrbanXrisis' solution looks good to me.
     
  9. Oct 30, 2004 #8
    Sorry I totally didn't see that it was horizontally kicked just that it was kicked.
     
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