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What is the initial speed of the ball?

  1. Sep 24, 2006 #1
    A ball is projected horizontally from the edge of a table that is 1.00m high, and it strikes the floor at a point 1.20m from the base of the table.

    a) What is the initial speed of the ball?
    b) How high is the ball above the floor when its velocity vector make a 45.0° angle with the horizontal?

    ...So far all I got is that it take .102 seconds for the ball to hit the ground and I think average speed is 11.8 m/s. Please help.
     
  2. jcsd
  3. Sep 24, 2006 #2

    radou

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    How did you get the 0.102 seconds? Doesn't look correct. Could you present the way you got that result? Since the ball is thrown horizontally, the equation of displacement for the y-direction is pretty simple.
     
  4. Sep 24, 2006 #3
    oh my bad. That's wrong. But then how would I find it?
     
    Last edited: Sep 24, 2006
  5. Sep 24, 2006 #4

    radou

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    From the equation y(t) = 1/2*g*t^2.
     
  6. Sep 24, 2006 #5
    ok im not getting what im supposed to do with this equation. can u help me out please
     
  7. Sep 24, 2006 #6

    radou

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    This equation represents the displacement of the ball in the y-direction (vertical), right? And you know the height of the table (i.e. the distance from the top of the table to the ground). So, just plug it in the equation and calculate the time it takes for the ball to fall on the floor.
     
  8. Sep 24, 2006 #7
    ok so it'd be like 1.0=1/2(9.81) x t^2? and then i jus isolate the t^2.
     
  9. Sep 24, 2006 #8

    radou

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    Exactly. After getting the time, use x = v0 * t for the initial speed.
     
  10. Sep 24, 2006 #9
    Alrite so i got 2.21 seconds. What's v0?
     
  11. Sep 24, 2006 #10

    radou

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    You know the displacement in the x-direction equals 1.2 m, and you just calculated the time. And you saw what is written in my post above. I think that's enough. :smile:
     
  12. Sep 24, 2006 #11
    k so initial speed is 4.42 m.s?
     
  13. Sep 24, 2006 #12
    no ... t = 2.21 seconds is incorrect, it should be t = 0.452 seconds.
     
  14. Sep 24, 2006 #13
    ok yea i got that. so then initial speed would be .542 m/s.
     
  15. Sep 24, 2006 #14
    no again ... (delta x)/(time) = speed

    1.2/0.452 = ?
     
  16. Sep 24, 2006 #15
    2.65 m/s. damn im stupid. thx bro. appreciate it. now i jus gotta figure out b.
     
    Last edited: Sep 24, 2006
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