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Homework Help: 2-d kinematics problem

  1. Sep 27, 2010 #1
    1. The problem statement, all variables and given/known data
    If you can hurl a ball so that its initial speed is 30 m/s, what is the widest river it can cross?

    v1 = 30m/s

    2. Relevant equations
    I am not sure with only the one variable given.

    3. The attempt at a solution
    Start with a x and y component diagram?
  2. jcsd
  3. Sep 27, 2010 #2


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    Yes split the inital velocity into vertical and horizontal components.

    Now consider vertical motion, when the ball lands on the ground again, the displacement is zero. Use y=vyt_0.5gt2 to get the time for this motion.

    Now horizontally the Range (how wide your river is) is given by R=vxt

    that 't' in the equation for 'R' is the time for the motion above.
  4. Sep 27, 2010 #3
    Umm.. I don't understand. How would the 't' answer the question? Wouldn't I need a 'd' since it's asking how wide the river could be?

    edit: SOrry I don't really understand what I'm even supposed to draw in the diagrams of the components
  5. Sep 27, 2010 #4


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    Solving for the 't' would get the 'd'.

    Draw the 30m/s at an angle θ and split it into vertical and horizontal components.
  6. Sep 27, 2010 #5
    Can I get something step by step please? I drew the components but I don't get why I even have to have both an x and the y. I just have two sets of axes and a theta angle at 90degrees
  7. Sep 27, 2010 #6


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    You will see why you need it soon.

    Did you get your components as

    y: 30sinθ

    x: 30 cosθ

    These are your initial velocity components

    Now consider vertical motion: y=vyt-0.5gt2

    When the ball hits the other side of the river, y = 0. So solve for 't' and tell me what you get.
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