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Homework Help: Question on motion in two dimensions:

  1. Oct 5, 2006 #1
    OK, ive been trying to get problems like these where you are given the angle (but not the velocity) of the direction in which a projectile is fired. I have tried as many ways as i could conceive using kinematics, but i keep getting stuck. If anyone could just give me an idea how to go about getting a little more info so i can complete the problem, it would be greatly appreciated.

    Barry Bonds hits a home run so that the baseball just clears the top row of bleachers, 23.6 m high, located 130 m from the home plate. The ball is hit at an angle of 42° to the horizontal, and air resistance is negligible. Assume that the ball is hit at a height of 1.00 m above the ground. Find the following values.
    (a) the initial speed of the ball

    (b) the time at which the ball reaches the cheap seats


    (c) the velocity of the ball when it passes over the top row
    (____i+____j m/s)

    (d)the speed of the ball when it passes over the top row

  2. jcsd
  3. Oct 5, 2006 #2
    I think a good first step would be to use some trig to figure out the vertical distance the ball travels
  4. Oct 5, 2006 #3


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    Homework Helper

    After drawing a diagram, write down the equations of displacement of the ball for the x and y direction separately. What do you know about the initial position of the ball? What do you know about the final position of the ball? Eliminate the variable t from one of the equations, to get the initial velocity. The rest should be easy.
  5. Oct 5, 2006 #4
    how would you do this, since the path would be parabolic?

    EDIT: the total displacement is 131.9498, still dont get how im gunna figure it out tho, i drew everything out but cant seem to get it still...
    Last edited: Oct 5, 2006
  6. Oct 5, 2006 #5
    i tried what radou said, and it (seemed) to work. i got the values for Vxi and Vyi, then found Vi from that. it all worked out the way it should have an di checked my answer. these are the values i got:

    Vxi = 174.9323 m/s
    Vyi = 33.7752 m/s
    Vi = 235.3946 m/s

    t = .7431 seconds

    with these values, i was able to calculate that the ball was at the following points at t= .7431: x = 129.9922 (which is right, its supposed to be 130, just some round off error). y = 23.3926 (should be 23.6, again, roundoff error). Since those values are so close, webassign should have no problem accepting those answers, but it says theyre wrong...
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