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Homework Help: Asymmetric initial velocity calculation

  1. Feb 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A rock is thrown upwards at an angle of 35.0° to the horizontal. The
    rock hits a signpost 15.0 m away at a point 2.00 m above the level
    from which it was thrown.
    Calculate the initial velocity of the rock.

    2. Relevant equations

    d = v * t
    d= displacement, v=velocity, t=time


    x= displacement at t
    x0 = initial position
    v0 = initial velocity

    3. The attempt at a solution
    It appears that the x displacement is related:
    (1) 15=Vx0 x t

    (vx0 is the initial x-component velocity)
    because the x velocity is unchanged

    The y velocity is related:
    (2) 2=vy0t +1/2gt^2

    where vy is the initial y vector component

    (3) vx = v0 cos 35
    (4) vy = v0 sin 35

    It appears possible to substitute for vx, vy so that
    15= v0 x cos 35 x t
    2= v0 x sin 35 x t + 1/2 gt^2

    They look like simultaneous equations but I do not obtain the expected result :(
    Help and advice gratefully received.
  2. jcsd
  3. Feb 11, 2009 #2


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    Homework Helper


    15 = Vo*cos35 * t
    2 = Vo*sin 35 * t - 1/2 gt2
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