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Homework Help: Reviewing Newtonian Mechanics

  1. Aug 29, 2013 #1
    b]1. The problem statement, all variables and given/known data[/b]
    1. Two seconds after being projected from ground level (y=0 m), a projectile is displaced
    x=40 m horizontally and y=53 m vertically above its launch point. What are the (a) horizontal
    and (b) vertical components of the initial velocity v of the projectile? (c) At the instant the
    projectile achieves its maximum height above ground level, how far is it displaced horizontally
    from the launch point?

    2. Relevant equations

    3. The attempt at a solution

    I am working on part a). I was able to determine the horizontal component; however, I am unable to ascertain the vertical component of velocity. I tried applying kinematic equations, but with no avail. I tried to employ a symmetry argument, by finding the velocity acquired as the projectile falls to earth from a vertical distance of 53 m, but then I realized that I don't know the velocity at this point, nor can I suppose that the speed is zero, because it isn't.

    What should I do?
  2. jcsd
  3. Aug 29, 2013 #2
    There is a formula relating initial velocity, uniform acceleration, displacement and time. You know three of these, thus should be able to find out the remaining unknown.
  4. Aug 29, 2013 #3
    Are you speaking of this formula: vi = (y -.5at^2)/t. If so, how can I apply it when I need to set t = 0, in order to determine the initial velocity?
  5. Aug 29, 2013 #4
    At what value of ##t## do you know ##y##? Why would you let ##t = 0## instead?
  6. Aug 29, 2013 #5
    I figured that I would set t = 0, because that is the instant whose velocity I am trying to find.
  7. Aug 29, 2013 #6
    At ## t = 0 ##, ## y = 0 ##, so your equation is ## 0 = v_i \cdot 0 - g \cdot 0^2 / 2 ##, which is useless. But you are given data at ## t ## different than 0, so use that.
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