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Homework Help: Projectile magnitude and angle question

  1. Sep 11, 2005 #1
    Hello, I am currently taking AP physics and require some assistance with a problem. I have to find the magnitude and angle of the original velocity of a projectile, normally this would be no problem for me but the original height is not zero which is giving me some problems.

    My givens are:
    t= 4.5s
    dx= 45m
    h= 1.6m

    I believe I can get the x component with:
    delta x=vox t +1/2gt**2
    45m=vox(4.5s) + 4.905(4.5s**2)
    so vox= .4334

    If I did the above correctly all I would need now would be the y component, the problem is I have no idea on how to find it.

    Any assistance would be greatly appreciated.
  2. jcsd
  3. Sep 12, 2005 #2
    Please post the exact question as it appears in your book. Your method of finding the x velocity is wrong. Think, what direction is the acceleration?
  4. Sep 12, 2005 #3
    The problem is:
    A football player punts the football so that it will have a "hang time" (time of flight) of 4.5 s and land 48 m away. If the ball leaves the player's foot 160 cm above the ground, what initial velocity must the ball have?

    Also, yes I did do teh x component wrong. I think it should be:
    delta x=vxt + xo
    48=vx4.5 + 0
    so vx= 10.66
  5. Sep 12, 2005 #4


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    Homework Helper

    Yup, you got the correct x-component. v_x = 10.66 m / s.
    Now your goal is to find the initial y-component.
    First, you know that the ball has a 'hang time' of 4.5 s, and it's 1.6 m above the ground. What equation should you use to find out the initial velocity with the given information?
    Note that the ball has an acceleration of -g (downward).
    So you can use:
    [tex]x = x_0 + v_it + \frac{1}{2}at ^ 2[/tex]
    to find v_i (the initial y-component).
    x_0 is the ball's initial position (or height), ie: 1.6 m.
    When the ball hits the ground, x = 0.
    With the x-component and the y-component, can you find out the ball's initial velocity?
    Viet Dao,
    Last edited: Sep 12, 2005
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