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Homework Help: Projectile motions problem

  1. Oct 6, 2011 #1
    1. The problem statement, all variables and given/known data
    A projectile is fired with an initial speed of 16.0 m/s at an angle of 45.0^\circ above the horizontal. The object hits the ground 9.00 s later.

    What is the direction (below +x) of the projectile's velocity at the instant it hits the ground?

    Tf=9 seconds
    magnitude of velocity an instant before it hits the ground: 77.7 m/s
    max height: 301.53 m/s

    2. Relevant equations

    Xf=Xi+Vxi(tf-ti) + 1/2 A(tf-ti)^2

    Yf=yi+Vyi(tf-ti) + 1/2 A(tf-ti)^2
    3. The attempt at a solution
    I got 45 degrees and I got that from finding the xcomponents and ycomponents
  2. jcsd
  3. Oct 6, 2011 #2


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    Homework Helper

    If it took 9 seconds to hit the ground, it must have been fired from the top of a rather tall building/high hill.

    At that speed, at 45 degrees it would hit the ground [flat ground scenario] in a little over 2 seconds.
  4. Oct 6, 2011 #3


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    Staff: Mentor

    In light of what PeterO has written, I'd check that 45 degrees angle. It would have been 45 degrees after 2 secs of flight, so if it took another 7 secs, then its vertical speed would have been considerably more, making the angle steeper.
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