Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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?

    Vo=16m/s
    a=-g
    θ=45
    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

    -gsin(θ)
    Vxf=Vxi+a(tf-ti)
    Xf=Xi+Vxi(tf-ti) + 1/2 A(tf-ti)^2
    Vxf^2=Vxi^2+2a(Xf-Xi)

    Vyf=Vyi+a(tf-ti)
    Yf=yi+Vyi(tf-ti) + 1/2 A(tf-ti)^2
    Vyf^2=Vyi^2+2a(Yf-Yi)
    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

    PeterO

    User Avatar
    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

    NascentOxygen

    User Avatar

    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook