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Homework Help: Projectile Motion

  1. Feb 1, 2010 #1
    1. The problem statement, all variables and given/known data

    I need help with these 2 problems:

    A hunter aims directly at a target (on the same level) 81 m away. If the bullet leaves the gun at a speed of 225 m/s, how far below the target will the bullet hit?

    A rifle bullet is fired at an angle of 15.2° below the horizontal with an initial velocity of 154 m/s from the top of a cliff 63.9 m high. How far from the base of the cliff does it strike the level ground below?

    2. Relevant equations



    3. The attempt at a solution

    Vx = 225 m/s, x = 81 m
    Vyo = 0 m/s ?
    a = -9.8 m/s/s, t = .36 s (from Vx = x/t)

    so I used y = Vot + at^2 to find y, but that's not right...

    and for the second one

    Vx = 148.613 m/s
    Vo = -40.377 m/s, a = -9.8 m/s/s, y = -63.9 m
    using the same equation, t = 1.221 s
    and V = x/t, x= 181.456 m

    again, not right. help please.
     
  2. jcsd
  3. Feb 1, 2010 #2

    berkeman

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    Staff: Mentor

    In the first one, you're just missing a 1/2 in your equation here:
     
  4. Feb 1, 2010 #3
    Wow, how embarrassing. However, in the 2nd one. I now get 201.965 m, which is not right?
     
  5. Feb 1, 2010 #4

    berkeman

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    Staff: Mentor

    Could you please show your new work for the 2nd one?
     
  6. Feb 1, 2010 #5
    well, using the correct equation y = Vo + 1/2at^2 and keeping everything the same
    Vx = 148.613 m/s
    Vo = -40.377 m/s, a = -9.8 m/s/s, y = -63.9 m
    I found t = 1.359 s
    and V = x/t, x= 201.965 m...
     
  7. Feb 1, 2010 #6

    berkeman

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    Staff: Mentor

    The bullet in the 2nd problem is fired downward, so it's initial y velocity is not zero...
     
  8. Feb 1, 2010 #7
    I meant y = Vot + 1/2at^2
    I'm sorry if I was not clear, but Vyo = -40.377 m/s
     
  9. Feb 1, 2010 #8

    berkeman

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    Staff: Mentor

    That looks mostly right then, unless there is a small rounding error or something. Is it not the right answer? If not, could you please write out each step again, and I;ll do a more careful check of the math. (I'm bailing for a couple hours though -- will try to check back a bit later)
     
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