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Homework Help: Prjectile Motion

  1. Apr 30, 2010 #1
    A rifle fires a bullet at 300 m/s at a target that is up on a wall 100 m away. The rifle's line of sight creates a 30 degree angle with the horizon. What initial angle does the rifle need to be directed to hit the target?

    The formulas I thought I needed are:

    Vy=ViSIN(30) Here my Vi is the V-"not"

    I found that the height of the target is 57.75 m, and the hypotenuse is 115.5 m...

    I was told to find time then use a kinematic equation. So I found t=.33. I came up with several different angles in several dfferent ways and they are all too large... My prof said that it is only raised a very little bit. I have had MANY failed attempts at this and just can't figure it out.

    I appreciate any help possible!
  2. jcsd
  3. Apr 30, 2010 #2


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    Whenever solving problem for inclined plane, consider axis along the inclined plane as x-axis and perpendicular to it as y-axis. Then
    Vi(x) = Vi*cos(θ-α)
    a(x) = -gsin(α)
    Vi(y) = Vi*sin(θ-α)
    a(y) = -gcos(α) where α is the angle of inclination and θ is the angle of projection along the horizontal.
    Now proceed as usual in the projectile motion.
    Last edited: Apr 30, 2010
  4. Apr 30, 2010 #3
    I am just working in circles... just come up with the values I'm already given. I don't understand what formulas to use...
  5. Apr 30, 2010 #4


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    Time of flight T = 2vsin(θ-α)/gcosα
    Horizontal displacement OX = vcosθ *T
    Range along the inclined plane = R' = OX/cosα.
    Angle of inclination α is given. Now find θ.
  6. May 1, 2010 #5
    Thank you so much for your help! I think I finally got it :)
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