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9 pm, and still dont understand

  1. Oct 22, 2007 #1
    1. The problem statement, all variables and given/known data
    A rifle bullet is fired at an angle of 30 degrees below the horizontal with an initial velocity of 800 m/s from the top of a cliff 80 m high. How far from the base of the cliff does it strike the level of ground below
    g=9.8
    initial velocity- 800 m/s
    y=80 m


    2. Relevant equations
    People i ask say..
    R=(initial velocity squared x sin(2 theta)
    g




    3. The attempt at a solution
    i did
    800^2 x sin2(30)
    9.8

    i get 56556.76106-which seems way to high
    Then i did Soh Cah Toa
    Sin=Opp/Adj
    56556.76106/80
    and got 706.9595133

    I really just dont understand what i am doing, and need help. bad. this is due tomorrow.
     
  2. jcsd
  3. Oct 22, 2007 #2
    Hello welcome to Physicsforums! Hope you will learn here and also teach us what you know!

    For the problem:
    (1)You must resolve the initial velocity into their X and Y coordinates using trigonometry.
    (2)Then use kinematic equations to solve the problem.
     
  4. Oct 22, 2007 #3
    I dont even know any of that.
    We never learned Kinematic equations.
    Im very slow at this, and am now regretting taking this class.
     
  5. Oct 22, 2007 #4
    Hey don't despair! I can teach you what I know. I am learning this myself.

    Kinematic Equations:
    S=Vot+0.5at^2
    S=Vft-0.5at^2
    S=((Vo+Vf)/2)*t
    Vf=Vo + at
    Vf^2=Vo^2 + 2aS

    S=Displacement in metres
    Vo=Initial Velocity in metres/second
    Vf=Final Velocity in metres/second
    a=acceleration in metres/second squared
    t=time in seconds

    For all kinematic problems, write your 5 variables down and write the known values in.

    Velocity is a vector quantity. It has magnitude and DIRECTION. direction is usually indicated by an angle as in your problem.

    Velocity of the bullet is 800m/s angle 30degrees below the horizontal. Draw a picture and see if you can resolve the velocity into X and Y coordinates.

    Have a look at this site:
    http://www.glenbrook.k12.il.us/gbssci/phys/Class/vectors/u3l2c.html

    Then we will see if we can solve the problem. Please come back.
     
  6. Oct 22, 2007 #5
    [​IMG]
     
  7. Oct 22, 2007 #6
  8. Oct 22, 2007 #7
    use the 2nd equation to find time

    keep in mind the bullet is being fired DOWN, so your y value must be negative

    when you find time, disregard the negative value

    then use the 1st equation to find x
     
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