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Homework Help: Projectile from a cliff - ugh!

  1. Feb 13, 2010 #1
    1. The problem statement, all variables and given/known data
    A projectile fired up into the air from the top of a 75m high cliff hits the ground 500m out from the base.
    Calculate the initial launch velocity (U), and launch angle (theta) of the projectile.


    2. Relevant equations
    1) v=u+at
    2) v2=u2 +2as
    3) s=ut+0.5at2


    3. The attempt at a solution
    Consider up to be positive direction.
    Consider vertical component (y component) of motion for entire flight:
    a=-9.8 m/s2
    s=-75m
    uy=Usin(theta)

    Sub into Eqn 3:
    -75=Usin(theta)t+0.5(-9.8)t2 #EQN A

    Consider horizontal component (x component) of motion for entire flight:
    a=0
    s=500m
    ux=ucos(theta)

    Sub into Eqn 3:
    500=ucos(theta)t #EQN B


    Now comes the problem... I have created 2 equations (A,B), but with 3 unknowns (u, theta,t). I think I need another equation ???

    This is REALLY bugging me, please help.
    Thanks very much.
     
  2. jcsd
  3. Feb 13, 2010 #2

    kuruman

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    Your thinking is correct. You do need more information. The answer to this problem is not uniquely defined with the information that is given.
     
  4. Feb 13, 2010 #3
    OK thanks, but the book gives the answer as 65m/s at an angle of 40degrees to the horizontal.
    Could it be that I need to create another equation from my data ??

    Thanks
     
  5. Feb 13, 2010 #4

    kuruman

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    As I said earlier, there are pairs of angles and initial speeds that give the same numbers. The numbers that are given by the problem do not uniquely define a solution. Try this:

    Assume that the angle is 40o and that the initial speed is 65 m/s.
    Solve your EQN B for the time of flight.
    Plug the numbers in the right side of EQN A; you should get -75 m.

    Now repeat but with an angle of 60o and initial speed 72.15 m/s and see what you get.

    The answer to this problem is not unique unless some other quantity is given to remove the ambiguity. You have created all the equations that can be created from what is given. To get another equation, you need more data.
     
  6. Feb 13, 2010 #5
    Thanks kuruman I see your point.
     
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