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Homework Help: Finding the intial velocity using vectors

  1. Oct 9, 2011 #1
    1. The problem statement, all variables and given/known data
    a fire hose sprays water with an initial velocity of 40ft/s at an angle of 60 degrees to the horizontal. Find the minimum initial velocity to hit a point on a building 15 feet to the right and 16 feet up.



    3. The attempt at a solution
    these are my equations but i feel like im missing something b/c im not sure how i would solve for velocity
    r(t)=(z*cos60)T i + (z*sin(60))-16t^2) j
    r'(t)=z*cos60) i + (z*sin(60)-32T) j
    r''(t)=-32j
     
  2. jcsd
  3. Oct 9, 2011 #2

    LCKurtz

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    Your equations look OK, with z as the unknown velocity. Use the x equation to figure out when it will have traveled the 15 feet to the wall. Plug that time into the y equation and see what z must be to make y = 16. And put the exact values of cos(60) and sin(60) in those equations.
     
  4. Oct 9, 2011 #3
    thats a lot easier said than done, are you sure thats the way to do it? b/c that math is just ugly and terrible
     
  5. Oct 9, 2011 #4

    LCKurtz

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    Yes, I'm sure. You wind up with a quadratic equation in the unknown z. A few square root signs in the answer never hurt anybody.
     
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