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Projectile moting and checking an answer with another equation doesn't work.

  1. Sep 18, 2012 #1
    1. The problem statement, all variables and given/known data

    Firemen are shooting a stream of water at a burning building using a high-pressure hose that shoots out the water with a speed of 25.0 as it leaves the end of the hose. Once it leaves the hose, the water moves in projectile motion. The firemen adjust the angle of elevation of the hose until the water takes 3.00s to reach a building 45.0 away. You can ignore air resistance; assume that the end of the hose is at ground level.

    a. Find the angle of elevation
    b. Find the speed and acceleration of the water at the highest point in its trajectory
    c.how high above the graound does the water strike the building and how fast is it moving just before it hits the building?

    2. Relevant equations

    x-xo=vox*t
    vx=vox
    y=yo +voy*t -1/2 *g *t^(2)
    vy=voy-g*t
    y-yo=(voy+vy)/2 *t
    vy^(2)=voy^(2) -2g(y-yo)


    3. The attempt at a solution

    a= 53.1 degrees
    x=xo+vox*t
    45.0m=25.0m/s*cos(theta)*3.00s
    Theta=53.1 degrees

    b=15 m/s and g=9.8m/s^(2)
    Vox=25.0m/s*cos(53.1)= 15m/s
    Aceleration is the same vertically.

    c=15.9m
    y=yo+voy*t -1/2*g*t^(2)
    y= 20m.s*3.00s-1/2*g*9.00s^(2)
    y=15.8m

    d=17.7m/s
    vy^(2)=voy^(2)-2g(y-yo)
    vy^(2)=(20m.s)^(2)-19.6m/s^(2)*15.9m
    vy=9.4m/s

    therefore Vf=17.7m/s by v=sqrt(vx^(2)+(vy)^(2))
    vx=15.0m/s


    My question is is that when I try to check my answer with other equations they do not come out the same. Why is that?
    For example:


    I want to get my heigth at the highest point

    vy^(2)=voy^(2)-2g(y-yo)
    0=(20m/s))^(2)-19.6*y i get y=20m

    y-yo=(voy+vy)/2 * t
    y-0=(20m/s+0)/2*3.00s here i get 30m

    How come these equations won't work to give me the actual height of 15.9m?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Sep 18, 2012
  2. jcsd
  3. Sep 18, 2012 #2
    1. You did not show your solution but just final results. How can someone guess what is wrong?
    2. The values without units are useless. It takes 3s or 3 hours to reach the building? The answer will be different for the two cases.
    3. Are you assuming that the water hits the building at the maximum point of the trajectory? The problem does not say so.
     
  4. Sep 18, 2012 #3
    There I fixed it. Yes I did assume that the water hits the building at the max point.
     
  5. Sep 18, 2012 #4
    No need to try it anymore. I've got it now. Thank you nasu.
     
  6. Sep 18, 2012 #5
    You are welcome.
     
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