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Projectile equation help

  1. Oct 21, 2009 #1
    english is not my native language , ill do my best

    1. The problem statement, all variables and given/known data
    someone wants to throw water to a flower that is to the distance of 5 m and at the same level as the place where the water comes from. air resistance doesnt count

    what is the velociity of the water so that it hits the flower, if the water is thrown with an angle of 60 degrees from the horizontal?


    2. Relevant equations



    3. The attempt at a solution


    already put the parametric equations of the movement :
    x=xo + voxt
    y=yo+voyt-1/2gt^2
    but now i cant do anithing else!

    and i cant find any way to find the velocity of the water( the inicial one i think)

    anione can help fast please? thanks a lot
     
  2. jcsd
  3. Oct 21, 2009 #2
    Re: projectile

    Are you able to write down the equations of motion for a projectile in terms of s, u, v and t?
    These are the standard equations one uses to solve such problems
    If you can do this, then you need to be able to split the motion into horizontal and vertical components and solve from there.
    Let me know how much of that made sense
     
  4. Oct 21, 2009 #3
    Re: projectile

    well first of all let me repost :D

    the parametric equations are :
    x=vo*costeta*t
    and
    y=vo*sinteta-5t^2

    then when vo*costeta*t=5, , when 5 is the reach, we can find the vo
    vo=5/0.5t

    but now how can i find the time for this equation? am i going well?
     
  5. Oct 21, 2009 #4
    Re: projectile

    aha , now ill try to do the position/time wich is:

    r(t)= vocostetat ex + vosintetat - 5t^2 ey

    now if i find the zeros of this function i can get the time right?


    EDIT : damn this way i still dont know the vo....
     
  6. Oct 21, 2009 #5
    Re: projectile

    ok so i restarted the exercise and now i got an answer, can you guys please try to see if this is the correct answer? because this exercise is not from any book with solutions

    so this is what i did:
    x=vocos60t
    y=vosin60t+5t^2

    vosin60t+5t^2=0 ( when it is vertically at the same spot as the flower)
    t=0 and t= 0,173vo

    then

    x=vocos60*t
    0,5vo*0,173vo = 5 (when x is 5, i calculated the vo)
    vo = 7,6 m/s



    what was your answer please answer this is really making my mind go crazy :D thanks in advance
     
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