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Homework Help: Non-Horizontally launched Projectile

  1. May 17, 2010 #1
    1. The problem statement, all variables and given/known data
    A ball is thrown from the ground (height=0) with an initial speed Vo at an angle of 45 degrees

    a)when does ball start falling to earth
    b)what is balls max height
    c)How long does it take to hit the ground again?
    d)where does the ball hit the ground?

    2. Relevant equations
    x=Xo + Uxt + (at^2)/2
    z=Zo + Uxt + (at^2)/2

    r=(0,0,0)
    u=Vo(cos45,0,sin45)
    a=(0,0,-g)

    3. The attempt at a solution

    x= Ut
    z=cos45t + (sin45t^2)/2

    Now I'm stuck lol
     
  2. jcsd
  3. May 17, 2010 #2

    rock.freak667

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    Well when the ball starts falling back down, what should the vertical velocity be?
     
  4. May 17, 2010 #3
    Yeah, I know that, velocity=0 right? but which equation do I make =0? what do I do after this?
     
  5. May 17, 2010 #4

    rock.freak667

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    Well you know that v2=v02+2as, v=v0+at as well as the equation you put. So which one will give you time when v=0?
     
  6. May 17, 2010 #5
    We don't have a value for Vo though. That's whats confusing me.
     
  7. May 17, 2010 #6

    rock.freak667

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    In that case, I suspect you should find your answers in term of v0
     
  8. May 17, 2010 #7
    Wtf, how would you do that?
     
  9. May 17, 2010 #8

    rock.freak667

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    You have vx=v0cos45 and vy=v0sin45. So when considering horizontal motion, you can write

    x=vxt as x=(v0cos45)t

    Like that.
     
  10. May 17, 2010 #9
    So the only way to do it is to have an unknown variable still there?
     
  11. May 17, 2010 #10

    rock.freak667

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    Yes, you were not given enough information to find v0
     
  12. May 18, 2010 #11
    So 0= t because anything divided by 0 is 0, correct?
     
  13. May 18, 2010 #12

    rock.freak667

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    Where did you get that from?

    You have three motion equations

    s=ut+1/2at2
    v2=u2+2as
    v=u+at


    For the first, part, which one will give you time when the final vertical velocity is zero?
     
  14. May 18, 2010 #13
    Not quite.
    It's undefined. Plug a number divided by 0 into your calculatior. It'll say Math Error or something like that..
     
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