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Initial Velocity of a projectile, how?

  1. Sep 13, 2009 #1
    Initial Velocity of a projectile, how???

    If I have a problem that says the x and y distance of a target from a starting position, and only gives the angle at which the particle is launched, how do I find the required initial Velocity for its launch?



    I've been given the following equations to work with:
    Vf = Vi + at
    xf = xi + Vit + (.5)at2
    Vf2 = Vi2 + 2ad




    I've spent hours trying to find a solution like this and can't seem to get anywhere. At one point I wrote out a huge equation with a bunch of trig stuff and it said a human threw a ball at over 513m/s so I know it must have been wrong.
    One line I have written in my work says:
    1.05=( Visin43 )/( Vicos43 ) - ( 4.9 * 9 )/( Vi cos 43 )
    but I'm just asking in general, how do you do this???
     
  2. jcsd
  3. Sep 13, 2009 #2

    kuruman

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    Gold Member

    Re: Initial Velocity of a projectile, how???

    If you are given the x and y distance, in general, you use the first equation to solve for t in terms of x then plug that in the second equation, That gives you y as a function of x with the initial speed and angle as parameters. Use the third equation to find the maximum height reached by the projectile, in general.
     
  4. Sep 13, 2009 #3
    Re: Initial Velocity of a projectile, how???

    I took what you said to heart and I still seem to just be making a mess.

    Here are a few lines from my work:
    1.05 = Vy(xf/Yix) - 4.9 ( xf2 / Vix2 )
    ...
    1.05 = 9 tan43 - 4.9 ( 80 / Vix2 )
    ...
    Vix = .136 ???
    ...
    t = xf / Vix
    t = 9 / .136 = 66.176

    seems like a long time for something to get to a target that close :-/


    Do you think you could show me or maybe just give an example?
    My values are [tex]\theta[/tex]=43(degrees), xi = 0, yi = 0, xf = 9, yf = 1.05

    thanks
     
  5. Sep 13, 2009 #4

    kuruman

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    Re: Initial Velocity of a projectile, how???

    Can you state the problem exactly the way it is given? Your interpretation of the given quantities may be in error.
     
  6. Sep 13, 2009 #5
    Re: Initial Velocity of a projectile, how???

    quoting is probably not a good idea but trust me they were pretty straight-forward
    There was even a diagram below the problem

    --
    A guy who's 2 meters tall, stands 9 meters horizontally from a ball-hoop mounted 3.05m high.
    He throws the ball at an angle of 43 degrees from the horizontal. How hard must he throught the ball (m/s) for it to go into the hoop but not hit the backboard?
    --
     
  7. Sep 13, 2009 #6

    rl.bhat

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    Re: Initial Velocity of a projectile, how???

    This equation is correct.
    But Vix is wrong. Check the calculation.
     
  8. Sep 13, 2009 #7
    Re: Initial Velocity of a projectile, how???

    extreme thanks

    I'm not sure what I did at this point because I failed so many times but when i fixed that mistake, I managed to get to a correct final solution
     
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