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Homework Help: Projectile Motion, only given horizontal distance

  1. Sep 22, 2014 #1
    1. The problem statement, all variables and given/known data
    The longest pass in history is 85.8m. Ignoring any air resistance effects, calculate the initial speed of football.

    2. Relevant equations
    Vx=Vo cosΘ
    Vy=VosinΘ -gt
    y= VosinΘt - 0.5gt^2
    3. The attempt at a solution
    I really don't know what to do here. I've looked through all my notes and book and haven't seen any problems like this (with no angle). Every time I try to rearrange and substitute equations I still end up with 2 unknowns. It would be great if someone could point me in the right direction.
  2. jcsd
  3. Sep 22, 2014 #2
    Are you sure 85.8m is all you're given? There will be many answers if you only have 85.8m as the distance because the magnitude of the initial velocity will be dependent on the angle. In this case, I'd first ask your teacher and if not, just choose a random angle 90>x>0 (most likely 45 degrees for simplicity).
    Last edited: Sep 22, 2014
  4. Sep 22, 2014 #3
    Yeah that's all the question had. You might be right about that, the "longest pass in history" could be implying a 45 degree angle since I remember him saying that a 45 angle would give the greatest range. It would have been nice for him to actually state that in the question though. I'll try to work it out tomorrow (it's 1 AM and I've been pondering it for far too long tonight) like that and see how it goes. Thanks for the help!
  5. Sep 22, 2014 #4
    Np, good night.
  6. Sep 23, 2014 #5
    Well I managed to work it out, you were right! I assumed an angle of 45 degrees and got the right answer pretty quickly after that using the range formula. Really appreciate the guidance!
  7. Sep 23, 2014 #6
    Yep I would assume that assuming 45 degrees would be the best way to approach the problem. Given the way its worded though, it would be a bit confusing.
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