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Homework Help: Projectile Motion - Hitting an edge

  1. Sep 12, 2009 #1
    1. The problem statement, all variables and given/known data
    I've made a picture of the situation:

    http://img11.imageshack.us/img11/7177/drawing1m.jpg [Broken]

    I need to find the expression for the start velocity v0. The ball should exactly hit the corner.

    The function should have the distance L (Which both is x-distance and y-distance to the point) and the angle a as variables.

    We can ignore air resistance.

    2. Relevant equations

    Normal projectile motion equations:


    3. The attempt at a solution
    I've been sitting with he assignment for 2 hours - trying both creative and stupid solutions!

    First attempt was setting x=L and y=L and find the expression for v0.

    I've also trying to set the v=0, but afterwards finding out the ball speed dosnt have to be zero.

    Nothing has worked out so far.

    Any suggestions?

    Thanks alot.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 12, 2009 #2
    Might try this approach...

    How far below the corner would the projectile strike if you aimed directly at the corner?
  4. Sep 12, 2009 #3


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    Hi Mcoroklo, welcome to PF>

    Your two starting equations are correct. Use the first equation to eliminate time in the second. That should give you what you need.
  5. Sep 12, 2009 #4
    ... Thanks a lot!

    I guess you guys know the feeling. A lot of time spend on outrageous thoughts, afterwards finding the solution is way simpler.
    I actually had your solution to Kuruman, checked for input (40deg and 10m). It gave complex numbers and I tried another approach. Ofcourse it should give complex numbers for each angle outside the span of 45-90..

    Thanks a lot to both of you.

    Looking forward to participate in the forum, hopefully helping people to! :-)
  6. Sep 12, 2009 #5
    Actually I read the problem wrong initially, I thought you were trying to solve for the angle given an initial velocity. But, if it helped, I am happy. The other posters approach is easier, my proposal was a conceptual way that might help with other similar problems. And it may have helped eliminate unecessary stuff if you were guessing and checking with numbers.
    Last edited: Sep 12, 2009
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