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Irritating vector problem

  1. Sep 21, 2006 #1
    Some dude on a cliff throws a ball with an unknown initial velocity. The ball is in motion for 2seconds before its instant velocity is mesured, being 24m/s [E 45 S]. Determine its initial velocity.

    i got

    g= -9,8
    t= 2
    instV= 24
    iV=? Vx2= 24cos45 = 17m/s
    Vy2= 24sin45 = 17m/s
    9,8=(17 - Vy1)/2.....Vy2=2,6m/s

    and thats all i can get, i have the y axe vector for the initial velocity and i cant seem to figure out anything further...help needed! lol
  2. jcsd
  3. Sep 21, 2006 #2
    Sounds like a poorly worded question. If on a cliff, then what is E 45 S? Those are horizontal plane directions. I am confused also.
  4. Sep 21, 2006 #3
    well i am assuming they mean 45 degrees under the horizontal axe of the top of the cliff
  5. Sep 21, 2006 #4


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    Staff Emeritus
    Science Advisor

    I agree with civil_dude, the question is poorly worded, unless the problem has redefined the compass orientations. N, S, E, W normally refer to directions (lateral) normal to the local gravity field, which is vertical (elevation). E45S represents a vertical plane, in that orientation.

    On the other hand,

    if the velocity has no vertical component and is horizontal, pointing in that direction, then one has a horizontal speed of 24 m/s at 2 seconds, which would be at the top of an arc. So it would be 48 m away and would come back to same elevation 2 sec later for a range of 96 m.

    Can one determine the altitude after 2 seconds, i.e. one needs to determine the angle the ball is thrown.

    One may neglect air resistance.
  6. Sep 21, 2006 #5
    Its simply initial velocity components along with final velocity comps. and a vertical accelerated component. The final velocity, after 2 seconds, is 24m/s directed at a 45 degree angle under the horizontal pane of the top of the cliff. I found the vertical and horizontal final velocities, and managed to find the vertical initial velocity component, but am completely lost as to how to find the horizontal initial velocity comp...
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