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Help with Vector Addition Problem

  1. Sep 12, 2015 #1
    1. The problem statement, all variables and given/known data
    In photo:
    https://gyazo.com/6348b5ffe368852ee9868a2ba0dd7e38

    2. Relevant equations
    In image

    3. The attempt at a solution
    In the boxes

    Where did I go wrong with the angle calculation?

    Thanks
     
  2. jcsd
  3. Sep 12, 2015 #2
    I don't know, you didn't show the Calc.
     
  4. Sep 12, 2015 #3
    https://gyazo.com/efc009de2080feedea420d6b2a5edab4

    Triangle on the right shows how I calculated -10° angle by using tan-1(-3.2/17.4), -3.2 being the change in y and 17.4 change in x after I added the vectors.
     
  5. Sep 12, 2015 #4
    Can't follow your work. Your original post gives the answer as 350. How did you come up with this number?
     
    Last edited: Sep 12, 2015
  6. Sep 12, 2015 #5
    I calculated the angle as -10°, but since the question asked for the angle counter-clockwise from the x axis, I thought it would be correctly expressed as 350.
     
  7. Sep 12, 2015 #6
    -10 degrees measured from what?
     
  8. Sep 12, 2015 #7
    From the positive x axis.
     
  9. Sep 12, 2015 #8
    Okay where did you get the -10 degrees from?

    You can find the coordinate of the player's position through your vectors. After that you can simply use geometry to determine the ccw angle, after that, 360 - positive angle = clockwise angle. The program may not want to accept negative angles, try the positive equivalent between 0 and 360.
     
  10. Sep 12, 2015 #9
    https://gyazo.com/efc009de2080feedea420d6b2a5edab4

    Triangle on the right shows how I calculated -10° angle by using tan-1(-3.2/17.4), -3.2 being the change in y and 17.4 change in x after I added the vectors.
     
  11. Sep 12, 2015 #10
    And yes, that's how I got 350° counter-clockwise. By doing 360 minus the clockwise angle (360-10).
     
  12. Sep 13, 2015 #11
    I think if you draw yourself a picture you will see that it can't be -10 degrees from the +ve x-axis.
     
  13. Sep 13, 2015 #12
    I got -10 from the X-axis counter-clockwise when drawing a picture, as I've explained. Do you have a better answer or suggestion?
     
  14. Sep 13, 2015 #13
    I could not understand your drawings. So my suggestion is to redraw a simple picture that summarizes the vectors and shows the -10 degrees.
     
  15. Sep 13, 2015 #14
    Untitled.png

    From this image, find the vector P.
     
  16. Sep 13, 2015 #15
    That wouldn't make sense, as the angle from initial to final points would be from the top of the red line on the y-axis to the final spot, not from the origin.
     
  17. Sep 13, 2015 #16
    I calculated the final point (17.4, -3.2) and drew a line from the origin (0,0) to this point, creating a triangle with the x and y segments. Using Pythagorean theorem I calculated the magnitude of this vector 17.7
    I then used inverse tangent to find the angle -10°
     
  18. Sep 13, 2015 #17
    Ok, I follow you now. I think you got it right. Maybe a small rounding error. Try and see if 349.6 works.
     
  19. Sep 13, 2015 #18
    Tried it, didn't work. I think the teacher probably just put the answer in wrong accidentally. Will ask tomorrow. Thanks for help.
     
  20. Sep 14, 2015 #19
    That is not the answer that I am getting at all.

    I am getting a positive angle of 24.213 degrees counter-clockwise.

    Again, if you're not showing your calculations it's impossible for us to tell you where you're going wrong.

    Your y-coordinate is wrong. The point is in the first quadrant.

    Bx=11.3137
    By=11.3137
    Cx=6.0622
    Cy=3.5

    Therefore the resultant vector is
    r=[Bx+Cx,By-Cy]
     
  21. Sep 14, 2015 #20
    I got the same answer as the OP.
     
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