Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Geometry/Discrete Math Velocity Vectors Help

  1. Feb 12, 2006 #1
    I'm working on this problem and it's frustrated me and I was wondering if I could get a spot of help on it:

    An airplane heads due south with an air speed of 480km/h. Measurements made from the ground indicate that the plane's ground speed is 528 km/h is 528 km/h at 15 degrees east of south. Calculate the wind speed.

    I have a diagram of what I worked out so maybe someone could show me what I did right and what I did wrong, that'd be nice.

    http://img497.imageshack.us/img497/6442/diag0ia.png [Broken]
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 12, 2006 #2


    User Avatar
    Homework Helper

    You've got your plane heading 15 deg south of east, not 15 deg east of south!
    15 deg east of south is at -75 degrees( that's 75 degrees clockwise from direction of the positive x-axis)
  4. Feb 12, 2006 #3
    Okay thanks. I'll try it again and see what I come up with.
  5. Feb 12, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    I would suggest using -j (-y) for S, and i (+x) for E.

    Consider the ground speed as the true speed. Resolve the ground speed vector into its S and E components. The air speed is 480 km/h, so the air must have a southward component to carry the plane with a faster ground speed.

    What are the S and E components of the ground speed vector?
  6. Feb 12, 2006 #5
    I figured it out. The problem were in the angles, so I redrew it like this:

    http://img490.imageshack.us/img490/842/diag3tf.png [Broken]

    Then I used cosine law to get the wind speed. Just thought I'd post the solution so people would know :D
    Last edited by a moderator: May 2, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook