1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Why is this certain angle 20 degrees?

  1. Jun 13, 2010 #1
    1. The problem statement, all variables and given/known data
    Problem #199
    http://img88.imageshack.us/img88/8008/scan0001vd.jpg [Broken]
    Solution
    http://img21.imageshack.us/img21/2131/199hg.jpg [Broken]

    2. Relevant equations
    Why is the angle from VB/A 20 degrees from the solution diagram? It would seem that I had to know that direction of VB/A had the same angle as VA in terms of the geometry (if two parallel lines are cut by a transversal, its alternating interior angles are equal). I just don't see how you can assume that.


    3. The attempt at a solution
    Since drawing a triangle is the first part, I don't have any "attempt" at it yet.
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jun 13, 2010 #2

    DaveC426913

    User Avatar
    Gold Member

    I'm not sure what you're asking.

    You are given that the bearing is 20 degrees. i.e. A takes a bearing of B and sees it is 20 degrees East of North, thus theta is 20 degrees.
     
  4. Jun 13, 2010 #3
    I saw that 20 degrees was given, I just don't see how the angle is 20 degrees on the solution diagram. Ship A observes ship B at 20 degrees, but how is VB/A also 20 degrees down from horizontal? Sorry for the confusion!
     
  5. Jun 13, 2010 #4

    DaveC426913

    User Avatar
    Gold Member

    It's been while, sorry, VB/A represents what part of the diagram?

    I'd assumed we only care about angle theta, which is 20.
     
  6. Jun 13, 2010 #5
    VB/A comes off the head of VA. I just don't see how its 20 degrees when VB/A and VA are connected as seen on the solutions diagram.
     
  7. Jun 13, 2010 #6

    DaveC426913

    User Avatar
    Gold Member

    Sorry, I hadn't looked at the second diagram.
    Well, the solution triangle is just a rearrangement of the starting configuration. The two vectors start off at 20 degrees, why would that change?
     
  8. Jun 13, 2010 #7
    Oh, I guess I made the wrong assumption. I was assuming the angle that VB/A made was not necessarily 20 degrees. I guess I'm confused (a little bit) still is because I can't see it geometrically. Like say for instance, I'm still on the assumption that the angle is not 20 degrees for VB/A and I label as an unknown, how would I geometrically prove that the angle is 20 degrees?
     
  9. Jun 13, 2010 #8

    DaveC426913

    User Avatar
    Gold Member

    You would not be able to solve the problem. You're given the angle because you need it.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook