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Question about 2 ships - vectors?

  1. Sep 6, 2011 #1
    1. The problem statement, all variables and given/known data

    Ships A and B leave port together. For the next two hours, ship A travels at 35 mph in a direction 20° west of north while the ship B travels 10° east of north at 40 mph.

    (a) What is the distance between the two ships two hours after they depart (in miles)?

    (b) What is the speed of ship A as seen by ship B (in mph)?

    2. Relevant equations

    3. The attempt at a solution

    I have no idea how to approach this problem. I believe it is vectors, however as I said I'm not sure. Any help would be greatly appreciated.
  2. jcsd
  3. Sep 6, 2011 #2
    Start by drawing 2 vectors from one point, lets say (0,0)
    -One vector will go at a 20° angle to the west (of north) and the other will go 10° east (of north)
    -Since it's 2 hours you can make the vector length double (70 and 80).
    -You will end up have 1 angle, 2 sides
    -You can now use the law of cosines to solve

    I believe this is correct, if not someone will correct me :p

    Edit: that's part A.
  4. Sep 6, 2011 #3
    Thank you, that made a lot of sense and I calculated part A correct. How would I calculate part B though? What does speed as seen by the other mean?
  5. Sep 7, 2011 #4


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    Pretend you were standing on boat B watching boat A. If both boats were travelling in tje same direction at the same speed, boat A would appear stationary to someone on boat B.
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