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: 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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook