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Homework Help: Relative motion of ships

  1. Oct 22, 2008 #1
    1. The problem statement, all variables and given/known data
    Two ships, A and B, leave port at the same time. Ship A travels northwest at 22 knots and ship B travels at 29 knots in a direction 40° west of south.

    (a) What is the magnitude the velocity of ship A relative to B?

    (b) What is the direction of the velocity of ship A relative to B?

    (c) After what time will the ships be 125 nautical miles apart?

    (d) What will be the bearing of B (the direction of B's position) relative to A at that time?

    2. Relevant equations

    V_aw=V_ab+V_b Where a is boat a, b is boat b, and w is the water-also aw is ship a's velocity with respect to the water, ab is the velocity of a with respect to b)

    3. The attempt at a solution
    I found the componets of the two vectors(with respect to a y axis thats positive in the north direction and an x axis positive in the east direction) as:
    A=(-15.55i+15.55j) or (22cos45+22sin45)
    B=(-22.22i+-18.64j) or (29cos40+29sin40)

    I've found these componets with respect to their own frames of reference, however the problems asks for the magnitude of the velocity of a with respect to b, so should I measure my angles of a from the b vector? I'm lost as to how to solve. Also I've come to the conclusion that this is addition( or subtraction) of vectors a and b, being that its a with respect to b however I'm not sure if it is B-A=C (with c being the velocity of a with respect to b)or A-B=C.
  2. jcsd
  3. Oct 22, 2008 #2
    So I'm looking for V_ab( velocity of a with respect to b), therefore the velocity of a(V_a) is equal to the velocity of b(V_B) plus the velocity of a with respect to b(V_ab) and I know both a and b so:




    however is is showing as incorrect(I used 7 since it asked for the magnitude)
  4. Oct 22, 2008 #3
    I also tried the vector equations:




    so when I seperate the equations I get:





    therefore V_ab=(6.67i+3.09j)
    so the magnitude of V_ab should be 7.35
  5. Oct 22, 2008 #4
    I really need help with this, its due in a few hours. =/
  6. Oct 23, 2008 #5


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Jharr94! Welcome to PF! :smile:

    West of south is between southwest and south.

    You're using south of west. :cry:
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