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Relative Velocity question

  1. Mar 3, 2009 #1
    I'm studying for an exam and this is one of the sample questions which I cannot figure out.

    1. The problem statement, all variables and given/known data

    Two ships A and B move with constants speeds 48 km/h and 60 km/h respectively. At a certain instant ship B is 30 km west of A and is travelling due south. Find:

    i) The direction ship A should steer in order to get a close as possible to ship B.

    2. Relevant equations
    Vab = Va - Vb

    3. The attempt at a solution
    Well, Vb = 0i - 60j
    Va = -48cosxi - 48sinxj
    and therefore
    Vab = -48cosxi + (60 - 48sinx)j

    I have drawn diagrams to help but I can't figure out the right angle. I tried differentiation to find the lowest possible value for j (they are already horizontally across from each other) but I got 1 for an answer which I know is not correct. Any advice.
  2. jcsd
  3. Mar 3, 2009 #2
    Have you tried relating each 'line' between objects as vector?

    The Bob
  4. Mar 4, 2009 #3
    I'm sorry, I'm not sure what you mean.
  5. Mar 4, 2009 #4
    When you drew your diagrams, you must have had a series of velocity vectors relating to the velocities of the ships. By using x = vt you could also obtain position vectors for each of the lines in your diagram. Doing this you could use normal addition of vectors to find your unknown, in this case the closest distance between A and B, and find an equation to be solved for it.

    As this may still be confusing then let me give you my first line and see if that helps:
    xAB = -vBt + 30i + vAt

    where xAB is to be the distance between A and B, vA is the velocity vector of A, vB is the velocity vector of B, 30i is the distance between A and B initially, t is time and I have assumed that the closest distance between A and B must be on the same trajectory as A will initally head to get to this point.

    That make any sense whatsoever?

    The Bob
  6. Mar 4, 2009 #5
    Bob, Your method I think will give the closest distance given a direction. (I think, haven't studied it yet). I don't know the direction. But I solved it. Using a relative velocity diagram you can derive a formula which relates the path (angle) of A to its angle. Then the cure all for all these max/min questions. Calculus. You can differentiate the values (sorry for being vague) and get the answer.
  7. Mar 4, 2009 #6
    Far enough. It would be hard, I admit, but possible to solve with the two unknown variables. However, if you are happy with your (far-easier-it-should-have-been-staring-us-in-the-face) method then cool. Glad you've got it and that I was no help whatsoever.

    The Bob
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