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Finding relative speed

  1. Oct 9, 2007 #1
    1. The problem statement, all variables and given/known data

    A ship cruises forward at Vs = 5 m/s relative to the Water. On deck, a man walks diagonally toward the bow such that his path forms an angle theta = 22 degrees with a line perpendicular to the boat's direction of motion. He walks at Vm = 2 m/s relative to the Boat.

    Here is the image


    At what speed does he walk relative to the water? Answer in units of m/s.

    2. Relevant equations

    I'm unsure at what equations to use.

    3. The attempt at a solution

    Do I find the horizontal movement then minus that number to 5 m/s? Hmm.

    Would I do cos 68 = adj/hypo. I got 68 because 90 - 22 = 68 and, I couldn't do cos 22 because it's not a right triangle when I draw in the "imaginary" lines. But what is adj and the hypo and the opposite lengths?

    I'm at a lost. Help please
  2. jcsd
  3. Oct 12, 2007 #2


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    Homework Helper

    You need to add the velocity of the boat relative to water... to the velocity of the man relative to the boat...

    take x to be the direction of the boat's motion..

    take y perpendicular to x...

    You know that the boat's velocity relative to water is:

    vxboat = 5m/s
    vyboat = 0m/s

    Now, get the x and y components of the man's velocity relative to the boat:

    what is vxman and vyman?

    The hypoteneuse of your right triangle is 2m/s. The angle is 22 degrees. calculate the two legs of the right triangle.
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