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Homework Help: Relative Velocity

  1. Nov 9, 2007 #1
    Two swimmers, Al and Bob, live on opposite shores of a 200.0 m wide river that flows east at 0.70 m/s. Al lives on the north shore and Bob lives on the south shore. They both set out to visit a mutual friend, Ted, who lives on the north shore at a point 100.0 m upstream from Al and 100.0 m downstream from Bob. Both swimmers can swim at 1.4 m/s through the water. How much time must Al wait after Bob sets out so that they both arrive at Ted's place at the same time? Both swimmers make their trips by the most direct routes.

    So, first I can find the time taken by Al.
    = 100/V_aw-V_wg
    =100/0.7= 142.85 Seconds

    Now, for Bob

    we could use trig to find the angle (beta) of his most direct route.
    but 1st we need to find theta.

    So using tri==== Tan(theta)= 200/100
    And we get theta as 63.4 degrees.
    Then we can find beta, since one angle is 90 and the other is 63.4.
    so beta will be= 26.5 degrees
    Then, Sin(26.5)= 0.7/hypotunese
    hence hypotunese= 1.56m/s
    This is bob's speed (most direct route)
    Now for his time, t= 200/1.56= 128.20 seconds.

    T1-T2= 142.82-128.20= 14.62 seconds.
    hence Al must wait for 14.62 seconds after Bob has left inorder for both of them to reach Ted's place at the same time.

    Does this look good?
  2. jcsd
  3. Nov 9, 2007 #2
    Your answer for Al is correct but you need to show that he can reach the other bank in that time, too.

    Regards Bob, the angle his most direct route makes with the banks is your theta and the angle it makes perpendicular to the banks is your beta (you miscalculate beta by 0.01 degrees).

    Bob's "most direct route" diagram can't be used as the vector addition triangle to find the velocity that he travels on that route. Imagine yourself as Bob, swimming. What direction would you have point your body so that addding your swimming velocity to the river's flow velocity, you travel on the most direct route?
  4. Nov 10, 2007 #3
    I guess, Bob will have to travel towards the north direction, and then let the eastwards water current carry him.
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