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Homework Help: Time for boat to make two sets of displacements

  1. Sep 30, 2008 #1
    1. The problem statement, all variables and given/known data

    I would really appreciate if someone could check my work for this poblem b.c i seem to be getting the wrong answer.

    Problem: A boat moves through a river at 6.1 m/s relative to the water, regardless of the boat's direction. If the water in the river is flowing at 2.4 m/s, how long does it take the boat to make a round trip consisting of a 246 m displacement downstream followed by a 194 m displacement upstream? Answer in units of seconds.

    First, I used the pythagorean theorem to find the resultant vector- 6.555150647m/s. Then, used distance=Velocity * time . I plugged in 246m for distance and 2.4m/s for the velocity to get 102.5 sec for time. I did the same thing with the second displacement to get 183.3333333s for time. Then I added them together for a total time of 183.3333333s.
     
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  3. Sep 30, 2008 #2

    madmike159

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    If its going down stream then up stream surly that means its going from A to B and then B to A.
    If its moving 6.1m/s relative to the water down stream it would be moving at 8.5m/s
    up stream it would be moving at 3.7m/s (unless it is taking 6.1m/s as its actual velocity, but then why would you have the speed of the water?).
     
  4. Sep 30, 2008 #3
    hmm i kinda follow, but how do you find the time?
     
  5. Oct 1, 2008 #4

    madmike159

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    With distance = velocity * time. If your having trouble with the boat moving relative to the water think of trying to measure the speed of a car from inside another moving car. If you were measuring car A (30mph) from car B (20mph) it would look like car A is going 10mph.
     
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