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Homework Help: Problem about relative motion

  1. Sep 15, 2010 #1
    I have posted the problem and and the solution of part a,b,c in file 1.doc. This is problem
    from ch. 4 of serway. It doesnt yet assume calculus. I have question about the part c.
    I did it using calculus. but the book's solution uses the kind of arguments which i am not able to understand. Is it possible to solve the part c without using calculus.


    Attached Files:

    • 1.doc
      File size:
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  2. jcsd
  3. Sep 19, 2010 #2


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    Homework Helper

    Hi issacnewton,

    Yes, part c can be done without using calculus. Please show your attempt and I or someone else can help you.
  4. Sep 20, 2010 #3
    hi al

    I think the combined velocity has to make the minimum angle with the direction which is perpendicular to the stream.... in that case we can guarantee that the distance traveled downstream will be minimum... but I dont see how to avoid calculus again....

  5. Sep 20, 2010 #4


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    That's right, and another way to say that is that the combined velocity you are looking for is the one that makes a maximum angle away from the stream's flow.

    So what are all these possible combined velocities? I would suggest when you draw your vector diagram, draw the stream velocity first, and then think about adding all possible boat velocities (the speed is set, so you can only vary the boat's velocity angle) to that. What do you get?

    There are still a few more steps; does that help?
  6. Sep 20, 2010 #5
    hi al

    yes , basically we have to increase the angle of combined velocity vector with the stream velocity vector.... I see what you are saying...... now the diagrams given in my attachment are making some sense.....so basically we fix the tail of boat velocity vector to the tip of the stream velocity vector and then rotate this boat vector, so that the combined velocity vector makes maximum angle with the stream velocity vector.....I see where the argument is going.......let me think on these lines..........
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