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Homework Help: Relative motion question

  1. Apr 3, 2013 #1
    If a man how is able to row on still water at 2.5m/s. If the water is moving at 1.5m/s, in which direction should he row.
    The answer is he should row at 53.1 degrees upstream but the question for an observer on the boat and another observer on land, how will the motion appear?
    For an observer on land will the man appear to be rowing along a perpendicular to both river banks ?? Or is it for an observer flowing smoothly with the river current ??
    Hence I would like to know how is motion will look like from three different points, from the land, from the water and from the boat !
    Thanks !!
    Last edited: Apr 3, 2013
  2. jcsd
  3. Apr 3, 2013 #2
    The observer co-flowing with the stream and the observer on the land will be observing the motion of the boat, that is clear.

    But what is the observer on the boat going to observe?
  4. Apr 3, 2013 #3
    The thing that is bothering me is with respect to an observer flowing with the river due east it will appear that the boat will be going vertically upstream. Of course the observer on the boat knows where he is going (2.5m/s at 53.1 degrees upstream. My question is on the ground/bank, what will a stationary observer see ??? Surely he can't see either of the two cases mentioned above ??
  5. Apr 3, 2013 #4
    The co-flowing observer cannot just see that the boat is going strictly against the stream. In the end, the boat DOES cross the river, and that will be seen by any observer.

    In any case, the observed velocity of the boat is its velocity in the co-flowing frame of reference plus (vector plus!) the velocity of the co-flowing frame relative to whatever frame the other observer is in.
  6. Apr 3, 2013 #5
    But if you draw a vector diagram the resultant of the boat and the river current, you will notice that the river current and the relative velocity vector are perpendicular !!
  7. Apr 3, 2013 #6
    The relative velocity is not perpendicular. It is at 53.1 degrees. The resultant velocity is perpendicular.
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