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Relative velocity of river swimmer

  1. Nov 21, 2014 #1
    in swimmer-river physics problems ,velocity of swimmer in still water=velocity of swimmer with respect to ground OR velocity of swimmer in still water=velocity of swimmer with respect to river
    which of these is correct .I am really confused.
    Last edited: Nov 21, 2014
  2. jcsd
  3. Nov 21, 2014 #2


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    I don't know anything about swimming. In flying, a pilot uses "ground speed" for navigation and "airspeed" for performance characteristics. They're related and both are necessary. I would expect swimming to be similar, since fluid dynamics rules both situations.
  4. Nov 21, 2014 #3
    i am talking in context of swimmer-river physics numericals.
  5. Nov 21, 2014 #4


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    Sorry... I just re-read your first post more carefully. In still water, there is no difference between the river and the ground as reference points, so both of your choices are correct.
  6. Nov 21, 2014 #5


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    Staff: Mentor

    In still water, they're the same so are both equally correct. If there's a current, then the problem is incompletely specified if it doesn't say which one is intended.

    In some problems, you will be expected to figure out which one is intended from the context.
  7. Nov 21, 2014 #6


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    There is no problem if you use the velocity through the water vector and the velocity of the water vector. The velocity over the ground is the vector sum. How is that confusing? Vectors work that way and sticking to the rules will give the right answer. (As long as we avoid relativistic situations or different geometries)
  8. Nov 24, 2014 #7
    I have done so many problems like these (river-boat). I have done those from I E IRODOV and S S KROTOV. There I took velocity in still water as velocity wrt river.
    Reason : if I am the river, to me I am at rest. To earth, I am flowing. Hence to me the boat moves with the velocity the sailor thinks he is moving....
    I have got all answers using this. I was confused initially.IF U HAVE A SPECIFIC QUESTION, I CAN SHOW YOU HOW TO USE RELATIVE VELOCITY CONCEPT.
  9. Nov 24, 2014 #8
    you are right in my textbook also velocity in still water is taken as velocity wrt river.But i am still not getting it' reason.
  10. Nov 24, 2014 #9


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    I guess "velocity In still water" is short for "velocity in the reference frame where the water is at rest", or for "velocity in the ground frame if the water wouldn't move relative to the ground".
  11. Nov 24, 2014 #10


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    Always remember the simple rule that absolute motion is relative motion plus dragging motion, where:

    Absolute motion => that of the swimmer with respect to the river's bank
    Relative motion => that of the swimmer with respect to the water
    Dragging motion => that of the water with respect to the river's bank

    With that in mind, solving that type of problems becomes very easy.
  12. Nov 24, 2014 #11
    1)Imagine you are the river.to you, you are at rest.
    2)Now velocity in still water means that the water is not flowing. The possibilities are that it can be with respect to ground or wrt river.
    3) If I am the ground, I can see the river flowing.Hence it is not wrt ground.
    4)If I am the river I feel that I am at rest. It is like travelling in a train on a very smooth track. You can't feel the train moving unless you look out of the window.Hence it has to be wrt river.
    5) If you still don't understand ask your physics teacher or refer " Halliday Resnick and Krane" book or any mechanics book by reputed authors.
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