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Simple Relativity

  1. Oct 18, 2007 #1
    1. The problem statement, all variables and given/known data
    A boat travelling with the current has a speed of 10km/h.
    The same boat travels against the same current at the speed of 6km/h.
    At what speed is the river flowing?


    2. Relevant equations



    3. The attempt at a solution
    10km/h - 6km/h = 4km/h ?
     
  2. jcsd
  3. Oct 18, 2007 #2

    learningphysics

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    No. That's not right.

    If the speed of the boat is b, and the speed of the river is r... what is the speed of the boat travelling with the river in terms of b and r... equate this to 10.

    this gives 1 equation..

    what is the speed of the boat travelling against the river in terms of b and r... equate this to 6.

    this gives a 2nd equation... so you have 2 equations with 2 unknowns.

    solve for r speed of the river.
     
    Last edited: Oct 18, 2007
  4. Oct 18, 2007 #3
    how do i do that? = S
     
  5. Oct 18, 2007 #4

    Dick

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    Do what learningphysics suggested. If the speed of the boat is b and the speed of the river is r, they add and give 10 if they are going in the same direction. They subtract and give 6 if they are going in opposite directions. Write down those two equations. And then solve them.
     
  6. Oct 18, 2007 #5
    b + r = 10, b = 10-r
    b - r = 6

    10-r-r = 6
    10-2r = 6
    4 = 2r
    r = 2 ?

    : )
     
  7. Oct 18, 2007 #6

    Dick

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    r=2. That says it all.
     
  8. Oct 18, 2007 #7

    learningphysics

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    Yup r = 2 km/hr is the answer.
     
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