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Calculating Distance and Time with two boats in a river

  1. May 11, 2012 #1
    1. The problem statement, all variables and given/known data

    Two boats, A and B, travel with a velocity of 4.90m/s across a river with a width of 72.9m. The river flows with a velocity of 2.50m/s. Boat A travels the shortest distance and boat B travels the shortest time. If both start at the same time, how much time will they take to cross the river?

    2. Relevant equations

    There are no equations given, but I was able to use this...
    Δd/v(boats)+v(river)=shortest time
    Δd/v(boats)-v(river)=shortest distance

    3. The attempt at a solution
    Boat A: (Shortest Distance)
    72.0m/(4.9m/s-2.5m/s)=72/2.4=30s

    Boat B:(Shortest Time)
    72m/(4.9m/s+2.5m/s)=72/7.4=9.7s≈10.0s

    My Answer: Boat A can make it in 30s while Boat B can make it in 10s. I did this on a test and missed all of the points possible. Can anyone please help me find my error ansd reach a resonable answer? Thank You!
     
  2. jcsd
  3. May 11, 2012 #2

    gneill

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    Staff: Mentor

    Neither boat travels directly with or against the current (so simply adding or subtracting the current speed to the boat's speed is not correct). You have to consider velocity components (vectors). Also, the distance is given as 72.9m and you've used 72m.
     
  4. May 12, 2012 #3
    Sorry, I meant to type 72.0 in the original problem instead of 72.9.
    Can you explain to me what you mean by vector components? I know what they are, I am just confused as to how you can apply them to this problem.
     
  5. May 12, 2012 #4

    gneill

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    Staff: Mentor

    A boat's velocity with respect to the river can have two components; one directed straight across the river, and one directed up or downriver. The component directed straight across moves the boat in the direction of the far shore. Only the one directed up/downriver can influence the boat's motion up/downriver.
     
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