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Relative Velocity Activiy

  1. May 29, 2008 #1
    1. The problem statement, all variables and given/known data
    I have a boat trying to cross a river with a current flowing ->

    The boat is attempting to make it straight across (90d). In order to do this I have to change the volocity to 119.38 and the angle to 100.3.

    So what is the speed of the current?

    2. Relevant equations

    3. The attempt at a solution
    I tried solving using: cos(100.3) = -current/119.38

    So -current = -21.345 but this is incorrect. Can anyone help?
  2. jcsd
  3. May 29, 2008 #2


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    You're off by a minus sign and perhaps a significant digit. In terms of the velocity of the current, -V_c = -21.345; thus, V_c = 21.345 to the right. In terms of speed, that's the magnitude of the velocity, always a positive number.
  4. May 29, 2008 #3
    Its still saying its wrong. It was the speed of the current but I thought thats what I was getting.
  5. May 29, 2008 #4


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    How is the problem worded? Is the current flowing exactly west to east, parallel to the x axis? Is the boat's velocity given as exactly 119.38 units? What do you mean when you say you had to 'change' the velocity to 119.38?
  6. May 29, 2008 #5
    Your task is to set the velocity of the boat so that it travels straight across the river. From the selected boat velocity, you can then compute the speed of the current.

    Instructions: (The little white spots are floating in the current.)
    (1) Drag the blue arrow tip to set the velocity of the boat relative to the water.
    (2) Press START to begin crossing the river.
    (3) If the boat misses the dock, press RESET to begin again with the same current.
    (4) You can select a new random value for the current with the NEW VELOCITY button.
    (5) When you have found the necessary boat velocity vector, use it to figure out the speed of the current. Enter your calculated value in the box labeled “current” below. Press the CHECK button to check your answer.
    (6) Keep trying until you get it right*, then press the SUBMIT button to record your score. Only one submission is allowed, but that’s enough, since you shouldn’t submit until you have it right.
    * For your answer to be excepted, it must be within 5% of the correct answer.

    Its an interactive learning thing.
  7. May 29, 2008 #6
    136.84/tan(angle) worked :P

    Thanks for the attempt at helping, much appreciated!
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