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DE: Unit Cancellations Not Making sense

  1. Jan 30, 2011 #1
    1. The problem statement, all variables and given/known data
    Consider a tank used in certain hydrodynamic experiments. After one experiment the tank contains 200 liters of a dye solution with a concentration of 1 g/liter. To prepare for the next experiment, the tank is to be rinsed with fresh water flowing in at a rate of 2 liters/min, the well-stirred solution flowing out at the same rate. Find the time that will elapse before the concentration of dye in the tank reaches 1% of its original value.

    2. Relevant equations
    [tex]\frac{dQ}{dt}[/tex]=rate in - rate out

    3. The attempt at a solution
    Why the heck do they cancel units incorrectly?
    [​IMG]
     
  2. jcsd
  3. Jan 30, 2011 #2

    Dick

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    Hard to say until you tell us what the heck Q is.
     
  4. Jan 30, 2011 #3
    Q is the amount of dye in the tank.
    dQ is the rate of change of the amount of dye in the tank.
     
  5. Jan 30, 2011 #4

    Dick

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    Then all the terms in your equation should have units of grams of dye per minute. I think the Q/100 l/min should be g/min. Probably a typo.
     
  6. Jan 30, 2011 #5
    Ok that's what I was hoping - thank you.
     
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