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Force and Velocity

  1. Oct 27, 2009 #1
    1. The problem statement, all variables and given/known data

    A 1 meter long hose of 8 cm diameter is conneted to a faucet. The end of this hose is connected to a second hose which is 4 meters long and 4 cm in diameter. At the open end of the second hose water flows out at a rate of 8 cm cubed/s.

    What is the ratio of the speed of the water flowing in the second hose to the speed of the water flowing in the first hose?

    2. Relevant equations

    P = F/A Not sure what other equations I could be using!

    3. The attempt at a solution

    At first I assumed v1/A1 = v2/A2, so (8 cm cubed/s)/(4pi) = (x)/(16pi), and thus the answer ought to be 1/4, but that is incorrect. I also tried to use squares, since I reasoned that a = (v^2)/r, but this gave me the equally incorrect ratio of 1/8. Help!
  2. jcsd
  3. Oct 27, 2009 #2


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    Welcome to PF!

    Hi FauxNews! Welcome to PF! :wink:
    hmm … so you think velocity is proportional to area?

    why? :smile:
  4. Oct 27, 2009 #3
    Thank you for the welcome!

    Well, I know that F1/A1 = F2/A2...correct? But I suppose that would not hold true for velocity, now that I think about it! I'm completely confused. :(
  5. Oct 27, 2009 #4
    tiny-tim's question is a hint to guide you on your way. The fact that A1v1 = A2v2 is only true because of the way the rate of flow of a fluid "works."

    So, go back and figure out why A1v1 = A2v2, and see if that helps you find the solution.

    Good luck!
  6. Oct 28, 2009 #5


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    Hi FauxNews! :smile:

    (just got up :zzz: …)

    Hint: conservation of mass. :wink:
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