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Equation to model dripping faucet or slowing roulette wheel?

  1. Sep 11, 2004 #1
    I've noticed when I take a shower that after I turn off the water, the water that's left in the pipe runs out in a regular pattern. At first it streams out continuously, and then as the amount of water decreases, the stream turns to a trickle, and then a drip, and the drip is the most interesting part. At first the drip is several times a second,


    then it slows to about once a second

    and then it slows to where there are several seconds between drips so that the entire pattern is

    you get the idea.

    I've noticed a similar pattern when watching wheel of fortune and seeing the wheel spin around, the ticking sound makes the same pattern as the wheel slows.

    The question is whether there is an equation that describes this pattern. In trying to develop one, I've come to a few other questions specific to my attempt.

    So, I'll leave the first question as "is there an equation that models the dripping of water (not from a leaking pipe) but from a pipe with a finite amount of water, or similarly, is there an equation that models the spinning of a slowing roulette wheel such as the wheel of fortune wheel or the price is right wheel."

    part of me thinks that this is simply a constant deceleration problem and the (1/2)at^2 nature is messing with my head, but I'm not familiar enough with physics to work out whether this hunch has merit.

    For my faucet, I've approximated the time between drips as being 3/2 times the previous time between drips. For example, if the first drip and the second drip were a second apart, the second and the third drip would be a second and a half apart. So, the time between drips 1 and 2 is 1, and between 2 and 3 is 3/2*1 and between 3 and 4 is 3/2*3/2 and between 4 and 5 is 3/2(3/2*3/2) = (3/2)^3 which is the geometric series for (3/2), which diverges for values greater than 1.

    I know this isn't an exact model, because the bathtub drips come to be approximately 6 seconds apart steadily, but by this point I was more interested in the question of finding an equation for the drips if the rate of dripping was given by the above geometric series.

    So I said that the rate of dripping versus the rate of drops (time as a function of drops) was equal to (3/2)^n, or
    dt/dn = (3/2)^n, and I tried to solve this differential equation, which I presumed for a given value of n, would be equal to the finite value of the geometric series for r!=1.
    Which is ( 1 - r^n ) / (1 - r) according to
    this site .

    Unfortunately, I couldn't get this to work out. So my second questions is can anyone get this model (albeit not the correct model for the motivating problem) to work out so that the function, total time t(n) for the nth drop is equal to (1-r^n)/(1-r).

    Thanks. I hope these problems are as appealing to some of you as they have been to me.
  2. jcsd
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