Date: Thu, 16 Sep 2004 15:51:52 -0600

Author: Jerry DiMarco

Subject: Re: Perform an experiment :)


At 04:21 PM 9/16/2004, you wrote:
>Awesome, Jerry.
>The only criticism I had of the McDonald's Prinecton paper was that he has
>the initial velocity nonzero, which isn't physical, but then I re-read
>more closely and sure enough, he discusses the initial acceleration and
>then takes it from there to find what he later refers to as the 'initial
>velocity' of the tank. I'm still a little amazed, though, that his result
>is borne out by experiment. Could your apparatus test his conclusions
>about the final state of the system as a function of the initial masses?

I'm not sure it's sensitive enough. The initial motion is very
slight. If I start adding mass to the boat it might become
unnoticable. I'd like to try a bigger hole first.
One thing I do agree with McDonald about is that the real physics is
going on in the motion of the water in the tank. This is the second time
at least that I've been stumped by a problem involving water draining out
of a container because I don't understand how the water flows. I did an
experiment awhile ago similar to what Adam described and got what I would
call preliminary results, indicating there is a vertical gradient of water
velocity that increases with depth. But I couldn't get long enough runs
(shallow tank), and then had to abandon the effort because it got busy
here. Does anyone know if this has ever been studied?


>Jason St. John 617.353.5980
>Boston University Physics Department
>Room 255, 590 Commonwealth Ave. Boston, MA 02215
>On 2004-09-16.13:58 sent:
> I already did a quick experiment just to see if it was possible. I
> decided
>to do it with a boat on water because I wasn't sure my carts had low enough
> I used a Coleman cooler for my tank (biggest tank I have), and a
> styrofoam
>boat that is twice as long as it is wide, that has 1" thick walls and
>bottom. I
>melted a ~ 5/8" hole through the bottom at one end with a heated pipe. A
>stopper was used to plug the hole from the bottom - it was pushed in only hard
>enough to seal.
> The boat was filled with water and allowed to stabilize, then I
> pushed the
>stopper out with the end of my pen. I did this several times, and each
>time the
>boat moved ever so slightly away from the drain initially, but did not seem to
>accelerate. It appeared only to get an impulse when the water began to flow,
>then coast until the water was almost out, at which time it reversed direction
>and seemed to accelerate for a brief period toward the drain. It definitely
>traveled at a higher speed in this direction, passing its starting point quite
> Not only was I surprised by this result, I was even more surprised
> that the
>motion matched the description given by the Princeton fellow (see Chele's
>email). He must have done the experiment too because his description is not
>complete. I don't think he understands everything that's going on either.
> If I have time, I'm going to try it again with a larger hole. If the
>effect is strong enough maybe I'll try it on a cart. Has anyone else tried?
> Jerry


Jerry DiMarco
Manager of Lecture Demonstrations and Instructional Labs
Montana State Univ., Physics Dept.
Bozeman, MT

Our Motto: "There's a demo in there somewhere."
From Thu Sep 16 21:31:25 2004