Physics Question, Is My Explanation Correct?

darthbob88 · Post by **darthbob88** » 2007-12-21 11:43am

I'm sorry I can't give video right now, so I'll have to make up for it with 1000 words.

My family has a fairly large sink for washing dishes in; several gallon capacity, easily, and of the usual construction, square in shape with the bottom sloping towards a drain at the center. We also have a large, round, heavy-bottomed saucepan, of a scale with the sink but smaller. After I'm through washing dishes, I will generally take the pan and put it in the water, lowering it until it nearly covers the drain and the water in the sink rises along the sides. When the pan is set in more-or-less straight and flat over the drain, it will remain there, held in place quite firmly. When it is set down unevenly over the drain, however, it will rapidly oscillate from side to side, like a metronome or a see-saw. This effect will last so long as there is water draining out of the sink, and so long as the level of the water is over the bottom of the pan. What causes this phenomenon?

My hypothesis is that the hydrodynamics involved are similar to those in venturis; as water drained through the gap between the bottom of the pan and the bottom of the sink, its velocity increased and its pressure decreased, forcing the pan to swing that way. As the pan swung thusward, cutting off the flow of water, the pressure and force holding it there was discontinued, and the pan swung back the other way in order to balance itself. This will continue so long as there is water flowing through the gaps between the pan and the sink. As soon as air injects itself into the situation, however, it will cease because the vacuum formerly filled by the pan is filled by air, and the hammering ceases.

darthbob88 · Post by **darthbob88** » 2007-12-21 11:44am

Ghetto edit: Thoughts, ideas, am I right, am I wrong?

Winston Blake · Post by **Winston Blake** » 2007-12-21 06:01pm

It makes sense to me. An alternative explanation would be that it's bouncing off the bottom of the sink because the water is 'glug-glug-glugging' instead of 'swirl-swishing', if you get what I mean. Like turning a narrow-necked container of water upside-down - it glugs and vibrates up and down.

darthbob88 · Post by **darthbob88** » 2007-12-25 12:31am

Winston Blake wrote:It makes sense to me. An alternative explanation would be that it's bouncing off the bottom of the sink because the water is 'glug-glug-glugging' instead of 'swirl-swishing', if you get what I mean. Like turning a narrow-necked container of water upside-down - it glugs and vibrates up and down.

Your explanation also seems to make sense; I'll have to test for it a few times, but I'll have opportunity over the Xmas break. Also, I didn't notice this earlier, but it seems to work well when the pan is way off center, though that proves nothing, and it "accelerates" up to cruising speed, going from thub, thub, thub to thub thub thub and ending at a sink-hammering THUBTHUBTHUB. This seems to support the theory that it's the result of an imbalance in forces, which builds to its full force, though it's mute on whether that's due to reduced pressure, or merely inefficient drainage.