The roaring thirtieth. Resonance.

Kitchen physics.

The sink in the kitchen, where I wash the dishes after breakfast and lunch, is equipped, in addition to the water drain pipe, with a powerful pump built into it. At first, I did not use it, considering it a purely American whim. But I soon realized that this is a completely reasonable improvement: It prevents clogging and clogging of the drain pipe. The pump creates a powerful flow of flowing water and thereby washes away possible “deposits” in the pipe.

I started using it twice a day: When washing dishes after breakfast and then lunch. At the same time, an interesting effect was found. When the sink is filled with hot water and I turn on the pump, after a short time the pump starts roaring, the whole sink and even the large board holding it begins to shake violently. After three to five seconds, this shaking and roaring stops. And after emptying the sink, there is a short roar sound and then everything calms down.

The volume of the sink is THIRTY LITERS!

The explanation is elementary:

The pump initially rotates its blade disk in the air and some vibration of it is in the region of relatively high frequencies. When the pump is loaded with a stream of water, the rotation frequency begins to decrease and passes through the natural oscillation frequency of the sink and the large kitchen board on which the sink is fixed. The coincidence of frequencies causes resonance in the pump– sink–board system and strong vibrations of the sink and board. But after a short time in a few seconds, the speed of the blade disc drops even lower and the driving frequency already decreases below the natural oscillation frequency of the sink and board. The vibrations stop. When the water is all sucked out, the pump begins to gain momentum again and again, but QUICKLY, passes a resonant frequency – a short “Roar”, because it is already gaining momentum not in a continuous stream of water, but in the air.

I conducted such experiments: I took a full sink of hot water after washing and rinsing the dishes, then, WITHOUT TURNING ON the pump, opened the drain hole, waited for a few seconds until all the water began to pour out with a “rapid jack” and only then turned on the pump.

Interestingly, there was NO roar and vibrations!

Why?

Yes, because the pump started working already at low speeds, pumping a large volume of water and therefore did not reach the resonant frequency. Only then, when all the water was draining away, there was a short “Roar” of the frequency passing through the resonant.

That’s the physics in the kitchen!

6 XI 2024