The truth is that although it seems so, the water in the drain doesn’t end up flowing any faster. The rate at which the water comes out of the drain depends on what’s above it, because all of that puts pressure on it. It’s like a piston, the more on top, the more pressure there is and the faster it comes out. It is defined by Torricelli’s theorem, which we have known since the 17th century. And in the drain, the water has the pressure of the air plus the pressure of the water itself; Because less water is left, the air pressure stays the same, but the water pressure decreases.
As the water flows down the drain, it has less thrust and loses speed. There comes a time when the speed drops so much that it stops. And there’s even a pool of water at the end that doesn’t even fall down. It looks great doing a simple experiment at home.
You take a plastic cup and make a hole in the bottom. You put a finger on the hole and fill the glass with water. If you take your finger away and let the water flow out of the hole, you can see that it flows out much faster at first, and as there is less water left in the glass, the speed decreases because the pressure also decreases. And in the end it comes out so slowly that you can see the drops separated from each other. As soon as you tilt the glass, it becomes perfectly clear that the water comes out at a certain angle at first, it’s like a parabola shot. Like kicking a ball, the arc will be larger or smaller depending on the speed at which the water exits. And what you see in this experiment is that the arc decreases when there is less water left, which is evidence that its speed decreases as well.
This is studied a lot in engineering. In addition to pressure, the rate at which the liquid descends down the drain (or any pipe) is also affected by other phenomena: due to surface tension, the water flowing through the middle is freer than that clinging to the wall , and this increases if the drain also arcs and causes not all the water to flow at the same rate; The increase in velocity creates turbulence at the same time, which also affects the shape and velocity with which the liquid descends. And on top of all that, the water can have a different viscosity because it contains soap or foam or other products. And all these physical phenomena have to be taken into account, so it’s a very complex problem.
But the question makes a lot of sense because we’re all under the impression that’s exactly what happens, that the water that goes down the drain ends up flowing faster. And that’s because when you watch the water fall down the drain, you see the top of the water, and it’s at a very low speed. You can’t see what’s happening down the drain, you can’t see the water falling down. But when it’s almost completely empty you can see the water falling and you can estimate the speed at which it’s flowing down, which wasn’t possible before because the water at the top was covering what was flowing below. And so when you finally see the water flowing down the drain, you get the impression that it’s going faster, when it’s not.
Ruth Laskoz She has a PhD in physics and is a professor and researcher at the University of the Basque Country.
Question sent by José Antonio Nuñez.
Coordination and writing: Victoria Bull
we answer is a weekly scientific advisory session hosted by the Dr. Antoni Esteve Foundation and the L’Oréal-Unesco program “For Women in Science” and answers readers’ questions about science and technology. They are scientists and technologists, members of AMIT (Association of Women Researchers and Technologists) who answer these questions. Send your questions to [email protected] or on Twitter #weanswer.
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