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Why does the water in my dog's bowl act like a compass? (feddit.org)
submitted 2 days ago by sbf@feddit.org to c/askscience@lemmy.world
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My dog eats and drinks out of metal bowls. I’ve noticed that when I turn around after filling up the water bowl in the sink, any spit bubbles or backwash on the surface of the water stay oriented the same way (relative to everything else, not me). Why is this? This behavior doesn’t change regardless of how fast or slow, careful or not I am. I’m not sure what kind of metal the bowl is, but it’s about the size and shape of half a basketball. Where I live, the tap water is pretty damn clean. If there are any extraneous factors I haven’t thought to mention, let me know.

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[-] MotoAsh@piefed.social 23 points 2 days ago* (last edited 2 days ago)

Probably more inertia than magnetism. The water isn't stuck to the bowl as a solid mass. Rotating with the bowl requires the water to drag on itself to move with the bowl. That takes time for enough force to transfer to really get the water rotating at any significant speed, so it merely looks like it's acting like a compass. If you keep rotating, the water will eventually catch up to the bowl's speed.

Mind, anything floating on the water could also actually act like a compass needle if it has a strong enough charge differential across it. That's not uncommon at all, it'd just be weird for spit or a collection of bubbles to have enough charge since they're bad at holding charge.

The test would be to spin around once or twice with the bowl and set it down. If it is not magnetic, the water should stay rotating very slowly and the slobber/etc will just continue to rotate a little and eventually stop and sit at a new orientation. If it IS acting like a compass, the floaters will slow down and then rotate back the few degrees the other way to stay oriented.

[-] kbal@fedia.io 11 points 2 days ago

Yes, just inertia explains what's described. I'm in the habit of drinking a hot beverage that sometimes has bits of stuff floating in it, and if there's one floating bit I want to slurp up right away the practical way to get it over to the side of the cup I'm drinking from is to turn around until I'm on the same side of the cup that it is. I rotate, the liquid doesn't. It's' true that "the water will eventually catch up" to the rotation of the bowl, but it might take quite a lot of rotations before it gains any appreciable momentum. The viscosity of water is relatively low.

[-] MotoAsh@piefed.social 2 points 1 day ago

Personally, I just got good at the cup swirl motion. Enough to get it rotating without the wave going over the top. Mostly just the lazier option but it is much less convenient to turn yourself when at a desk. lol

[-] notsosure@sh.itjust.works 9 points 2 days ago

Had the same experience. Turned out the house I was renting was haunted. We hired a psychic, costs us 1000s! We got rid of the problem eventually, but sometimes I wish I had received a better education.

[-] frongt@lemmy.zip 5 points 2 days ago

As mentioned, it's basically inertia, friction, laminar flow. You turn the bowl, but the contents are liquid, so the bowl moves around them. Only the water touching the bowl itself is moved through friction (and the water touching that water, and so on, diminishing the further you get from the bowl).

If you turn the bowl quickly, you overcome most of that friction, and less water moves. Turn the bowl slowly, and friction keeps the water together.

For a fun visual, in a still liquid, you can see that these rotations are reversible, allowing you to "unmix" a liquid! https://youtu.be/p08_KlTKP50

[-] ieatpwns@lemmy.world 4 points 2 days ago

I’m not a scientist but I think it’s because their saliva is very viscous and sticky so the bubbles on the surface dont move as easily as the water underneath.

[-] dwindling7373@feddit.it 5 points 2 days ago

I'm not a scientist but no.

Stuff on top of water will necessarily move alongside the water underneath it.

The solution lies in the viscosity of the water itself, being low enough that the motion of the molecules in touch with the rotatin bowl don't transfer enough force to the inner "layers" of water to offset its inertia.

ELI5: water be a slippery fuck, too heavy to spin at your whims.

[-] littletoolshed@lemmy.world 3 points 2 days ago* (last edited 2 days ago)

Is the surface, upon which the bowls rest, perfectly level?

[-] porksnort@slrpnk.net 2 points 2 days ago

Like lambs to the cosmic slaughter!

this post was submitted on 06 Oct 2025
32 points (100.0% liked)

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