# How the Wet-Dog Shake Gets Mammals Dry in No Time Flat

By Sophie Bushwick | August 16, 2012 2:39 pm

There’s a certain expression a wet dog wears as it trots up to you, a kind of gleam in the eye that says, “I’m about to shake so vigorously that in a mere 4 seconds, 70 percent of the water in my fur will fly off of my coat and on to you.”

But the wet-dog shake, though it’s an annoyance to us, may be a survival technique to dogs. The water that sticks to a mammal’s fur can lower its body temperature, causing hypothermia, so it behooves wild animals to get rid of all that water as quickly and efficiently as possible.

To find out just how efficient the wet-dog method is, researchers from the Hu lab at Georgia Tech filmed 33 different wet zoo mammals from rats to kangaroos to lions and tigers and bears (oh yes) with high-speed cameras and analyzed the motion of their bodies, skin, and fur. Their research was first published back in 2010, but their latest study, published in the Journal of the Royal Society Interface, improves their mathematical model of the wet-dog shake and reveals how much force the furballs can generate. (The paper is not yet online; we will provide a link when it becomes available.)

Dogs shake with a characteristic oscillation, twisting their bodies from side to side at a set frequency to generate so much centripetal force that water droplets go flying. The shaking animals observed in this study generated forces 10 to 70 times greater than gravity, a feat that was easier for the larger mammals.

This was because more massive animals have a bigger body radius: they can shake with a lower frequency and still generate as much centripetal force as the faster-shaking smaller animals. Loose skin helps, too, because it whips around the body and throws off more drops.

So if you own a big, jowly dog, you have even more reason to keep your distance after a dip.

Video courtesy of Nature News & Comment

CATEGORIZED UNDER: Living World, Physics & Math
• Jay29

Wow, great video. Never seen a mouse shake in slo-mo before — 18 to 27 times per second, amazing!

• John Lerch

The tuned frequency is just so the animal can impart enough motion to the lower parts. IOW notice the squeeze motion from head to toe; but there is no squeegee. So the sequential tuned motion is for the body as opposed to for the drops per se.
Hmm maybe I need to stop trying to be lean and hard and get some folds of fat.
BTW there is no such thing as a centrifugal force; the drops are going straight and the surface just curls away from it.

• Willy

I’m not a scientist, but I play one on the internet.

Anyone ever notice that humans have this too? It’s sort of a half-assed shake that lasts only a few seconds. It comes over you without your consent. Sometimes shortly after you’ve gone to the bathroom, or if you’re a little chilly. But it’s such a weak, ineffective effort that it seems to be something we’re evolving away from. I feel like we used to be able to shake more vigorously and started losing the ability when we were better able to control our body temperature through clothing, fire, etc.

• OldBob

It is the centrifugal force and not the centripetal force that causes to fly off the animal’s fur. The centripetal force tends to hold things close to the radial center of the rotation, while the centrifugal forces tends to act away from the center of the rotation.

Centrifugal force is very real. It is for force trying to pull a moving body away from the center of rotation.

• lonestarr53

People who have not studied science, should refrain from making ignorant comments, Mr. Lerch. Centrifugal force is a term used in physics to describe “an apparent force that acts outward on a body in motion around a center, arising from the body’s inertia.” (American Oxford dictionary) Besides, the term used in the article is “centripetal” which indicates a motion toward the center as opposed to away from the center…

• Leatherneck

twisting their bodies from side to side at a set frequency to generate so much centripetal force that water droplets go flying.

Actually, CENTRIFUGAL FORCE causes the droplets to flee the dog.

TC

• Sophie Bushwick

The centripetal-vs-centrifugal discussion is a bit more complicated than I want to get into in a comment. To put it as simply as possible, in the words of University of Washington physicist Andy Ganse, “Centripetal force and centrifugal force are really the exact same force, just in opposite directions because they’re experienced from different frames of reference.” He’s got a great explanation of reference frames, and how a rotating (and thus non-intertial) reference frame gives rise to the centrifugal force, in this blog entry: http://staff.washington.edu/aganse/blog/files/centrip.html

Or you can just go with the xkcd version: http://xkcd.com/123/

• http://www.elliotmabeuse.com dr_mabeuse

Mr. Lerch is correct in saying that centrifugal force, strictly speaking, is not a real fundamental force in physics, on a par with electromagnetism, the strong, the weak, and gravitational force. However the word’s still used because it’s a convenient name for the force directed on a body undergoing radial acceleration. It’s a shorthand way of speaking. That’s all.

Likewise Centripetal force is not a real force either. But it’s a handy way to name the force that keep our radially accelerating mass tethered to its center of rotation. Swinging a pail of water around your head, you can say it’s the “centrifugal force” that keeps the water in the bucket while your arm is the source of the “centripetal” force that keeps the pail from flying off into space.

The physics maven knows that Centrifugal and centripetal are shorthand terms and that the water really stays in the bucket because of Newton: it just wants to go straight. But the pail is constantly thwarting it

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