The Physics of a Wet Dog Shaking Itself Dry

Posted on October 27, 2010

It's something we've all wondered about: at what rate does a wet dog oscillate in order to get himself dry? Does a small dog oscillate faster than a large dog? And what about cats?

Scientists at the Georgia Institute of Technology set out to master the physics of how dogs shake their wet fur to get it dry (preferably while standing next to an unwitting human who then will become correspondingly wet). The physicists filmed a number of dogs and other animals shaking themselves dry and then developed a formula to describe the process. It turns out that smaller dogs do oscillate at a higher rate.

The researchers created a mathematical model to determine the centripetal force required for the dog to shake itself dry. The MIT Technology Review explains:

This model leads to an interesting prediction. If the animal has a radius R, the shaking frequency must scale with R^0.5. That makes sense, smaller animals will need to oscillate faster to generate forces large enough to dry themselves.

To find out whether that applies in nature, Dickerson and pals studied films of various animals of different sizes. They found that a mouse shakes at 27 Hz, a cat at about 6 Hz while a bear shakes at 4Hz. "Shake frequencies asymptotically approach 4Hz as animals grow in size," they conclude.

So, there you have it. This story reminds of how much we miss the show NUMB3RS and Professor Charlie Epps' explanations of the physics of everyday occurrences. The Wall Street Journal has a less technical explanation of what's going on:

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