** Apologies on not posting for a while. I hadn't noticed how long it's been! **
This is not a photo (!) but a video that I came across a couple years ago. The authors from Georgia Institute of Technology applied some basic physics to determine how quickly an animal has to shake in order to dry itself. Here's their website where you can find a more detailed description of their study, including the resulting published paper. Being a fan of applied physics, I really enjoyed this, especially because it is simple enough for an introductory physics student to understand.
The basic idea is to look at the forces acting on the water droplets in an animal's fur. The force exerted by air pressure and water tension work to hold the droplets in the fur. By shaking, the animal produces a force that acts to move the water droplets outward. This is called the centripetal force. When you are next in a car, notice how you move when the car makes a turn, especially if it's a quick turn. You should feel your body moving away from the pivot point. In other words, a right turn will cause you to move toward the left side of the car. This is also the centripetal force at work, and it occurs whenever a body is rotating.
The centripetal force produced is proportional to the angular frequency, or speed, of the rotation as well as the distance from the pivot point to the object that is rotating. For our rotating mammals, the pivot point is the center of the body and the object, the water/fur. Therefore, the distance is just the radius (or half the thickness) of the mammal's body. The conclusion that smaller animals must shake faster makes sense because of this proportionality.
Let's introduce the actual equation for rotational motion and centripetal force: Fc = m*r*w^2, where Fc is the force, m is the mass of the water droplet, r is the radius of the animal, and w is the angular frequency with which it shakes. Assume we want to keep Fc constant - that water droplets always require the same amount of force to remove them. Since a smaller animal has a smaller radius r, it will require a larger rotating speed w to compensate for the smaller r. Larger animals with larger radii, can shake more slowly to produce the same centripetal force as their smaller companions.
Here's a challenge: Next time you're at the beach or get out of the shower, try to shake yourself dry. Can you do it? If so, what do you think your shaking speed (oscillations per second) is? If not, what makes you different from the mammals that do dry by shaking?
References
Wet mammals shake at tuned frequencies to dry by Andrew Dickerson, Georgia Tech
