## How to Whirl a Squirrel off of a Bird Feeder

Squirrels are all right. They’re better than your average rodent, and they jump around and stuff. But if you have a bird feeder, you might hate them. These animals just don’t get that some food is set aside for birds. They don’t respect boundaries, and they’re not above wrecking your feeder to get the goods.

That’s why some people employ anti-squirrel technology. A company called Droll Yankees makes dispensers with names like the Tipper, the Whipper, and the Flipper. That latter one has a motor on the bottom and a weight-activated spinning perch. Birds aren’t heavy enough to trip the switch, but a squirrel is.

Now usually, a squirrel will jump off a bird feeder that starts to spin—but not the one in this viral video. You have to admire his spirit, actually. He hangs on till the bitter end, but it’s not enough and he catches some major air.

You know what’ I’m thinking? This is a perfect example of the forces involved in circular motion. Let’s take a look at some of the interesting physics questions here.

Why Does the Squirrel Fly Off?

So you have this furby attached to a spinning contraption. Clearly it’s not easy to hold on—but why? Is this all about centrifugal force?

Yes, it’s true that this deals with centrifugal force. It’s also true that most physics teachers hate using centrifugal force, because it’s conceptually dangerous for beginning students. Let me first describe the idea, and then I’ll tell you why it’s not included in introductory physics courses.

You know about centrifugal force, right? When you’re sitting in a car that’s turning left, you feel something pushing you to the right—away from the center of the circle that the car is moving in. (A turn is temporarily part of a circular motion.) That’s what centrifugal means—to flee (fugere) the center. It’s a force that pushes away from the center of a circle. The faster the car goes, the greater the force. The tighter the turn (i.e., the smaller the radius of the circle), the greater the force.

That’s what happens to the squirrel. As the rate of rotation increases, he gets pulled and stretched outward, away from the center, until his little paws can’t hold on and he loses contact with the bird feeder.

But wait! Centrifugal forces are different from the usual physics forces. We typically describe forces as an interaction between two objects. If you hold out an apple and let go, it will fall. That falling motion is due to a gravitational interaction between the Earth and the apple. But what is the force-paired object pushing on the squirrel? There isn’t one.

Another way at this is to think about what it is that forces do. A force acting on an object changes its momentum—where momentum is the product of mass and velocity. When you drop that apple, the gravitational force increases its speed as it falls, thus increasing its momentum.

So here’s a little thought experiment: Let’s say this apple starts 1 meter above the ground. If you drop it with zero initial velocity, it will move down with an acceleration of 9.8 m/s2, and it will take 0.45 seconds to hit the ground.

Now drop the apple again, but this time, do it inside an elevator that is just starting to go up. (You know the elevator is accelerating upward because you feel “heavier.”) If you measure the falling time, you’ll see that it now takes less than 0.45 seconds to hit the floor.

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