# Friday Sprog Blogging: taking physics for a spin.

Dr. Free-Ride: So tell me about that device of yours? How did you make it and what does it do?

Elder offspring: There is a cut piece of a drinking straw. You also need two pieces of tin foil and a long string.

Dr. Free-Ride: That’s really aluminum foil, isn’t it?

Elder offspring: They call it tin foil.

Dr. Free-Ride: Well, I’ve been meaning to have a word with “them” about that. Anyway, how did you assemble the device.

Elder offspring: First, you put the string through the piece of straw. Then, you pinch the pieces of foil into bow shapes (like bow-ties), and then you tie the ends if the string around the middles. Then, you crumple the foil into shapes as much like round bodies in space as you can.

Dr. Free-Ride: Did you start out with one piece of foil much larger than the other?

Elder offspring: Yes.

Dr. Free-Ride: And this device has no name?

Elder offspring: (in a robotic voice) That is correct.

Dr. Free-Ride: So, what can you do with this device?

Elder offspring: First, you can bring the larger body right up to the straw, so it doesn’t have any mobility. Then, if you hold onto the straw, you use mostly your wrist to swing it. The string goes around and the smaller ball goes around the larger one. I think it’s a third class lever.

Dr. Free-Ride: I am so behind on learning the ranks of simple tools. You told me this demonstrates “orbit”?

Elder offspring: Yes, the smaller one is orbiting the larger one.

Dr. Free-Ride: But that’s not all you can do with this device, right?

Elder offspring: Right. It can also demonstrate centrifugal force.

Dr. Free-Ride: First, before we talk about that, can you explain what centrifugal force is?

Elder offspring: The faster something’s going in a circle, the farther it gets away from its center. That’s basically what it is.

Dr. Free-Ride: I don’t mean to rock your world, but did you know that the official line in physics is that there’s really no such thing as centrifugal force? The real deal is something called centripetal force.

Elder offspring: Centripetal force?

Dr. Free-Ride: Don’t worry, learning science is a continuing process of using one model, then talking about what’s wrong with it and replacing it with another model. Just don’t be too shocked when they take your centrifugal force away.

Elder offspring: Even if it’s imaginary, we can still use this thing to demonstrate centrifugal force.

Dr. Free-Ride: OK, lay it on me.

Elder offspring: First you start out with the small ball sitting at the top of the straw and the big ball hanging down. Then, you start to spin the smaller ball —

Dr. Free-Ride: — again, with wrist action —

Elder offspring: — and slowly but surely, the smaller ball will go outward, bringing the bigger ball up towards the straw.

Dr. Free-Ride: Which is surprising because you’d think that the smaller object wouldn’t be able to move the bigger object?

Elder offspring: That’s right.

Dr. Free-Ride: OK, so the force of the small ball spinning in a circle ends up acting like a force out from the center of the circle?

Elder offspring: Or a pulley.

Dr. Free-Ride: I really need to set you to work making us some Rube Goldberg machines.

Elder offspring: You know I know what those are.

Dr. Free-Ride: And yet, how many have you built for me?

Elder offspring: Hmmm.

Dr. Free-Ride: Anything else you can demonstrate with this device?

Elder offspring: It can also be used as a weapon to whack people in the face.

Dr. Free-Ride: Not in my house, it can’t!

* * * * *

As with most demonstration apparatuses from the field of physics, this one was temperamental when we tried to get some pictures of it in use. This was a source of frustration to the elder Free-Ride offspring, who I tried to reassure.

Dr. Free-Ride: You know, if we need to, we can settle for a still picture of the device and detailed descriptions of what it’s supposed to do.

Elder offspring: But we need to give proof! It’s no good to claim a device does something if you can’t prove with a photo that it really does!

Younger offspring: But the sprog blog readers will know you’re not lying.

Elder offspring: Look, if I claimed to have seen a Martian, would they believe it? Not if there was no picture to prove I really did.

Younger offspring: But seeing a Martian would need more proof than describing how your device works. Especially since people could build their own and try it out.

Verily, sometimes it feels as though I have the scientifically-oriented blogosphere in my kitchen.

1. #1 Doug Fort
January 1, 2010

Here’s Randall Monroe on centrifugal force: http://xkcd.com/123/

2. #2 Galen Evans
January 1, 2010

when you hear “Elder offspring: Look, if I claimed to have seen a Martian, would they believe it? Not if there was no picture to prove I really did.” that means you have won at parenting.

3. #3 Shannon
January 1, 2010

I hope my future kids turn out as awesome as yours!

4. #4 George D Turner
January 2, 2010

Here is a classic on ficticious forces and other goodies.
http://www.archive.org/details/frames_of_reference

5. #5 Isis the Scientist
January 3, 2010

Look, if I claimed to have seen a Martian, would they believe it?