Blog post: swansont: Don't You Fly Off the Handle

It’s Plane To See …

Joe runs a popular blog with lots of neat stuff, but I have to pick a nit here. Well, honestly I think it's more than a nit.

This is vague, at best, but at worst it reinforces a misconception about circular motion. If by "outward" you mean straight out, i.e. radially, then mass in circular motion does not want to fly outward. The only force on the mass is inward, which is gravity. If you "snipped the rope" or turned gravity off, the mass would want to go in a straight line, tangent to the circle it was traveling in. Now, that line increases in distance from the center, so in that sense the motion is outward, but I think the vagueness of such wording is counterproductive, because it can reinforce a misconception so easily.

You might ask, "What about the centrifugal force?" Well, that's a fake force we put in place when we are in a rotating coordinate system, but want to pretend. We pretend that we can still apply Newton's laws, and we pretend our motion is a straight line. As such, the net force has to be zero, so we pretend there's a centrifugal force that balances gravity. But that force really isn't there, as forces are defined. It's an illusion of being used to inertial reference frames and trying to make sense of motion when suddenly you're not in an inertial frame. (The Coriolis force explanation is similar to this, owing to the rotation of the earth)

And that batter flying off the egg beater? Take a look at the pattern it makes on the bowl. Is it perfectly round dots, which is what you'd expect from flying straight out, or is it somewhat elongated dots or streaks, from something moving in a tangential straight line, hitting a curved surface? (This week on CSI: Kitchen)
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