One of the worst teaching tools physicists use (and they almost all do it) is to tell students,
There’s no such thing as centrifugal force.
What can you do when the top physics education website says, “It is important to note that the centrifugal force does not actually exist. We feel it, because we are in a non-inertial coordinate system.” There’s a very funny comic over at xkcd that goes as follows:
Well, what’s the deal? What really goes on, physically, and what causes a centrifuge to work? Is your physics teacher right, or is there more to the story than “the centrifugal force does not actually exist?”
So, your physics teacher is partially right. When an object moves in a circle, it’s moving, at any instant, tangentially to the circle, and it’s the centripetal force, causing an acceleration towards-the-center, that keeps it moving in the circle.
But that’s not the end of the story, or I wouldn’t be posting this. Do you remember Newton’s third law? It says for every action, there’s an equal and opposite reaction. That is, for every *force*, there’s an equal and opposite force. Take the photo below:
Notice how the man’s face getting pummeled is not only experiencing a force from the fist, but the fist experiences an equal and opposite force! (And if you don’t believe me, go punch a brick wall.) Well, if there’s a centripetal force acting on an object, pulling it towards the center, then there’s a centrifugal force from the object reacting to it, pushing things away from the center. If it’s a ball attached to a string, the string pulls the ball towards the center, and the ball pulls the string away from the center. If it’s a wall pushing a person towards the center, the person pushes back against the wall, pushing it away from the center, like so (that’s why the walls need to be sturdy):
So, that’s an example of a real centrifugal force. But beyond that, how does a centrifuge actually work, with only centripetal forces acting on the stuff being centrifuged?
Centrifuges spin really fast, causing the stuff inside to separate out according to density, with the most dense pushed out the farthest, towards the bottom, and the least dense winding up at the top, closest to the center. Because we don’t know the difference between gravity and any other acceleration or force, centrifuging something is basically like turning the knob on gravity way up — by up to a factor of 15,000, depending on the centrifuge! In zero gravity, things don’t sort themselves by density, but in a high enough field of gravity, they do. Even solids. Like this thing:
The centripetal force of the sides of the centrifuge push back just like the seat of your chair pushes back against you when you’re in a gravitational field. The faster the centrifuge, the harder the sides push back; and the whole thing acts like an enhanced version of gravity. Well, what does this have to do with the Solar System? Sir Arthur Eddington once described all the life on Earth as follows:
We are bits of stellar matter that got cold by accident, bits of a star gone wrong.
While it’s true that all of the elements on Earth that we know and love (except for hydrogen and helium) were formed in stars, the Sun is almost all hydrogen and helium, and the planets are almost exclusively heavier elements! How did that happen? Was it an accident, as Eddington suggests? No; it was centrifugal force pushing the heavier elements (like carbon, oxygen, nitrogen, iron, phosphorus, silicon, etc.) away from the center relative to the light ones!
So the Earth, and for that matter, all other planets, are made out of denser elements than stars are. And the reason is all due to a force that your physics teacher probably told you doesn’t exist!
UPDATE (January 29, 2008): It occurred to me that some of you might like a way to *test* this. It’s well known that solar systems form from dusty disks, known as proto-planetary disks. If what I’ve just articulated is correct, the material closest to the center of the disk should be preferentially less dense than the material farther away. We don’t have a dusty protoplanetary disk around our sun, but we have an analogous, dusty disk around one of our larger planets:
Any volunteers to test it out?