Centripetal vs. Centrifugal force, & the Solar System

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Centripetal vs. Centrifugal force, & the Solar System

January 28, 2008 on 10:24 pm | In Astronomy, Gravity, Physics, Solar System |

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:
xkcd's James Bond's Doom!

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.
uniform circular motion

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:
Newton's 3rd law in action!

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):
The Gravitron is a Human Centrifuge!

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?
Laboratory Centrifuge

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:
layers of the Earth

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:
Look at the dusty disk!

Any volunteers to test it out?

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I just realized that if your blog title was shortened to an ackronym, it’d be SWAB. Was that an intentional backronym? You astro-types seem to love those….

Comment by benhead — January 29, 2008 #
Yes, yes it was intentional, and also fortuitous. Now I can make awful pirate jokes galore. Where does a pirate keep his Buccaneers? Under his buckin’ hat! (Ha ha ha…)

Comment by ethan — January 29, 2008 #
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Pingback by Universe Today » Astrosphere for February 6, 2008 — February 6, 2008 #
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Your explanation is still misleading regarding “centrifugal” force. While it is true that Newton’s Third Law is in play (the string pulls centripetally on the ball, so the ball pulls back outward equally) this outward force acts on the string and not on the ball. Thus, there is no centrifugal force—a force acting on an object throwing it outward due to its circular motion.

This really comes down to properly defining the “system of interest”. If the ball is your system, then the only force acting on it is the string, pulling inward. No centrifugal force. If your system is the ball+string, then the only force acting on it is the “pin” holding string in place. Again, outward forces.

If your system is the string, then, yes, there is an inward force (pin) and an outward force (ball) but all it does is increase the string tension since the two forces balance—there’s nothing pushing the string outward.

But, even if you want to quibble and call this outward force on the string a centrifugal force, it still does not act on the ball.

So, when you move to your centrifuge example, it is erroneous to state that there is anything pushing the material in the tubes outward. The only possible centrifugal force would be the force of the fluids+solids on the tube.

The best description of what’s happening in a centrifuge is that the material is given some initial velocity (and since the distances from the axis are relatively small, we could even pretend that everything in the tube is moving with about the same tangential velocity). Then the amount of force needed to keep them in a circle is proportional to the mass/radius. The more massive (higher density) stuff needs more force, which the fluid around it can not provide—therefore it moves to a larger radius of curvature until it collides with the tube bottom that can provide the necessary force. This is much the same as taking a curve too fast; if your tires-on-the-road can’t give you enough centripetal force, eventually the guardrail will.

The dense stuff eventually gets so close-packed it pushes the less dense stuff inward; those solids then provide the centripetal force for the less dense solids, etc.

Once again, from an inertial reference frame, no centripetal force is required. And we can find no object that is physically providing an outward force on the objects.

Comment by David Klassen — February 13, 2008 #
David,
Centripetal and centrifugal forces are easily confused, and I know that when I was in High School, College, and Graduate School, I was taught it incorrectly three times.
Centrifugal force on the stuff in a centrifuge is an effective force. Think about it in terms of buoyancy; just as the less dense stuff floats to the surface in a pool, the less dense stuff floats to the inside in a centrifuge.
I agree that it’s important to think about what forces are acting on what objects, but everything I’ve written in the above article is both correct and self consistent. In the centrifuge, the material inside is receiving a “Normal force” from all the material beneath it, and the least dense stuff will rise to the top. That’s all.
Also, remember that you’re assuming that the stuff being centrifuged can be treated as a point. But if I put you in a centrifuge, your feet would feel a much greater force than your head would, since they feel the centrifugal force from everything above them!
Hope this answers your question, and thanks for your curiosity!
Ethan

Comment by ethan — February 13, 2008 #
hi… nice to read ur blog …..
it has very informative info ……abt centrifugal force and …….
but i have one query when we derive equation for sedimentation velocity we consider frictional , buoyancy force ….but we didnt consider centripetal force ….but centripetal force is always counteract to centrifugal force …..
can u help me out ..
my email id is
biophysics.tushar@gmail.com

Comment by tushar — July 16, 2008 #
Tushar,

You will be okay; the frictional force is always there, and buoyancy forces are, effectively, the combination of gravitational and centripetal/centrifugal forces.

Did you know that if the Earth rotated five times per hour instead of once per day, the oceans and rocks at the equator would be thrown into space, overcoming the Earth’s gravity? How’s that for centrifugal force??

Comment by ethan — July 17, 2008 #
Sorry, just a little question? I am not too knowledgeable about physics or whatever other field this might be in,but I was wondering, as you stated that things heavier than the Sun were thrown out or pushed out . I can see that happening ,but at the same time I was told that the center core of the Sun was made of Iron, but that could be a wrong theory . How about the Earth. The center of the diagram has the solid center then the liquid center , but wouldn’t the solid be more dense than the liquid. and how about the air being lighter than the water why is that being pushed upward. and Centrifugal and centripetal are the principals of Gravity, right? So it is Centripetal forces keeping us to the ground , is that right ? So how heavy does something have to be to be thrown out?I guess there might be a equation for that . and another question would (in theory) the planets in general be denser than the ones closer to the Sun ?
.

Comment by Adrian — July 18, 2008 #
You’re right on this account, Adrian. When you get close to the center of a bound object, gravity is much stronger than centripetal/centrifugal force, and so the dense thing sinks to the center. When you’re far away, if your velocity exceeds gravity, the opposite is true. In reality, things are distributed in some very complex balance dependent on initial conditions. Also, Saturn’s rings aren’t a solidly rotating disk either, so I’m not sure my analysis applies.

Comment by ethan — July 18, 2008 #
You say:
“Did you know that if the Earth rotated five times per hour instead of once per day, the oceans and rocks at the equator would be thrown into space, overcoming the Earth’s gravity? How’s that for centrifugal force??”

That’s just the Earth’s gravity not providing enough centripetal force to stop the fast moving surface travelling off in to space.

…and it would still be rotating once per day, as that’s what a day is :-p

Of course, ignoring further pedantry about a day not being a full rotation, but the sun returning to above the same point… err I mean line of longitude.

It’s a minefield this expressing an opinion thing, isn’t it.

Comment by Chris — August 9, 2008 #
what will happen if the force between the star and planet suddenly stops?

Comment by afshan ali — August 26, 2008 #
what will happen if the force betwwen planet and starsuddenly stops?

Comment by afshan ali — August 26, 2008 #
Would you agree that during the golf swing the spine becomes the centripetal force and the club becomes centrufugal force because of the sockets and joints in the body? As long as the spine is moving with the elbows hinging correctly you can maintain these forces.

http://www.stevewozeniak.com

Comment by Steve Wozeniak PGA — September 2, 2008 #
Steve,

I think that’s strictly a point-of-view question, but there’s no doubt that the forces must balance somewhere; there’s no net force on a golfer from taking a golf swing.

Not that I know from experience; I am lousy at golf!

Comment by ethan — September 4, 2008 #
I think this blog as a good example of ‘bad science’. We were taught to steer clear of centriful force discussions. (Full stop). There is no string connecting the moon to the earth (last time I checked). There is no magical balancing act between gravitaional attraction and inertia (reluctance to changes in states of motion). There is a world of misguided notions re: orbital mechanics - it’s right in there with zero-point energy ‘anti’ gravity schemes or strategies. The moon is in reality constantly falling towards the earth (or more accurately the earth is accelerating toward the moon and the moon is accelerating toward the earth (consistent with Newtons Law of Universal Gravitation and Newtons 2nd Law) and to really round things out state that this all is centered about a point known as the Barycenter of this two-body system. The moon and the earth are indeed accelerating toward one another as has been the case for the past 4 billion years. Where the moon to lose it’s velocity component (normal to the direction of gravitional attraction) it would no longer miss the earth (in it’s continuous 4 billion year-old fall) and a condition where the two bodies would attempt to occupy the same space at a velocity approximately consistent with that of the escape velocity (17 km/sec).

Comment by Terri Philips — November 29, 2008 #
Terri,
The point isn’t that you should steer clear of centrifugal force discussions. Why would you not discuss something that’s a real, easily-observed physical phenomenon that students are familiar with?

The hard part is explaining it properly, which is what I’ve attempted to do here. Orbital mechanics is one of the things I’m kind of a scientific expert on, and so I have a particular interest in it. The Moon and Earth are unique in our solar system; no other inner, rocky planet has its own large moon, and so we’ve got something truly remarkable to study right in our backyard.

Comment by ethan — November 29, 2008 #
[…] Centripetal vs. Centrifugal force, & the Solar System | Starts With A Would you agree that during the golf swing the spine becomes the centripetal force and the There is no magical balancing act between gravitaional attraction and inertia […]

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Comment by Sharri Mcmasters — September 6, 2011 #
so inshort please tell me centrifugal force is exist in space aur zero garvity aur not?

Comment by areeba siddiqui — February 6, 2012 #
Centrifugal means to flee from the center.

Thus if a centrfifugal force was acting upon the particles being centrifuged and there was nothing to prevent them from doing so, they would travel along a path from the center of the circle of rotation if the force causing them to rotate was abruptly stopped.

But this is not the case. They would travel along a tangent to the circle of rotation, thereby proving that the force acting upon them is not a “centrifugal” force, but a force which causes them to rotate in a circular path.

Thus there is no centrifugal force acting on the items undergoing the circular motion.

The centrifugal force is the reactive force on that which is causing the rotation.

Comment by Kevin Moreton — February 19, 2012 #
This guy messed it all up… the reactive aspect of centripetal force is cetrifugal force, but this is not the kind of centrifugal force this clown act ever touches upon. He speaks of inertial centrigual force and thats it, and passes it off as if it is reactive centrifugal force. An example of the reactive centrifgal force would be a ball tied to a string, and the other end of that string being connected to, say, a pin somewhere. Then it starts rotating. For every force there is an equal and opposite force, but that doesn’t mean it has to exist at the same point. The string pulls inward on the ball, so there MUST be an outward force… but now on the ball like this moron said! It comes on the pin, there is an outward force pulling on the pin called cantrifugal force. Perhaps if the guy who wrote this wasn’t such a failure he would have gotten in to a decent university and learned the CORRECT laws of physics, instead of being taught the wrong thing 3 times. What a joke this was.

Comment by mike — September 12, 2012 #
“David Klassen” has given a fitting answere which is really good and understandable. But, in spite of being a Novice to this subject, I wonder why there is a big fight in Blog with terminologies? I remember one thing for sure here, we must bring in Mr. Einstein also in to account and to visualize his Frame in to reference. Although, both forces are inseparable, Centripetal force [mostly Perpendicular to rotational Axis]and the Centrifugal force [mostly Perpenicular to roational Axis; but..subjected to gravity or any Third Vectors nearby] which causes the conflict in your GOOD BLOG. Because of the third resultantforce, lighter ones fly above and.. denser ones are thrown to outer orbit.PLEASE CORRECT MY VIEW… Horizontally & Vertically.
Vanarajan M Greetings De, India

Comment by M Vanarajan — January 23, 2013 #

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Centripetal vs. Centrifugal force, & the Solar System

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