0123: "Centrifugal Force"

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0123: "Centrifugal Force"

Postby Shoofle » Mon Jul 03, 2006 12:23 pm UTC

You cannot stop me, Mr. Bond. I will launch my satellite armed with a laser which will shoot the moon and Mars, destroying both of them and preventing humanity from ever being able to colonize! I merely wish for the destrution of the human race, you see!

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Postby Spritzido » Mon Jul 03, 2006 1:21 pm UTC

Mr. Bond is so off the mark on this one. Centrepetal force is the name given to the force/forces that pulls a mass towards a centre, which, coupled with a straight line velocity, causes a change in direction around the point the centralpetal force originates.

Since Mr. Bond is being pushed away from the centre of the device while being contained by a circular construct, inhibiting the inertia's ability to push him away from the centre (resulting in the compression of body tissues against said circular construct), it's obvious it's centrifugal force.

Although, by the look of his constraints, it seems he could shift his position to the outside of the circular construct causing the centrifugal force to break his attachments to the construct, either by breaking the bindings or by ripping off his limbs.

I'm probably wrong about the physics stuff btw. (Biology, woo!)

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Postby Gozer » Mon Jul 03, 2006 3:30 pm UTC

The favorite, most memorable line from my high school physics teacher refers to this very subject. To disprove that you are being forced (accelerated) outward, he suggested letting go and hopping while riding a merry-go-round and exclaiming,

"Centrifugal force, TAKE ME!"

and see which way you go. (It's not nearly he same without the vocal emphasis and full-body gesticulation.)

He did mention the difference for a rotating reference frame, but quickly suggested that we forget that we heard it.

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Postby Wark » Mon Jul 03, 2006 5:42 pm UTC

If I remember my phsyics correctly (and that's a pretty big if), centripetal force is what is required to keep you on the circular path. If there's nothing there, you just go off in a tangent. In the comic, the force of the centrifuge pushing back at Mr. Bond (the "normal" force, if I remember my terminology correctly). At very high speeds, the normal force will be so great as to effectively squish him to death.

In actuality, the term "centrifugal force" wouldn't be so bad, since it's actually the centripetal force provided by the centrifuge that's crushing him. But that's just confusing.

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Postby CactusEater » Mon Jul 03, 2006 6:38 pm UTC

i reckon that the bates family out at sealand have one of them

they are so bond villainy its wrong
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Postby Scott » Mon Jul 03, 2006 8:50 pm UTC

Aside from really liking this comic (which I do!) I've found that it's even funnier when I picture Sean Connery saying Bond's lines. I'm not sure if Connery was Bond for Dr. No - it may have been Roger Moore, but Sean Connery's voice adds a new dimension of funny.

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Postby Matt » Tue Jul 04, 2006 6:19 am UTC

Scott wrote:Aside from really liking this comic (which I do!) I've found that it's even funnier when I picture Sean Connery saying Bond's lines. I'm not sure if Connery was Bond for Dr. No - it may have been Roger Moore, but Sean Connery's voice adds a new dimension of funny.


I thought this was referencing Goldfinger.

I thoroughly enjoyed the comic even though I can't contribute to the science discussion. Yet.
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Postby RealGrouchy » Tue Jul 04, 2006 8:34 am UTC

Um, I think you're all wrong (including you, Mr. Bond!).

The centrifugal force from rotation pushes Mr. Bond away from the centre.

The centripetal force exerted by the physical contraints of the centrifuge pushes Mr. Bond towards the centre.

It is the conflict and combination of both forces that will cause Mr. Bond's overdue demise.

Did I expect Mr. Bond to escape that false dichotomy on his own?

No, I expect him to die! Mwahahaha!
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Postby Shoofle » Tue Jul 04, 2006 12:14 pm UTC

See, the problem I've always had with the "centrifugal force does not exist" thing is that when you are on something that is rotating, you are accelerating toward the outside. Even if this is just an effect of other forces, that doesn't make what happens go away, and centrifugal force is the name for the effects. Well, what makes the effects happen. But yeah! Centrifugal force exists inside a rotating frame of reference. I mean, does coreolis force exist? Although that might be something else entirely. Uhh, just assume I'm right, please. But I think coreolis force might be a result of rotating frames of reference.

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Postby xkcd » Tue Jul 04, 2006 4:06 pm UTC

But yeah! Centrifugal force exists inside a rotating frame of reference. I mean, does coreolis force exist? Although that might be something else entirely. Uhh, just assume I'm right, please. But I think coreolis force might be a result of rotating frames of reference.

The same set of transforms that show you the centrifugal force dropping out of the equations also produce a second term. This is the Coriolis term.

The Coriolis force is pretty badass.

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Postby Gozer » Tue Jul 04, 2006 8:04 pm UTC

Shoofle wrote:See, the problem I've always had with the "centrifugal force does not exist" thing is that when you are on something that is rotating, you are accelerating toward the outside.


But that's just it, you are not accelerating towards the outside. You are accelerating inwardly. You feel like you are being pushed outside, due to your frame of reference. However, what you really feel is the force of the merry-go-round, centrifuge-of-death, etc. forcing you inwards. It's same as accelerating in a car. When you step on the gas, you feel like you are being pulled backwards into the seat, but you are actually being pushed forward.

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Postby radiantthought » Fri Jul 07, 2006 4:54 am UTC

Gozer wrote:The favorite, most memorable line from my high school physics teacher...


hah... the most memorable thing my highschool physics teacher did was on the last day of class just before saying goobye he walked around to the front of his desk, turned sideways so the class was looking at his profile, jumped in the air and extended his legs straight out in front of him and touched his toes with his hands in midair.(like this) To this day I have scarcely seen anything so completely out of character and amazing.
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Postby Sitnaltax » Fri Jul 07, 2006 5:09 am UTC

I printed this one out and stuck it to my cube wall; so did my roommate, a mechanical engineer. It's probably the funniest safe-for-work strip so far.

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Postby davean » Fri Jul 07, 2006 6:58 am UTC

Sitnaltax wrote:I printed this one out and stuck it to my cube wall; so did my roommate, a mechanical engineer. It's probably the funniest safe-for-work strip so far.


Did you actually see any cloths on ether of them? Well, ok, I guess hat guy is fairly well covered, I mean, where isn't a hat sufficient for being street legal? Bond on the other hand, well, prove he's properly clothed.

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Postby Vonkwink » Fri Jul 07, 2006 8:02 am UTC

Since Mr. Bond is being pushed away from the centre of the device while being contained by a circular construct, inhibiting the inertia's ability to push him away from the centre (resulting in the compression of body tissues against said circular construct), it's obvious it's centrifugal force.



This is normal force from the wheel in which he is spinning. Not centrifugical force.

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Postby xkcd » Fri Jul 07, 2006 8:36 am UTC

The centrifugal force is pushing various levels of his body downward, though. If you step onto a high-gravity planet, it's not wrong to say the force of gravity made you collapse, even though you would've stayed upright if not for the planet. The centrifugal force is what's pushing his body up against the wheel --> the centrifugal force is what's crushing him.

I'm okay with that.

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Postby Shoofle » Fri Jul 07, 2006 12:08 pm UTC

Ok, so the centrifugal force is the force moving you toward the outside, right? Well, if you consider your velocity vector at some point along the circle, say at 0 radians, and the centrifuge is spinning clockwise (we are looking at it top-down) then the vector is entirely tangential to the centrifuge. When Mr. Bond is pulled around the circle by the centrifuge, his inertia keeps him moving in the same direction (except that he is being held in by the centrifuge). The inertia keeping him on the outer edge is the centrifugal force - it is exerting a constant force outward.

Everything in this post is subject to the realization of how silly it may or may not be. Do not trust it, I thought it up as I wrote it.

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Postby Penguin » Fri Jul 07, 2006 2:00 pm UTC

Shoofle wrote:I mean, does coreolis force exist? Although that might be something else entirely. Uhh, just assume I'm right, please. But I think coreolis force might be a result of rotating frames of reference.


It doesn't as such... it's called the Coriolis Effect in every oceanography textbook I've ever seen. And I've seen a lot of them.

I think "centrifugal effect" might be a more appropriate term here, because there's no acceleration outward and therefore no outward force. As an object (or person...) moves around a circular path, their acceleration points RADIALLY INWARD. Their instantaneous velocity always points tangentially off the circle thanks to linear inertia, but a constant linear velocity does not imply the presence of a force; only acceleration can do that.

There is absolutely no force pushing you to the outside. At all. You aren't "pushed" to the outside of a spinning circle, you simply continue moving in a straight line while the circle continues to rotate. Keep your reference frames straight.

I swear if I have to explain the difference between acceleration and velocity once more, I will differentiate the perpetrator until they figure it out themselves.

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Postby Shoofle » Fri Jul 07, 2006 2:20 pm UTC

Eh, whatever. I was writing by the seat of my pants.

Also, if centrifugal force is completely nonexistent, what is holding whatever up against the side?

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Postby Penguin » Fri Jul 07, 2006 5:23 pm UTC

Like on one of those Round-Up rides at the carnival? Your own mass, honestly.

You, as a chunk of mass, have inertia. [Remember Newton's Laws on the subject?] Inertia is kind of a nebulous concept that expresses an object's tendency to remain in the state of motion it's in (possessing a constant velocity - and don't forget a constant velocity can equal 0!) unless an unbalanced force is exerted on it, which causes acceleration: a change in an object's velocity. (It's also wise not to forget here that a change of direction IS a change in an object's velocity, since velocity is a vector quantity!)

So, once you've been put in motion with the floor of the cylinder-thing (so that your velocity with respect to the ride is 0) through friction with the bottoms of your shoes and the like, your body wants to remain in motion, right? It wants to continue on the straight-line path of its instantaneous velocity. But the walls of the cylinder are in the way, and when you hit them, they exert a force on you (in the radially inward direction - it's the centripetal force! ooooh!). The centripetal force the wall exerts on you causes you to accelerate in the radially inward direction... which is what makes you go around the circle rather than shooting off tangentially.

I think that the reason people so commonly believe in a centrifugal force is because it's hard to grasp that the wall (an inanimate object!) is exerting a force on you (a person!) that causes you to accelerate inward. Part of it is also the reference frame - standing inside that thing, it sure feels like you're getting "pushed" up against the wall. If you look down from above, it's a little bit easier to see that you actually move at the wall, and the wall is what stops you from going in a straight line like you'd like to.


... If that makes any sense at all. I'm not so great at explaining physics to people, especially without diagrams and equations. I like math better, where coordinate systems are allowed in equations and there's no air resistance or friction to worry about.


EDIT: Here, Wikipedia has a little bit to say that might elucidate it for a few of you with better examples.
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Postby xkcd » Fri Jul 07, 2006 6:37 pm UTC

There is absolutely no force pushing you to the outside. At all. You aren't "pushed" to the outside of a spinning circle, you simply continue moving in a straight line while the circle continues to rotate. Keep your reference frames straight.

[explanation of centripetal acceleration cut]

I'm sorry, but this is exactly the misunderstanding/oversimplification the comic is trying to refute. Yes, if you stay in the inertial frame of reference, there is no centrifugal force, and all acceleration is caused by the wall pushing on you.

But if you work in a frame of reference which is rotating with the wheel, you see everything being accelerated with a force equal to m*omega^2*r in the outward direction, where omega is the angular velocity of the frame of reference. Although this force is only a result of the frame-of-reference transform and not caused by a physical effect, it is an actual force in the sense that it is an acceleration of a mass. If you're seeing something having both an m and an a, then it damn well better have an F. F=ma.

This isn't the only one of these reference-frame forces. There's also the coriolis force, which you mentioned. It's a little trickier and is proportional to how fast an object is moving, but again, in a rotating reference frame, it's an observed acceleration of a mass, so it is absolutely okay to call it a force. (There's actually a third, called I think the Euler force, that comes into play when the speed of rotation is changing.)

The reason people argue against this (as you do here) is that they think the rotating reference frames aren't a valid way to analyze the system, since they can get pretty complicated. But although the specific mathematical details get tricky, this 'force' is a rigorously defined concept that in fact happens to match up reasonably well with the intuition of the little kid on the merry-go-round who says he's getting pushed to the outside.

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Postby xkcd » Fri Jul 07, 2006 6:57 pm UTC

I mean, does coreolis force exist? Although that might be something else entirely. Uhh, just assume I'm right, please. But I think coreolis force might be a result of rotating frames of reference.

It doesn't as such... it's called the Coriolis Effect in every oceanography textbook I've ever seen. And I've seen a lot of them.

I think "centrifugal effect" might be a more appropriate term here, because there's no acceleration outward and therefore no outward force.

The concept of a 'force' is pretty rigorously defined in physics -- a force exists when a mass undergoes a change in velocity. This is Newton's second law. F = m*d^2x/dt^2, or F = ma (for the pedantists, the best form of this might be F = dp/dt where p is momentum, because this jives better with relativity.)

I like math better, where coordinate systems are allowed in equations

Physicists in general love changing reference frames because it so often makes our equations simpler. So let's bring them into the equations and see what happens. When our reference frame is rotating along with the centerfuge, so it appears to be stationary, we see everything rotating with it begin to accelerate outward. This acceleration is called, in physics, the "centrifugal force". We also see anything moving undergo some additional deviations. This is the "Coriolis force".

This transform is really useful in calculating the path of artillery shells on the Earth's surface. If you do it in an inertial reference frame, working out where the shells will go is a hellish problem. But if you work in the rotating reference frame of the spinning Earth, it's easy to calculate the contributions of the Coriolis force and the centrifugal force on your shells, and you can generate firing tables quite easily.

Interesting anecdote from physics class which I think is probably true: the British navy found that when they fired shells about 20 kilometers, the force pushed their shells off-target by about 50 meters to the right, which was enough to miss. (We rechecked this in class and found it to be right, I think). So they had firing tables that told them to aim about 50m to the left in addition to all the other stuff.

But then the British went to war down in Argentina or wherever, and discovered that they were missing their targets consistently, shells landing 100 meters to the left. This was because their firing tables were calculated for roughly Europe's latitude in the northern hemisphere, and they had forgotten to recalculate them when they decided to start shooting down in the South Atlantic.

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Postby davean » Fri Jul 07, 2006 11:30 pm UTC

Penguin wrote:You, as a chunk of mass


Most people's one and only capacity.

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Postby paige42 » Mon Jul 10, 2006 1:27 pm UTC

Penguin wrote:I swear if I have to explain the difference between acceleration and velocity once more, I will differentiate the perpetrator until they figure it out themselves.

Best. Threat. Ever. I need to start using that on the intro physics students I tutor.

I was going to throw in my two cents on the centrifugal force and rotating reference frames, but it seems that xkcd is doing fine without any help from me.

I have also heard the story about the navy forgetting to redo their calculations for southern latitudes, but it was also in anecdote-during-physics-class form, so I have no idea whether it's true.

Also, I thought about posting a copy of this comic on the Society of Physics Students' door, but when I got there, someone already had. I have no idea who it was, though.

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Postby Penguin » Mon Jul 10, 2006 3:14 pm UTC

This is the first time someone has actually *enjoyed* my threats. Wow. I feel strangely emasculated, which is largely odd because I'm female.
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Postby xkcd » Mon Jul 10, 2006 6:26 pm UTC

Also, I thought about posting a copy of this comic on the Society of Physics Students' door, but when I got there, someone already had.

That is the best thing I've ever heard.

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Postby kira » Tue Jul 11, 2006 1:33 am UTC

I'd like to present this thread as Exhibit A of why physics sucks and we should all stick to theoretical mathematics.

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Postby Penguin » Tue Jul 11, 2006 1:53 am UTC

Agreed!
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Postby xkcd » Tue Jul 11, 2006 3:11 am UTC

I'd like to present this thread as Exhibit A of why physics sucks and we should all stick to theoretical mathematics.

Made any progress on that axiom of choice lately, Kira? I'm having some trouble with your Banach-Tarski paradox

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Postby kira » Tue Jul 11, 2006 3:54 am UTC

At least the axiom of choice is INTERESTING. And I choose not to discuss it.

The Banach-Tarski paradox is ALSO interesting and has many neat repercussions (including confusing mathematicians).

The centrifugal/centripetal force debate, on the other hand, makes me want to gouge my eyes out.

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Postby paige42 » Tue Jul 11, 2006 6:29 am UTC

kira wrote:physics sucks and we should all stick to theoretical mathematics

Aww... can't we all just be friends? I like theoretical mathematics and theoretical physics! I haven't had enough math to have studied the axiom and paradox you're talking about, but they seem pretty cool to me.

kira wrote:The centrifugal/centripetal force debate, on the other hand, makes me want to gouge my eyes out.

It's simple:

If you're standing on the outside and watching something else rotate, you talk about a centripetal force acting on the rotating thing. If you're the one doing the rotating, then from your point of view, there is a centrifugal force acting on you instead of a centripetal force.

But to each his or her own, I guess. I don't want to force physics down anyone's throat, especially if it might result in eye-gouging.

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Postby kira » Tue Jul 11, 2006 7:12 am UTC

That's not simple! That's like saying that if I look at my dog from the front, I call her a poodle and if I look at her from the side, I call her a Volkswagon Golf.

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Postby xkcd » Tue Jul 11, 2006 2:26 pm UTC

If you're standing on the outside and watching something else rotate, you talk about a centripetal force acting on the rotating thing. If you're the one doing the rotating, then from your point of view, there is a centrifugal force acting on you instead of a centripetal force.

That's not simple! That's like saying that if I look at my dog from the front, I call her a poodle and if I look at her from the side, I call her a Volkswagon Golf.

Kira, I think your problems run deeper than either of us suspected.

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Postby davean » Tue Jul 11, 2006 3:53 pm UTC

Your problems stem from giving a name to something thats not there. If you stuck to only naming thats that inherantly existed, you'd be all set. If its view dependant, it doesn't actually exist.

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Postby paige42 » Tue Jul 11, 2006 10:48 pm UTC

davean wrote:Your problems stem from giving a name to something thats not there. If you stuck to only naming thats that inherantly existed, you'd be all set. If its view dependant, it doesn't actually exist.

Of course it "inherently exists". It absolutely must exist. If something is changing its speed or direction of motion, it is accelerating; if it is accelerating, there is a force acting on it. It just depends on whether you prefer saying that you yourself are rotating, or that the universe is rotating around you.

Let's try a different example: if I'm riding on a train at a constant velocity, I have two ways of looking at the situation. I could say that the train is moving north and the town outside the window is at rest, or I could say that the town is moving south and the train isn't moving at all.

These two views are equally valid as far as the equations are concerned. If you throw a ball from the train and calculate something about it - where it will land, how much time it will spend in the air, or whatever other boring things they make you do in intro classes - you will get the same answer either way.

Likewise, a person on a merry-go-round will get the same result whether they use an inertial (not rotating) reference frame and a centripetal force, or a rotating reference frame and a centrifugal force. The same thing happens either way; we just have two ways of talking about it.

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Postby xkcd » Tue Jul 11, 2006 10:52 pm UTC

Likewise, a person on a merry-go-round will get the same result whether they use an inertial (not rotating) reference frame and a centripetal force, or a rotating reference frame and a centrifugal force. The same thing happens either way; we just have two ways of talking about it.

Though note that the two forces don't really correspond. In the rotating frame, the centripetal force can either be there (then called the normal force from the side of the centerfuge) or not there (letting the guy fly off)

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Postby kira » Wed Jul 12, 2006 12:58 am UTC

But. But. When you're on a train, the town isn't moving south! The train is moving! I don't care if the equations are equivalent, saying that the town is moving south is tantamount to heresy!

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Postby davean » Wed Jul 12, 2006 1:49 am UTC

kira wrote:But. But. When you're on a train, the town isn't moving south! The train is moving! I don't care if the equations are equivalent, saying that the town is moving south is tantamount to heresy!


I'm not moving east, Cambridge is moving west and can stop when I get there.

paige42: No, acceleration and force "exist", they both can exist in any dirrection. These other things are interpritations of effects and hence not valid, as are all opinions.

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Postby xkcd » Wed Jul 12, 2006 2:48 am UTC

paige42: No, acceleration and force "exist", they both can exist in any dirrection. These other things are interpritations of effects and hence not valid, as are all opinions.

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Postby Shoofle » Wed Jul 12, 2006 2:55 am UTC

Like, nothing is real, man. It's all just part of the consciousness, man. Man.


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