Difference between revisions of "123: Centrifugal Force"

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(→‎Explanation: Thorough explanation of centrifugal force for laymen (i.e. long overhaul))
(Undo revision 47532 by Quicksilver (talk) Please focus only on the comic, but I did add the incomplete tag.)
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==Explanation==
 
==Explanation==
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[[Black Hat]] has strapped {{w|James Bond}} to a centrifuge and claims the centrifugal force will be lethal. Bond objects that there is no such thing, but just centripetal force.
  
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In this comic, [[Black Hat]] has strapped {{w|James Bond}} to a centrifuge and claims the centrifugal force will be lethal. Bond objects that there is no such thing, but just centripetal force. This is a common misconception among science teachers which will be addressed thoroughly in this explanation.
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Issac Newton was the first to announce that any body will be in a stable state of motion iff ([[1033: Formal Logic|if and only if]]) no force is applied. No motion at all is a stable state, as is constant translation. Imagine yourself on a bicycle: if you are going fast and do not brake, only the friction of the air, tires and internals of the bike will slow you down if you are not going uphill.
  
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Newton's famous {{w|Newton's laws of motion#Newton's second law|second law of motion}} states that the net force acting on a body is equal to its mass times its acceleration (if its mass is constant). The law is a {{w|Euclidean vector#In physics and engineering|vector}} law, however, so both force and acceleration are vector quantities. (Vectors have not only a size but a direction, and are usually represented as arrows.) Acceleration is defined as the {{w|Derivative|rate of change}} of velocity with respect to time, where velocity is a vector. Therefore, whenever the length of the velocity vector (known as the speed) changes '''or''' the direction changes, there must be an acceleration, and therefore a net force.
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Now ride your bike on a circular track, at considerable speed. You will feel the "centrifugal" force, which is actually a centripetal force that you are applying to leave your straight course. That is what moving along an orbit really is: constantly changing the direction of movement, which needs a constant force.
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Now, consider a body moving in a circle at a uniform speed. Although the length of the velocity vector is uniform, it is constantly changing direction. This means that there must be an acceleration and therefore a net force on the body. It turns out that the acceleration and force are both pointing ''inward'' to the center of the circle of motion. This force is the real {{w|centripetal force}}.
 
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The sun exerts an inward gravitational force on the Earth to keep it moving in its near-uniform near-circular motion. Another example of a centripetal force is as follows: if you were to hold one end of a string whose other end is attached to a ball, and then swung the ball around over your head, you would be supplying the inward force to keep the ball moving in its circle.
 
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Now, Newton's {{w|Newton's laws of motion#Newton's first law|first law}} states that an object at rest will stay at rest and an object in uniform linear motion will stay in its motion unless acted on by a nonzero net force. Any {{w|frame of reference}} for which this is the case is termed an ''inertial'' reference frame. (A reference frame is simply a "point of view," if you will; observations are made from the observer's reference frame.) Evidently, a reference frame moving with a constant speed and direction with respect to another reference frame is inertial via this law.
 
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But what if you are rotating with respect to an inertial frame? We know from experience (having been in a car as it has rounded a turn, or any similar turning motion) that objects at rest do ''not'' stay at rest; they are flung ''outward'' as if acted on by a force. It turns out that it is as if a force equal in magnitude to the centripetal force but pointing ''outward'' is acting on all objects in the rotating frame. This is called the {{w|centrifugal force|centrifugal force}}. Since this force and the resulting motion are due to the frame's being non-inertial instead of some unbalanced force, the centrifugal force is termed a {{w|fictitious force}}.
 
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The term "fictitious" implies that it does not really exist, and therefore zealous science teachers can go out of their way to claim that they are not real.
 
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But, since James Bond is rotating with respect to Black Hat and any other stationary onlookers (who are all in an essentially inertial frame; see the parenthesized paragraph below), he will experience a real centrifugal force. As the centrifuge rotates faster, the centripetal force needed increases, therefore the centrifugal force he feels increases, and eventually the force will {{w|G-force#Human tolerance of g-force|crush}} and kill him.
 
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A brief summary of the points of view:
 
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;Observing frame (Black Hat, us, etc.)
 
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: James Bond is moving in a circle, and is therefore accelerating. The force keeping him there is an inward force of contact against the centrifuge, a centripetal force. (Via Newton's {{w|Newton's laws of motion#Newton's third law|third law}}, since the centrifuge is pushing Bond inward, Bond is pushing the centrifuge outward. The centrifuge's material is strong enough not to break under this force, however.)
 
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;James Bond's non-inertial frame
 
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: In James Bond's frame, Bond is at rest. He is kept there by two forces: the above-mentioned inward force of contact against the centrifuge, and an ''outward centrifugal force''. He feels both forces.
 
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Note that, from our frame, we do not consider Bond to be acted on by a centrifugal force, since the one centripetal force accounts for his circular motion entirely. Conversely, Bond needs it to account for his staying at rest. This is, as stated above, why the centrifugal force is termed "fictitious," or sometimes, a "pseudo-force."
 
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(The observing frame is not technically inertial either, as it is on a planet which is rotating and revolving about a star which is moving non-inertially. The resulting pseudo-forces are very, very minor, however, and can only be detected by more sensitive instruments [e.g. a {{w|Foucault pendulum}}].)
 
  
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Black Hat argues that within a spinning inertial system, "centrifugal" force is real. Here is why: to transform equations to a subsystem, everything inherent to the system as a whole must be subtracted, including the centripetal forces, which leaves a centrifugal force on the other side of the equation. Wikipedia hints that while the centripetal force is universal, the centrifugal force is bound to the specific inertial system.
  
 
The final statement by Black Hat is that said by {{w|Auric Goldfinger}} in {{w|Goldfinger}} in response to James Bond's question "Do you expect me to talk?"
 
The final statement by Black Hat is that said by {{w|Auric Goldfinger}} in {{w|Goldfinger}} in response to James Bond's question "Do you expect me to talk?"

Revision as of 18:17, 25 August 2013

Centrifugal Force
You spin me right round baby, right round, in a manner depriving me of an inertial reference frame. Baby.
Title text: You spin me right round baby, right round, in a manner depriving me of an inertial reference frame. Baby.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect:
Please include the reason why this explanation is incomplete, like this: {{incomplete|reason}}
If you can address this issue, please edit the page! Thanks.
Black Hat has strapped James Bond to a centrifuge and claims the centrifugal force will be lethal. Bond objects that there is no such thing, but just centripetal force.

Issac Newton was the first to announce that any body will be in a stable state of motion iff (if and only if) no force is applied. No motion at all is a stable state, as is constant translation. Imagine yourself on a bicycle: if you are going fast and do not brake, only the friction of the air, tires and internals of the bike will slow you down if you are not going uphill.

Now ride your bike on a circular track, at considerable speed. You will feel the "centrifugal" force, which is actually a centripetal force that you are applying to leave your straight course. That is what moving along an orbit really is: constantly changing the direction of movement, which needs a constant force.

Black Hat argues that within a spinning inertial system, "centrifugal" force is real. Here is why: to transform equations to a subsystem, everything inherent to the system as a whole must be subtracted, including the centripetal forces, which leaves a centrifugal force on the other side of the equation. Wikipedia hints that while the centripetal force is universal, the centrifugal force is bound to the specific inertial system.

The final statement by Black Hat is that said by Auric Goldfinger in Goldfinger in response to James Bond's question "Do you expect me to talk?"

The title text is inspired by Dead or Alive's famous song from 1985, "You Spin Me Round".

Transcript

[James Bond is strapped to a giant wheel suspended from the ceiling. Black hat is standing next to two levers.]
Black hat: How do you like my centrifuge, mister Bond? When I throw this lever, you will feel centrifugal force crush every bone in your body.
[Same scene, but a closer shot.]
Bond: You mean centripetal force. There's no such thing as centrifugal force.
Black hat: A laughable claim, mister Bond, perpetuated by overzealous teachers of science. Simply construct Newton's laws in a rotating system and you will see a centrifugal force term appear as plain as day.
[Closer shot, only Bond's head is visible.]
Bond: Come now, do you really expect me to do coordinate substitution in my head while strapped to a centrifuge?
Black hat: No, mister Bond. I expect you to die.
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Discussion

Are you allowed to describe a force acting upon you when you are in an accelerating reference frame? I'm pretty sure you're not. The explanation says that from bond's point of view, he is at rest. Well, sort of. If you're in an accelerating car you can tell that you're not at rest because your inertia seems to be "pulling" you backwards. There's nothing actually pulling you, though. 108.162.219.202 05:24, 30 December 2013 (UTC)

According to general relativity, that inertial "pull" is indistinguishable from being at rest with a force being applied. In the rotating frame, this apparent force is the centrifugal force. 199.27.128.62 05:58, 4 February 2014 (UTC)
the explanation is correct, and you can describe forces acting on you in non-inertial frames. If you take Bond to be the origin of a rotating frame of reference then the position of Bond will be (0,0,0) at all times. So in that frame of reference, Bond is at rest (not "sort of at rest, really at rest). The equation of motion for Bond is 
   F + Fe + Fw + Fc = ma = 0
(F is external force, Fe is the force due to angular acceleration of the frame (relative to some inertial frame), Fw is centrifugal force and Fc is coriolis force )
Since the sum of the three "fictious" force are nonzero, and Bond is at rest in this frame, the force F must also be non-zero. This force F is the inward push of the centrifuge. In the moving car example, you can't tell if you are accelerating or if there is a massive graviational field pulling you backwards. From your perspective the experience is identical. If you take this idea and run with it you get general relavitity141.101.70.67 11:30, 6 August 2014 (UTC)
I believe the OP is referencing the vestibular system. This is what allows humans to feel acceleration. The actual physics at hand is regarding reference frames, not the ability of the body to detect acceleration. In regards to the question of "Are you allowed to describe a force acting upon you when you are in an accelerating reference frame?", the answer is yes. You can pick whichever reference frame you wish, but we tend to pick the one that simplifies the calculations the most.Flewk (talk) 06:44, 25 December 2015 (UTC)

"Apparent force" is the best term to use to describe centrifugal force, and could be inserted in the text to clarify. 172.68.142.89 21:14, 14 June 2018 (UTC)

"The surface of the Earth approximates an inertial frame."

This isn't correct at all. If you're standing on Earth, you're experiencing an acceleration of 9.8 m/s^2. 108.162.238.11 00:56, 12 April 2018 (UTC)

Actually, that is incorrect. Right now i'm in my desk chair, not accelerating. The force of gravity is cancelled out by the force my chair exerts on me to maintain this status quo. You're correct that it's not an inertial frame, but that is because the force of gravity, not some acceleration 141.101.77.20 12
14, 6 July 2018 (UTC)
Noting that gravity _is_ an acceleration. In fact, a case could be made that it is the dominant acceleration experienced by normal matter with the possible exception of dark energy.Iggynelix (talk) 12:09, 30 September 2020 (UTC)

It's NOT appropriate that lazy science teachers "lie to children" in that and various other ways. The kids can understand far more than the teachers assume, so that their choices of limitation are self-fulfilling prophecy, producing ignorant victims of the public school system. — Kazvorpal (talk) 01:39, 21 September 2019 (UTC)

It's also inappropriate and perhaps just as ignorant to assert that such oversimplifications only occur in _public_ schools. There are lots of private school teachers extolling the same simplification.
Fewer of them do, because the parents have the power to hold them to higher standards. — Kazvorpal (talk) 18:42, 29 January 2024 (UTC)
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