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Author Topic: Why is centrifugal force NOT gravity?  (Read 18008 times)

Offline Torino10

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Why is centrifugal force NOT gravity?
« on: 18/03/2011 19:45:55 »
Centrifugal acceleration os not the same as gravity, straight acceleration is. It should also be noted that the centrifuge accomodations module was not designwd as a place for astronaughts to sleep butwas designed as a science lab to determine the effects pf microgravity and various levels of simulated gravity experiments on biological and non biological chemistry. The Columbia disaster got this module bumped from the launch schedule. I am a strong supporter of a replacement centrifuge for the ISS, it is known that gravity has adverse effects on embryonic development in mammals, if the ultimate goal of manned space flight is self sustaining off world colonies then we need to know how much gravity and if centrigugal acceleration is an adequate substitute with it's strong coroilus effects.
« Last Edit: 21/03/2011 19:38:37 by chris »


 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #1 on: 18/03/2011 19:50:29 »
Centrifugal force is not the same as gravity because there is no such thing as centrifugal force. It's an imaginary force. The only force in this context is centripetal force.
 

Offline Phractality

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Re: Why is centrifugal force NOT gravity?
« Reply #2 on: 18/03/2011 20:55:22 »
Centrifugal force is not the same as gravity because there is no such thing as centrifugal force. It's an imaginary force. The only force in this context is centripetal force.

That's what we've all been told by high school physics teachers with a BA in education, but it's not true. In fact, it's a question of frame of reference. In a rotating frame of reference, like Planet Earth, centrifugal force is real. We usually consider the weight/mass ratio measured by a spring scale or gravity pendulum to be the actual gravitational field strength. In the rotating reference frame of Earth, that is true; but in the sidereal reference frame centered on Earth, the spring scale is measuring the gravitational force (a vector pointing to the center of Earth) plus the centrifugal force (a vector point away from Earth's axis of rotation).
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #3 on: 18/03/2011 21:12:57 »
Centrifugal force is not the same as gravity because there is no such thing as centrifugal force. It's an imaginary force. The only force in this context is centripetal force.

That's what we've all been told by high school physics teachers with a BA in education, but it's not true. In fact, it's a question of frame of reference. In a rotating frame of reference, like Planet Earth, centrifugal force is real. We usually consider the weight/mass ratio measured by a spring scale or gravity pendulum to be the actual gravitational field strength. In the rotating reference frame of Earth, that is true; but in the sidereal reference frame centered on Earth, the spring scale is measuring the gravitational force (a vector pointing to the center of Earth) plus the centrifugal force (a vector point away from Earth's axis of rotation).

But that ignores the fact that the Earth is spinning on its axis. If you choose to imagine that the Earth is not spinning on its axis, I'm sure you'll have little difficulty imagining lots of other imaginary forces.

BTW, what's wrong with a BA? Do you have an uber-douber BA or something?
« Last Edit: 18/03/2011 21:16:47 by Geezer »
 

Offline Phractality

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Re: Why is centrifugal force NOT gravity?
« Reply #4 on: 19/03/2011 00:25:19 »
I have temporarily withdrawn this post so I can review the definitions of centrifugal, centripetal and centrifical. I may have gotten them mixed up a bit.

EDIT, 3/19: Don't know when I'll repost this. My brain went on strike, and I accidentally dumped my copy of the original post.
« Last Edit: 19/03/2011 23:15:00 by Phractality »
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #5 on: 19/03/2011 02:16:41 »
I think you are correct is saying that centrifugal force is sometimes used in problem solving, but I think it's really just a name given to the reaction to centripetal force, all of which is thoroughly confusing! If there really was a centrifugal force, presumably it would tend to lift us radially from the surface of the Earth.
 

Offline Torino10

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Re: Why is centrifugal force NOT gravity?
« Reply #6 on: 19/03/2011 03:45:52 »
My apologies for saying Centrifugal force, I meant to say centrifugal acceleration, but regardless my point was that spin simulated gravity is not the same as actual gravity as implied in the Naked Astronomy podcast, I love the show by the way, but as I have taken a personal interest in the Centrifuge Accommodations Module that had been manufactured by JAXA and had been sitting in a parking lot at the Tsukuba space center in Japan which has sustained significant damage from the quake.

newbielink:http://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module [nonactive]

newbielink:http://www.spaceflightnow.com/news/n1103/11earthquake/ [nonactive]

My heart goes out to the people of Japan and I wish a better future and swift recovery for the JAXA space center in Tsukuba.
 

Offline CliffordK

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Re: Why is centrifugal force NOT gravity?
« Reply #7 on: 19/03/2011 06:11:36 »
Ahhh...

I was wondering.

Centripetal Force is the force towards a center of rotation which could be gravity.

Centrifugal force is the force away from the center of rotation which is derived from changing the direction of momentum from a tangential to a curve.

If you tie a ball to a string and spin it around your head.
The centrifugal force keeps it at a horzontal.
The centripetal force is that exerted by the string opposing the centrifugal force.
If you then release the string...  the ball continues on a tangent.  So...  you release it to the side when you want it to go forward.

As far as the artificial gravity induced by a spinning object.

No, it is not the same as "real" gravity as it is not the attraction of masses towards a center.

If you were in a circular device in orbit that was 1 mile in diameter, spinning, you might not be able to tell the difference between artificial gravity and real gravity as long as you never looked out the window (perhaps you could have artificial stabilized windows). 

In the case of the ISS Centrifuge Accommodations Module.  It looks like it is about 2 meters in diameter.  So, if you stood up, your head would be in Zero-G, and your body would be at 1-G.  It might give you the oddest sensation. 

Laying down, though, you would get the effect of a 1-G bed. 

Would you get dizzy?

I don't know.

I suppose dizziness really is the transition from a spinning environment to a non-spinning environment.  So, you would probably be fine as long as you were in the module, but it is quite possible that you would have a moderate dizzy feeling every time you transferred from the spinning module to the zero-g modules.

As far as your bones.
They wouldn't know the difference between gravity and artificial gravity.
 

Offline yor_on

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Re: Why is centrifugal force NOT gravity?
« Reply #8 on: 19/03/2011 13:11:35 »
As our balance is concentrated to the ears (That's why deaf people sometimes have problems with their balance) I would expect that you would notice a centrifugal 'force'. It's called the vestibular system and use a combination of fluid and sensitive hairs that feel the fluid move. and in a angular motion they will probably register something 'wrong', but if you had a radius that was big enough it might be different? But in that small environment I think they would complain.

"Our vestibular system works with other sensorimotor systems in the body, such as our visual system (eyes) and skeletal system (bones and joints), to check and maintain the position of our body at rest or in motion. It also helps us maintain a steady focus on objects even though the position of our body changes. The vestibular system does this by detecting mechanical forces, including gravity, that act upon our vestibular organs when we move. Two sections of the labyrinth help us accomplish these tasks: the semicircular canals and the otolithic organs.

"The semicircular canals are three fluid-filled loops arranged roughly at right angles to each other. They tell the brain when our head moves in a rotating or circular way, such as when we nod our head up and down or look from right to left. the otolithic organs, which are two fluid-filled pouches called the utricle and the saccule  tell the brain when our body is moving in a straight line, such as when we stand up or ride in a car or on a bike. They also tell the brain the position of our head with respect to gravity, such as whether we are sitting up, leaning back, or lying down.

Like the semicircular canals, the utricle and the saccule have sensory hair cells. These hair cells line the bottom of each pouch, and their stereocilia extend into an overlying gel-like layer. On top of the gel are tiny grains made of calcium carbonate called otoconia. When you tilt your head, gravity pulls on the grains, which then move the stereocilia. As with the semicircular canals, this movement creates a signal that tells the brain the head's position." From here Balance.

 
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #9 on: 19/03/2011 17:33:17 »
Clifford's ball and string model is a safer bet that the gravitational model, because that raises the ugly question of whether gravity even produces a force, and if there is no force, the discussion about centripetal versus centrifugal becomes moot.

In the ball and string case you can see that it is necessary to maintain a force in the string to prevent the ball flying off at a tangent, and that's the centripetal force. (Of course, there has to be a reaction to that force because there always is a reaction to any force.) If you eliminate the centripetal force by letting go of the string, there are no other forces acting on the ball, and the ball continues on a tangential path. If there was an actual centrifugal force, the ball would still have a radial velocity component, so it would not follow a tangential path.

I think that's the basic argument.
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #10 on: 19/03/2011 19:42:36 »
Centripetal Force is the force towards a center of rotation which could be gravity.
Right.
Quote
Centrifugal force is the force away from the center of rotation which is derived from changing the direction of momentum from a tangential to a curve.
Only if you're in a rotating frame of reference, like you're in a centrifuge or on a Merry-go-round or cornering in a car. Or the Earth's rotation is significant for what you're doing.
Quote
If you tie a ball to a string and spin it around your head.
The centrifugal force keeps it at a horzontal.
No, that's an inertial reference frame, you're not moving so centrifugal force doesn't appear in that case, that's just centripetal force and momentum.

Quote
No, it is not the same as "real" gravity as it is not the attraction of masses towards a center.
That's right.
Quote
If you were in a circular device in orbit that was 1 mile in diameter, spinning, you might not be able to tell the difference between artificial gravity and real gravity as long as you never looked out the window (perhaps you could have artificial stabilized windows). 
Actually if you drop something in that situation the coriolis effect kicks in and it falls behind you anti-spinward, about an inch. You would almost certainly know that you're spinning, and even know which way the axis is. The coriolis force comes out of the same equation that gives you the centrifugal force. In a rotating reference frame centrifugal force is a static force, but coriolis appears when things move.
Quote
Would you get dizzy?
Anything more than a couple of rpm would do that.
Quote
As far as your bones.
They wouldn't know the difference between gravity and artificial gravity.
Yes, that's right, they're both inertial accelerations.
« Last Edit: 19/03/2011 19:46:32 by wolfekeeper »
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #11 on: 19/03/2011 23:14:37 »

Only if you're in a rotating frame of reference, like you're in a centrifuge or on a Merry-go-round or cornering in a car. Or the Earth's rotation is significant for what you're doing.


It's probably just me, but I always think there is something distinctly dodgy about that argument.

If you know you are rotating, you know full well that you are only subject to a real centripetal force, and that any force you experience that maintains your circular motion is merely a reaction to centripetal force (like the force a car door exerts on you when you go round a corner) so you know you just happen to be rotating within an inertial frame.

If you cannot observe that you are rotating it's not easy to detect that you really are in a rotating frame of reference, although you could be, so you might easily infer that you are actually in an inertial frame, in which case you have no idea that the forces you experience are centripetal or centrifugal. They are just forces.

« Last Edit: 19/03/2011 23:16:14 by Geezer »
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #12 on: 20/03/2011 00:43:03 »
No, no, you can always tell you're in a rotating reference frame using a gyroscope.

Actually centrifugal forces aren't forces- you can't measure them (directly)!

If you fall under a centrifugal force, then your accelerometer measures zero, just like it does with gravity.

When you're sitting here right now, the acceleration you feel is upwards; your chair is fighting gravity and accelerating you upwards. That's what accelerometers measure! They can't measure gravity at all. And they can't measure centrifugal force or coriolis force either. These are all inertial forces, and accelometers can't detect them, and neither can you; not directly- they don't make your arms heavy or anything.

Also centrifugal force and coriolis force are actually pseudoforces; notably they don't obey Newton's third law!!!
 

Offline lightarrow

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Re: Why is centrifugal force NOT gravity?
« Reply #13 on: 20/03/2011 12:32:58 »
Also centrifugal force and coriolis force are actually pseudoforces; notably they don't obey Newton's third law!!!
Not even gravity. Not even the force of an EM radiation on an electron.
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #14 on: 20/03/2011 18:07:03 »

Actually centrifugal forces aren't forces- you can't measure them (directly)!


Of course you can. If you are in a rotating frame of reference you will be able to measure the force quite easily.
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #15 on: 20/03/2011 18:26:05 »
If you're holding an accelerometer for example, on Earth, it says that it's being accelerated upwards, and it's the same in a rotating situation. It's measuring the force of the floor/chair holding you up against gravity, not the gravity itself.

All parts of an accelerometer accelerate at almost exactly the same rate under gravity or centrifugal or coriolis, it can't really measure those accelerations at all.
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #16 on: 20/03/2011 20:36:16 »
If you're holding an accelerometer for example, on Earth, it says that it's being accelerated upwards, and it's the same in a rotating situation. It's measuring the force of the floor/chair holding you up against gravity, not the gravity itself.

All parts of an accelerometer accelerate at almost exactly the same rate under gravity or centrifugal or coriolis, it can't really measure those accelerations at all.

I see. So how would you explain what is slamming you into the wall of a centrifuge if it's not a force? I'm pretty sure your backside will be able to quantify something pushing against it. I understand we typically refer to that sort of phenomenon as a "force".
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #17 on: 20/03/2011 21:01:27 »
That's not a centrifugal force, that's a normal force that follows Newton's third law.

Note that the centrifugal force is acting whether or not you're resting on something, and if you're not resting on something, then it's not directly detectable (although you can infer it from the motion and sum of the applied forces.)
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #18 on: 20/03/2011 21:32:44 »
That's not a centrifugal force, that's a normal force that follows Newton's third law.

Yes. I agree with you on that. You might also consider it to be the reaction to centripetal force if you are rotating in an inertial frame.

But you previously described that force as a centrifugal force here. At least it seemed that you did.

Quote
Centrifugal force is the force away from the center of rotation which is derived from changing the direction of momentum from a tangential to a curve.
Only if you're in a rotating frame of reference, like you're in a centrifuge or on a Merry-go-round or cornering in a car. Or the Earth's rotation is significant for what you're doing.
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #19 on: 20/03/2011 22:21:20 »
Sorry, my bad, I misread your post.

From the non rotating point of view (the non rotating reference frame) The 'force' that pushes you into the wall is your momentum; the wall 'slams into you' as your momentum tries to make you go in a straight line.

In the rotating reference frame the force/acceleration pushing you away from the central axis (which does the same thing) is the centrifugal force. But it's not a real force. The real force is the centripetal force on your back from the wall, and the reaction to that (your back pushing on the wall), which is called the reactive centrifugal force. But the reactive centrifugal force isn't acting on your back, it's acting on the wall; it's different from the centrifugal force that we've been talking about, which acts on you. The reactive centrifugal force is in all frames, rotating and non rotating, and is a proper force.
 

Offline Phractality

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Re: Why is centrifugal force NOT gravity?
« Reply #20 on: 21/03/2011 00:09:27 »
I'm still in negotiations with my brain to get it back on the job; I think it wants less physics and more phun. Back to the subject:

If you want a non-rotating reference frame on Earth, try a west-bound bullet train, on smooth horizontal track, fast enough to match Earth's rotation at its latitude. That will eliminate most centrifugal, centripetal or centrifical force, whatever you choose to call it. Of course, you are still orbiting the sun, but that is 365 times less angular speed than the daily rotation.

If you weigh a kilogram mass on a properly calibrated spring scale in a sidereal reference frame, you can then calculate the correct mass of Earth from Newton's universal law. (Most spring scales are calibrated to show mass, instead of force, in the local gravity.) His formulas are not valid in the rotating reference frame of Earth. Or you can use a gravity pendulum instead of a scale.

Dr. Who has a solution to the question of whether he's on a space station with centrifugal (or centrifical, or centripetal) gravity. He always had a yoyo in his pocket, just in case. If the yoyo exhibits Coriolis effect, he's on a rotating platform. Of course, a 24 hour day makes the yoyo much too crude. You might need a Foucault pendulum.
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #21 on: 21/03/2011 02:46:41 »

From the non rotating point of view (the non rotating reference frame) The 'force' that pushes you into the wall is your momentum; the wall 'slams into you' as your momentum tries to make you go in a straight line.


 ;D No, it's not. Momentum is not a force. It's centripetal force that constrains your mass to follow a circular path so that your linear momentum is converted into angular momentum. As soon as the centripetal force is eliminated, your angular momentum becomes linear momentum again.   
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #22 on: 21/03/2011 03:08:07 »
That's right, that's why I put it as 'force' instead of force, in a rotating reference frame ordinary linear momentum appears/requires two accelerations to which you can ascribe two pseudoforces to explain the motion within that frame. I mean in a rotating reference frame you would intuitively expect things to still go in a straight line, but they tend to curve, and that can be represented as these accelerations/forces.

Incidentally, linear momentum doesn't convert into angular momentum, they're different things.
« Last Edit: 21/03/2011 03:09:46 by wolfekeeper »
 

Offline Geezer

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Re: Why is centrifugal force NOT gravity?
« Reply #23 on: 21/03/2011 03:19:56 »
Incidentally, linear momentum doesn't convert into angular momentum, they're different things.

Yes. In one case mass rotates around an axis whereas in the other, mass travels in a straight line, or did you mean something different?

Anyway, if you can can convince me that momentum is not conserved when the string breaks, you'll get my attention  :D
 

Offline wolfekeeper

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Re: Why is centrifugal force NOT gravity?
« Reply #24 on: 21/03/2011 03:31:30 »
Normally most people talk about angular momentum in terms of rotation, but if I'm measuring angular momentum around my finger and a car drives past it 10ft away at 30 mph, in a perfectly straight line, then from the definition of angular momentum the car has (a constant) angular momentum around my finger, even though nothing is rotating. And if I pick a different finger, the angular momentum is typically different; but the linear momentum is unchanged.

Conversely, something that is rotating, each bit/atom/molecule that's moving has linear momentum as well as angular momentum, and you calculate the two completely separately- one can't turn into the other; they're just different, and independently conserved.
« Last Edit: 21/03/2011 03:34:37 by wolfekeeper »
 

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Re: Why is centrifugal force NOT gravity?
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