Can centripetal and centrigugal force act together?

[sophiecentaur]

Which 'homo' am I supposed to be attacking? Mr Scales was my hero! He taught Physics like no other could. And I wouldn't attack YOU - you might be bigger than me. ;) - and you are telling me to take a running jump off a tall building, too.

My point was that, at an early age, I was given all the arguments about things not 'flying off' due to centrifugal force - which I, of course, appreciate because it could mean a big misconception and a false prediction. BUT, there is a force, which can be felt and measured and it IS in the direction opposite to the centripetal force. Furthermore, the direction of motion, when the central force is removed, asymptotes towards the 'away' direction even if it starts off tangential. The force's realness or otherwise is at another level of sophistication and it just seems over precise to make too much of a thing of it. I just wonder how many of the teachers who have shouted and screamed about it have given it as much thought as this thread has.

 

[D H]

WMr Scales was my hero! He taught Physics like no other could.

How are we to know who your Mr. Scales was? I thought you were referring to Junius Scales, the only Mr. Scales I could find who did something noteworthy in the early 60s.

My point was that, at an early age, I was given all the arguments about things not 'flying off' due to centrifugal force - which I, of course, appreciate because it could mean a big misconception and a false prediction.

Teachers (and textbook authors) who centrifugal force to explain orbits are, in my opinion, doing an incredible disservice to their pupils. For one thing, that explanation typically goes side-by-side with a drawing of a planet/satellite moving in circular path around the Sun/Earth. That circular path implies that the teacher or author is looking at things from the perspective of an inertial frame of reference. There is *zero* centrifugal force in this frame. The only perspective from which a circular orbit has a centrifugal force that exactly opposes the gravitational force is a rotating frame of reference in which the planet/satellite is stationary.

For another thing, orbits in general are not circular. They are elliptical.

Here are nine elliptical orbits with the same semi-major axis, eccentricity = 0.1 to 0.9 in steps of 0.1, as viewed from the perspective of an inertial frame:

With this inertial perspective, the shapes are simple and well-known (ellipses that share a common focus, and in this case, share a common semi-major axis). The descriptions of the orbits are once again simple and well-known (Kepler's laws). The equations of motion are yet again simple and well-known (Newton's second law + Newton's law of gravitation).


Here are the same orbits, but this time viewed from the perspective of a frame rotating with the mean motion of these orbits:

With this rotating perspective, the shapes are complex (this family of curves may have a name, but I don't know what it is). The descriptions of the orbits are complex, as are the equations of motion (Newton's second law + Newton's law of gravitation + centrifugal force + coriolis force).

BUT, there is a force, which can be felt and measured and it IS in the direction opposite to the centripetal force.

There is no measurable centrifugal force. The centrifugal force, if there is one, depends on the frame in which the picture is drawn. Whether you want to call it "real" is one thing. That it is measurable is quite another. It isn't.

 

[sophiecentaur]

Did I ever say that orbits are explained using centrifugal force?
'My' centrifugal force is the one you feel when you are on a fairground ride. It's there - I have felt it.
'My' Mr Scales was also notable to all his students; Google doesn't know everything.

 

[D H]

'My' centrifugal force is the one you feel when you are on a fairground ride. It's there - I have felt it.

That's not a centrifugal force. You feeling a centripetal force on those fairground rides.

Think about it this way: You are going around in a circle on those rides. That means your acceleration vector is pointing toward the center of the circle. That in turn means some force directed toward the center of the circle is acting on you, and that is by definition a centripetal force.

 

[sophiecentaur]

I know very well that the seat of the ride is pushing me inwards. The sums are quite clear. But I FEEL a force pushing my soft bits towards my back- 'outwards'. That force is away from the centre. Centrifugal, in fact. It is there just as much as the force of the ground on my feet. They are both reaction forces due to an acceleration. Why is this such a big deal? I just give a force a name and people get twitchy.

 

[D H]

I know very well that the seat of the ride is pushing me inwards. The sums are quite clear. But I FEEL a force pushing my soft bits towards my back- 'outwards'. That force is away from the centre.

As you knew Mr. Scales back in 1962, I assume you also knew Mr. Dodge Charger with a 426 Hemi back in 1966. If not that, pick some other 1960s muscle car. Those muscle cars weren't all that great on winding roads, but on a straightaway, DANG! Step on the gas and those puppies not only made you feel a force pushing your soft bits towards the back, they made you feel a force pushing every single one of your bits toward the back -- and right through the seat.

(Nostalgia time: My best car ever was a 1967 Plymouth Satellite that I bought in 1977 for all of $400. Unfortunately, two years later some old lady in a VW Rabbit ran a red light and front-ended me. My tank totally demolished her rabbit, but her rabbit did manage to crack my radiator and twist my suspension. End of an era ...)

Back to the topic at hand: Those muscle cars made you feel your bits pushed to the back when you stepped on the gas and accelerated forward. The car accelerated most of you forward. Your soft bits however, are only loosely connected to the rest of you. Those soft bits maintained their original momentum for a short time while your not-so-soft bits accelerated with the car. That pushed-to-the-back feeling resulted from you being accelerated forward. What you felt from those muscle cars of yore is exactly analogous to what you feel on one of those fairground rides.

 

[sophiecentaur]

Sorry to hear about the Plymouth. Insurance companies just don't understand about emotional attachment. "Beyond economic repair" is a phrase which they often apply to priceless objects.
Look, we're only arguing about a name. Because of Newton 3 we can always say that these forces appear in pairs. Mostly, we only consider and name one of the pair and I agree that centrifugal force is not the way to go in explaining what makes things go in a curve.
My fave car was a Lotus Super Seven, which was a bit like a roller skate. It could do 0 - 60 in less than 6 seconds (soft bits and all) with a 1500 Ford Cosworth engine. It woudn't do more than 100 and even at that it was screaming. Great at traffic lights in town until some brute in a Corvette Stingray left me behind in a cloud of dust. Really cut me down to size - then I bought a Ford Escort. . . .

 

[A.T.]

But I FEEL a force pushing my soft bits towards my back- 'outwards'. That force is away from the centre. Centrifugal, in fact.

You don't feel forces directly. You feel deformations of your body. Theses deformations occur because the centripetal force is applied non-uniformly to parts of you body. So what you feel indirectly is the centripetal force, not the centrifugal force. You feel a force pushing your back inwards against your soft bits.

If the centripetal force is suddenly gone and you fly off tangentially, the centrifugal force in the rotating frame is still there and accelerates you away from the center, but you don't feel any load. This shows that you cannot feel the centrifugal force.

 

[sophiecentaur]

OK - say we've got a conker on a string, whirling around. There MUST be two forces on each element of the string, keeping it taught. Those constitute the 3law pair I'm talking about. The must both exist or the string would be slack.
Yes, 'everyone' (at least you and I) knows the conker will fly off tangentially. That's not the issue. If I am on the conker, that's the frame I'm interested in and I guess I can change my interest when the string is cut - but I'm not going to cut the string.

What would you call the force pulling outwards on the string, then? (Bearing in mind that it does exist and is directed away from the centre and it needs a name).
I don't think you realize that we are arguing about semantics and not Physics.

 

[Doc Al]

What would you call the force pulling outwards on the string, then? (Bearing in mind that it does exist and is directed away from the centre and it needs a name).

Why does every force need a special name? Note that you are talking about an outward force on the string, not on the object. (Viewed from a rotating frame, centrifugal force acts on the object.) The string and object exert forces on each other. If you want to call that force on the string something, don't call it the 'centrifugal force'--that term already has a specific meaning in physics. (Some use the term 'reactive centrifugal force'--use that at the risk of confusing people.)

 

[jambaugh]

so how do you conclude the nature of this force,as to where which acts,centripetal or centrifugal,i mean generally,as in books especially in engineering concepts,it is highly unclear.

I always kept them straight by remembering the 'f' in centrifugal stands for 'flee' so it is the apparent radial outward force in a rotating frame while a centripetal force is the force typically opposing the centrifugal and effecting the circular motion. (The latin root fugere means 'to flee' also use to define fugacity in thermodynamics.)

 

[D H]

What would you call the force pulling outwards on the string, then?

Tension.

I don't think you realize that we are arguing about semantics and not Physics.

Misusing names can lead to confusion. The term centrifugal force has a specific meaning.

 

[sophiecentaur]

Tension.

Doesn't tension operate in both directions in the string? I think that's a cop-out.

Misusing names can lead to confusion. The term centrifugal force has a specific meaning.

I guess that's the best argument against using it when it's not appropriate. So what can we call it? Tension is just not specific enough - because it doesn't describe the force of my body against the seat on the ride.

 

[rcgldr]

Wiki has updated it's "reactive centrifugal force" article:

http://en.wikipedia.org/wiki/Reactive_centrifugal_force

Perhaps it's not common usage in physics, but it is common usage in English, which is a larger audience, and I doubt the term centrifugal force is confusing to anyone in physics, who would know that it's a reactive force in a standard inertial frame.

 

[sophiecentaur]

So they now have "reactive centrifuges" in Biology labs? Fair enough. Do you think it will catch on?

 

[A.T.]

What would you call the force pulling outwards on the string, then? (Bearing in mind that it does exist and is directed away from the centre and it needs a name).

That is the reactive centrifugal force (a 'real' interaction force that exists in every frame):
http://en.wikipedia.org/wiki/Reactive_centrifugal_force

Not to be confused with centrifugal force (a 'fictional' inertial force that exists only in the rotating frame):
http://en.wikipedia.org/wiki/Centrifugal_force_(rotating_reference_frame)

 

[rcgldr]

In some cases which forces are real or reactive get a bit fuzzy. Here is an example of a radio control glider dynamic soaring in circles at speeds up to 375 mph (the mentioned 392 mph pass wasn't caught on video):

http://www.youtube.com/watch?v=WaQB16ZaNI4&fmt=18

While circling, the air exerts a centripetal force on the glider, causing the glider to accelerate inwards, following a circular path. This coexists with the glder exerting a centrifugal force on the air, causing the air to accelerate outwards in a spiraling path. Here the glider's outwards reactive force coincides with the outward force the glider exerts onto the air, and the air's inwards reactive force coincides with the inwards force the air exerts onto the glider.

Similarly imagine a rocket in space void of gravitational effect, using it's thrust to follow a circular path. The spent fuel exerts a centripetal force on the rocket, causing the rocket to accelerate inwards, following a circular path. This coexists with the rocket exerting a centrifugal force on the spent fuel, causing the spent fuel to accelerate outwards in a spiraling path. Here the rocket's outwards reactive force coincides with the outward force the rocket exerts onto the spent fuel, and the spent fuel's inwards reactive force coincides with the inwards force the spent fuel exerts onto the rocket.

update - so which of the forces in these examples are "fictitious"?

 

[D H]

Stop that, Jeff!

 

[sganesh88]

Here the glider's outwards reactive force coincides with the outward force the glider exerts onto the air, and the air's inwards reactive force coincides with the inwards force the air exerts onto the glider.

They coincide because they are one and the same. Same is the case with the rocket example too. When you hit a wall, there is only one force exerted by you on the wall. You seem to think there are two- namely the force exerted by you on the wall and the reactive force you exert on the wall. This isn't true. We cannot absolutely classify a force as action or reaction. Its just that forces observed from an inertial frame occur in pairs of action and reaction.

so which of the forces in these examples are "fictitious"?

Terming a force as fictitious depends on the reference frame. If you have observed the above two events from an inertial frame, then all the forces you measure are real.

 

[jambaugh]

If you want to argue semantics then let's make it clear what the debate is.

*** In one context "centripetal" and "centrifugal" are just qualifiers for resp. negative radial and positive radial directions and you can also speak of centripetal velocity or centrifugal displacements as well as further qualify with "reactive" or "applied" or whatever. In such context the two words are redundant (which isn't necessarily bad) as we could as easily speak of positive centrifugal and negative centrifugal or similarly negative centripetal and positive centripetal.

*** In another context we resolve the vector acceleration of a particle in polar coordinates:

\vec{a}= \ddot{\vec{r}} = (\ddot{r}-r\omega^2) \hat{r} + (r\dot{\omega} +2\dot{r}\omega)\hat{\theta}
(\omega = \dot{\theta})

We get two sets of terms.

The coordinate accelerations:
\vec{a}_{cord}= \ddot{r}\hat{r} + r\dot{\omega}\hat{\theta}
and the components emerging from rates of change of our basis (local frame):
\vec{a}_{frame} = -r\omega^2 \hat{r} + 2\dot{r}\omega\hat{\theta}

We can then express Newton's 2nd law in two forms:
\vec{F} = m\vec{a}
or
\vec{F}_{eff} = m\vec{a}_{coord}
where the l.h.s. is an "effective force" which is the "physical force"
plus the "pseudo-forces" or "fictional forces" we get by subtracting out mass times the frame accelerations.

They are the coriolis force:
\vec{F}_{cori}= -2m\dot{r}\omega\hat{\theta}
and the centrifugal force:
\vec{F}_{cnf} = mr\omega^2\hat{r}

Now when considering systems where the coordinate acceleration is zero the centrifugal force must be canceled by a radial component of the "physical force" which component we call the "centripetal force".

*** Now we can also adopt a third context not totally distinct from the second one wherein we consider a uniformly rotating frame which itself may be resolved in rectangular or polar coordinates or some nastier system. We may even use an origin distinct from the center of rotation. In this context when using non-rectangular coordinates we will again need to resolve out coordinate and "local frame" components of acceleration while also taking into account the effects of the over-all time dependency of the rotating coordinate system.

It is thus useful (and consistent with Einstein's equivalence principle) to treat the coriolis and centrifugal forces due to the global frame rotation as "physical" e.g. a form of gravity. We will also have as before local frame components of the acceleration which we can again refer to as "pseudo-forces" if that is our inclination.

Typical examples in this last context are ballistics and the dynamics of a hurricane given the rotation of the Earth.

Now I think it is silly to argue over "fictitious" vs. "real" forces especially given GR where all gravitational forces are just as "fictitious" as the coriolis and centrifugal ones here. The important point is correct bookkeeping. We must be sure that Newton's 2nd law (F - ma =0) gets transformed correctly when we change coordinate systems.

Personally I am for dropping the "fictitious" or "pseudo" qualifiers in physics. (Though they may still be appropriate in engineering.) We should properly recognize that Einstein's equivalence principle goes both ways and such distinctions are meaningless. After all in Kaluza-Klein theory EM forces end up being "fictitious" as well. Consider also covariant momenta and gauge transformations when we define forces as rates of change of momenta.

 

[YellowTaxi]

In some cases which forces are real or reactive get a bit fuzzy. Here is an example ... While circling, the air exerts a centripetal force on a glider, causing the glider to accelerate inwards, following a circular path. This coexists with the glder exerting a centrifugal force on the air, causing the air to accelerate outwards in a spiraling path. Here the glider's outwards reactive force coincides with the outward force the glider exerts onto the air, and the air's inwards reactive force coincides with the inwards force the air exerts onto the glider.

Exactly, centrifugal/centripetal is just action/reaction between 2 bodies. After all, it's not possible to have action/reaction with just one, - the 2 always pull or push on each other. Which is the action and which the reaction is just an argument of semantics.

Similarly imagine a rocket in space void of gravitational effect, using it's thrust to follow a circular path. The spent fuel exerts a centripetal force on the rocket, causing the rocket to accelerate inwards, following a circular path. This coexists with the rocket exerting a centrifugal force on the spent fuel, causing the spent fuel to accelerate outwards in a spiraling path. Here the rocket's outwards reactive force coincides with the outward force the rocket exerts onto the spent fuel, and the spent fuel's inwards reactive force coincides with the inwards force the spent fuel exerts onto the rocket.
update - so which of the forces in these examples are "fictitious"?

both, it's all down to individual perception, ie which frame of reference you're using. And we could probably argue that all forces are fictitious if we follow Einstein's lead. And don't forget that Newton's first attempt at putting circular motion into a mathematical formulation involved the centrifugal not centripetal force.

 

[D H]

Exactly, centrifugal/centripetal is just action/reaction between 2 bodies. After all, it's not possible to have action/reaction with just one, - the 2 always pull or push on each other. Which is the action and which the reaction is just an argument of semantics.

Centrifugal force (rather than the archaic "reactive centrifugal force" that Jeff likes) does *not* obey Newton's third law. It is not just a matter of semantics.

 

[rcgldr]

Centrifugal force (rather than the archaic "reactive centrifugal force" that Jeff likes) does *not* obey Newton's third law.

In the examples I mentioned with circling glider and rocket, where is Newtons third law being violated?

In a sense, virtually all forces involving accelerations are "reactive". It's just that in most cases, one or more of the objects involved is attached some massive object (such as the earth) where that massive object is treated as if it had infinite inertia, and the momentum effects on that massive object are ignored.

 

[Doc Al]

In the examples I mentioned with circling glider and rocket, where is Newtons third law being violated?

Are you intentionally missing D H's point? Your examples have nothing to do with centrifugal force (as used in physics), only 'reactive centrifugal force'. (At this point I can only think that you are yanking our chains by purposely leaving off the label 'reactive'.)

 

[rcgldr]

Your examples have nothing to do with centrifugal force (as used in physics), only 'reactive centrifugal force'.

and 'reactive centripetal force'. The point of those examples (circling glider or rocket) was to demonstrate that sometimes both centripetal and centrifugal forces can be 'reactive'. In those examples, the centripetal force is just as 'reactive' as the centrifugal force. The other point of this was to demonstrate that just because a force is 'reactive' doesn't mean that force isn't real. If anything, the term 'fictitious' is the mis-leading term often used when describing certain forces.

 

[Doc Al]

The point of those examples (circling glider or rocket) was to demonstrate that sometimes both centripetal and centrifugal forces are reactive. In those examples, the centripetal force is just as 'reactive' as the centrifugal force.

In your examples you use the term 'centrifugal force' when you really mean the archaic 'reactive centrifugal force'. (The two terms are not related, except in their etymological origins.) It seems as if you are doing this on purpose to confuse things.

The other point of this was to demonstrate that just because a force is 'reactive' doesn't mean that force isn't real.

Huh? Any force that is 'reactive'--meaning that it is part of a Newton's 3rd law pair--is clearly a real, agented force.

If anything, the term 'fictitious' is the mis-leading term often used when describing certain forces.

What's misleading is your using the term 'centrifugal force' when you are really talking about the archaic 'reactive centrifugal force'.

 

[rcgldr]

I thought that the 'reactive' part of the term centrifugal force would be assumed when used in an inertial (non rotating or otherwise accelerating) frame, so I didn't think it was needed. In the broader world of the English language as opposed to physics termonology, centrifugal just means outwards. I don't see any need to avoid the term as long as it's not being used incorrectly to refer to what is really a centripetal force.

Again my point was that a centripetal force can also be reactive, but no one seems concerned about prefixing it with term 'reactive' in those cases. It's also possible to have an outwards (I'll avoid the term centrifugal) force that is non-reactive, such as rocket thrust perpendicular to direction of travel to change the eccentricity of an orbit.

I'll try to be more careful about my wording and use the term 'reactive centrifugal force' when it seems appropriate in any future posts I make here.

Getting back to the OP, is it possible to have a centripetal force without a 'reactive centrifugal force'? Take the case of a two object system in space, void of any external gravitational effects. The only gravity is that from the two objects. Assume the simple case where the two objects move in a circular path around a common center of gravity, each on the opposite side from the other about the center of mass. Gravity is the centripetal force in this case, but how does the 'reative centrifugal force' play a role in this case? Replace gravity with a string, but otherwise the same two objects, circling about the same center of mass as the same speed, and then the argument could be made that 'reactive centrifugal force' is what is maintaing the tension in the string, but going back to the gravity case, what role does 'reactive centrifugal force' play?

 

[sganesh88]

but going back to the gravity case, what role does 'reactive centrifugal force' play?

It makes the other body to revolve around the common COM.

 

[rcgldr]

two body system ... string case ... gravity case, what role does 'reactive centrifugal force' play?

It makes the other body to revolve around the common COM.

Unlike the string case, the force and rate of acceleration is a function of distance between the two objects, and not the velocities. The fact that v²/r = the rate of acceleration due to gravity is why the objects move in a circular path, but the velocity has no affect on the force or accelerations between those objects.

In the string case with no gravity, you have two sets of Newton third law force pairs: the ends of the string exert a centripetal force to each object, and each object exerts a reactive centrifugal force on its end of the string.

In the gravity case, you have one set of a Newton third law force pair, the equal and opposing gravitational force experienced by each object towards the other, unrelated to any 'reactive centrifugal force'. Their relative motion has no effect on the forces involved (ignoring speed of gravity issues). I think this is going to be the case when ever forces like gravity or electrical charge that don't require physical contact are involved.