Can centripetal and centrigugal force act together?

[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.