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We say that centrifugal force is fictitious, yet we still use it in some problems. If the centrifugal force is equal and opposite to the centripetal force wouldn't that make the net force zero?

Qmechanic
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Santosh Khatri
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    The title of the question should serve as a title to the whole question and one should probably avoid writing the text of the question in literal continuation of the phrase/sentence in the title. In other words, the body of the question should be such that it can convey some meaning on its own and need not be read in literal continuation of the title. –  Mar 19 '19 at 02:01
  • @DmitryGrigoryev While the titles are similar, I would say that question is somewhat different and is asking about the misconception of the centrifugal force as a reaction force to the centripetal force and starting with the assumption that the forces do not cancel out. I think it's a related question rather than a duplicate. I should also note that the OP of this question didn't choose the title it currently has. – BioPhysicist Mar 19 '19 at 10:08
  • I am really sorry for title.I keep in mind for next time – Santosh Khatri Mar 19 '19 at 12:04
  • You can't simultaneously have a real and a fictitious force, so the concept of "cancelling" cannot apply. – Carl Witthoft Mar 19 '19 at 14:52
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    @CarlWitthoft Yes you can. In fact the object does exactly what expect it to do. Remain motionless (within the rotation frame of reference where the Centrifugal force appears). – Aron Mar 19 '19 at 14:55
  • @Aron Well yes, but working in a rotational frame is rarely helpful. – Carl Witthoft Mar 19 '19 at 14:58
  • @CarlWitthoft I'm sure car designers would disagree with you, and you should thank them that you don't fall out of the car, and instead are kept stationary in the rotating frame of reference of your car turning a corner. – Aron Mar 29 '19 at 04:23

2 Answers2

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The centrifugal force is a "fictitious force" that appears when working in a rotating coordinate system. Basically (together with the Coriolis force) it's the imaginary force that would, if real, make objects move with respect to a fixed, non-rotating coordinate system in the same way that they actually move (due to inertia) with respect to the rotating coordinates that we're using.

If all that sounds really confusing, please see the second half of this earlier answer I wrote, where I try to explain this in more detail (and with pictures!).

Anyway, the point of these fictitious forces is that they let us apply the same laws and formulas of Newtonian physics in a rotating frame of reference as we would in a non-rotating one, and still get physically correct results, as long as we remember to also include the effect of those imaginary forces on all objects.

For example, in a normal non-rotating coordinate system, a stationary object will remain stationary if (and only if) all the forces acting upon it cancel out, so that the net force acting on the object is zero. In a rotating coordinate system, an object which is stationary with respect to the coordinates (i.e. rotating along with them around the same axis at the same speed) will remain so if (and only if) all the real and imaginary forces acting on it cancel out, leaving an (apparent) net force of zero.

Here's a simple example, taken from the answer I linked above. Imagine two spheres floating in space near each other. If you do nothing, they'll just keep floating there. If you push each of them in different directions, then they'll each float in the direction you pushed them, away from each other. But if the spheres have been tied together with a string, then the tension of the string will exert a centripetal force that will curve their trajectories into circles:
Illustration of circular motion due to a centripetal force

Now, if we look at the same system of two spheres and a string in a coordinate system which is rotating along with them, then the spheres will look as it they were motionless. But clearly something is still pulling the string taut (and, if it's elastic, stretching it), counteracting the tension force that is pulling the spheres together. We call this apparent force (which is really just inertia, hidden by the fact that our coordinate system is rotating) the "centrifugal force":
Illustration of centripetal and centrifugal forces in a rotating coordinate system

In this example, since the spheres are motionless with respect to the rotating coordinate system, and since the centripetal and centrifugal forces balance out, they will remain motionless with respect to the rotating coordinates — i.e. they will continue to rotate around the same axis at the same speed.

Ilmari Karonen
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First, it must be stated that Newton's laws only hold in inertial frames. What this means is that accelerations must arise from forces (Second Law), and these forces arise from interactions (Third Law). The issue with rotating frames is that accelerations arise when forces of interactions are not present, so Newton's laws do not hold in rotating frames.

However, the second law ($\mathbf F=m\mathbf a$) is nice to use since it tells us how to determine the position and velocity of a body given initial conditions. Therefore, we define "fictitious" centrifugal and Coriolis forces in order to keep this framework. They are "fictitious" because they are an artifact of the rotating reference frame rather than interactions, but they are not fake (for example, they are very real for anyone going around a sharp turn in a car). Essentially we have opted to abandon the third law in order to keep the second law.

Now, onto your specific inquiry: If you are in a rotating frame, and there is a force equally opposing the centrifugal force, then yes the net force is zero (assuming no Coriolis force either). Therefore in the rotating frame there is no acceleration of the object in question.

Of course, if you looked at the scenario from an inertial frame you would have a non-zero acceleration of the object as there is now a non-zero net force that is the centripetal force.

BioPhysicist
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    You should emphasize that Newton's laws are defined for inertial frames and that the treatment of non-inertial frame by applying inertial pseudo-forces is a lash-up to lets us apply the machinery of Newtonian mechanics to situations other than those for which in which the subject finds its natural expression. Otherwise you invite misunderstanding. – dmckee --- ex-moderator kitten Mar 19 '19 at 02:24
  • @dmckee I agree those are good points to make. I have added information pertaining to this. Thanks for the suggestion. – BioPhysicist Mar 19 '19 at 03:24
  • "Essentially we have opted to abandon the third law in order to keep the second law" That's one way of thinking about it, but you could equally well argue that we invented "gravity" to avoid abandoning the third law and keep the second law (and also keep Euclidean geometry) in the real world we live in, and inventing other so-called "fictitious forces" in non-inertial reference frames isn't really much different. – alephzero Mar 19 '19 at 10:02
  • @alphazero I usually try to stay in the framework specified by the tags. But that is a good point. – BioPhysicist Mar 19 '19 at 10:04