I Is calling fictitious forces "not real" just about terminology?

[renobueno4153]

TL;DR Summary

- Did I understand this concept right?
- If I hear a force is fictitious I think there is like an illusion making me think there is smth happening for the wrong reasons but fictitious forces like the corioles force have actually consequences in the real world. So my question is "real force vs. fictitious forces" just about differentiating two ideas or can the "illusion" of fictitious forces explained by the influence of real forces?

Hi there, im studying nanoscience at the university in Basel.

Today I looked at the topic of intertial and non-inertial reference frames and the existence of fictitious forces.

I understand that you call forces real in physics if they appear in interplay. Meaning that a force is real when there is the "actio" partner to the "reactio" partner. If this condition is not satisfied the force is not real.

I also understand that if you specifically look at non-inertial reference frames you can observe accelerations/forces which you wouldn't in inertial reference frames. And to satisfy newtons first law of motion (F=ma) you need to mathematically adjust that to keep that statement true. So in order to describe the accelerations and forces appearing in non-inertial reference frames you introduce the concept of fictitious forces.

So my question is:
- Did I understand this concept right? (Feel free to call everything wrong out! I dont care about brutal honesty I really want to know!)
- If I hear a force is fictitious I think there is like an illusion making me think there is smth happening for the wrong reasons but fictitious forces like the corioles force have actually consequences in the real world. So my question is "real force vs. fictitious forces" just about differentiating two ideas or can the "illusion" of fictitious forces explained by the influence of real forces?

 

[jbriggs444]

So my question is:
- Did I understand this concept right? (Feel free to call everything wrong out! I dont care about brutal honesty I really want to know!)

Yes. I agree with pretty much everything you have written about the distinction between real and fictitious forces. A real force (or "interaction force") is one that has a third law partner. A fictitious force will (in almost every case^(*)) not have a third law partner.

- If I hear a force is fictitious I think there is like an illusion making me think there is smth happening for the wrong reasons but fictitious forces like the corioles force have actually consequences in the real world.

Here I disagree somewhat. If you look at it in a certain way, the Coriolis force has no real world consequences. One could, in principle, describe the rotating Earth and the weather patterns on it in terms of a non-rotating coordinate system beneath which the ground and winds are whipping past at hundreds of meters per second. This description would make all the same experimental predictions (for wind, rain, barometric pressure, etc) as the more standard model in which the Earth is regarded as stationary.

We adopt the rotating coordinate system because it is convenient. We do not adopt the inertial coordinate system because it would be inconvenient. The "realness" or "fictitiousness" of the modelled quantities is unimportant to that decision.

So my question is "real force vs. fictitious forces" just about differentiating two ideas or can the "illusion" of fictitious forces explained by the influence of real forces?

It is just a choice of coordinate system. One often picks a coordinate system that makes calculations easier. Sometimes a uniformly accelerating system is convenient. Sometimes a uniformly rotating system is convenient. Sometimes a rotating system with rotational acceleration may be convenient and one then has to factor in the Euler force.

(*) When you learn General Relativity, you will be exposed to the idea that gravity is a fictitious force. Despite it having a third law partner. But one needs curved space time to make that idea work out. In the mean time the Newtonian approximation with gravity as a real interaction force is what everyone uses.

 

[PeroK]

The simplest explanation, IMO, is this:

If you measure an object to be accelerating, then there are two different reasons for this:

1) The object is subject to a real force.

2) You are accelerating (subject to a real force) and the object is not subject to that force.

(Note that, of course, it could be a combination of the two.)

Real forces are real - they can directly be measured; fictitious forces are not real - they are needed to balance the books.

The classic example is the scenario of an accelerating car. You feel the real force that accelerates you from the seat. If you look out the window, in your reference frame, the houses are accelerating "backwards". There are no new forces on those houses causing them to accelerate in the opposite direction from the car. Instead, you need to include fictitious forces to explain the relative motion of the houses, from your accelerating reference frame.

 

[wrobel]

Spoiler: just a personal experience

Fictitious forces are completely useless and they just obscure the process of problems solving.

 

[PeroK]

PS and, as you are at rest in your own reference frame, you have to add the same (*) fictitious force to yourself in order for Newton's second law to hold in that accelerating frame. I.e. you need to add a fictitious force equal and opposite to the real accelerating force you feel. Then the forces on you are balanced in your reference frame.

(*) Note that the magnitude of a fictitious force generally depends on the mass of the object.

 

[wrobel]

Fictitious forces appear as a result of the following banal mathematical trick.
Consider the second Newton relative an inertial frame:
$$m\boldsymbol a=\boldsymbol F.$$ Assume that we have a non-inertial frame as well.
Then the acceleration is presented in accordance with well-known kinematic formula:
$$\boldsymbol a=\boldsymbol a_c+\boldsymbol a_e+\boldsymbol a_r,$$
here $\boldsymbol a_c$ is a Coriolis acceleration;
$\boldsymbol a_e$ is an acceleration of transport;
$\boldsymbol a_r$ is an acceleration relative the non-inertial frame.
Substituting one formula to another we have
$$m \boldsymbol a_r=\boldsymbol F-m\boldsymbol a_e -m\boldsymbol a_c.$$
The last two terms in the right hand side are so called fictitious forces.

 

[robphy]

Possibly enlightening

Frames of Reference (featuring Hume & Ivey)
by Richard Leacock
go to 17:05 for the rotating frame

 

[renobueno4153]

Possibly enlightening

Frames of Reference (featuring Hume & Ivey)
by Richard Leacock
go to 17:05 for the rotating frame

HOLY SMOKES THATS SICK. Ok this video hooked me so much that I watched the whole 20min.

So I can see now that fictitious forces are depended on the reference frame. But do we have an explanation where that change in motion comes from? To be precise: We use fictitious forces to explain how the change of motion came to be. Meaning in the perspective of a non-initial reference frame, we think a force must have changed e.g. the path of an object. And I want to understand where this change comes from in the first place. Does this question make sense, its quiet late here in Switzerland. I cant let go just yet, I need answers! :D

 

[renobueno4153]

Thx to all of you guys! Much appreciated!

 

[jbriggs444]

HOLY SMOKES THATS SICK. Ok this video hooked me so much that I watched the whole 20min.

So I can see now that fictitious forces are depended on the reference frame. But do we have an explanation where that change in motion comes from? To be precise: We use fictitious forces to explain how the change of motion came to be. Meaning in the perspective of a non-initial reference frame, we think a force must have changed e.g. the path of an object. And I want to understand where this change comes from in the first place. Does this question make sense, its quiet late here in Switzerland. I cant let go just yet, I need answers! :D

I would say that nothing physical needs to change its state of motion. It is just coordinate systems. Scribbles on pieces of paper. This part of the coordinate system over here right now is moving differently from that part of the coordinate system over there back then. [There can be complexities with comparing velocities here and now against velocities there and then, but leave that for another day. In the flat space time of Newton and special relativity such comparisons are not problematic]

If you are measuring the velocity of a physical object using a non-inertial coordinate system as a reference, you may see that the coordinates for the object's position may not change evenly even if the object is subject to no interaction forces. Its coordinate acceleration may change in magnitude and direction as the coordinate system shifts beneath it.

Of course, we often choose to use non-inertial coordinate systems because they track the motion of some physical object. Like a car, carousel or planet we are riding in. We may lock the x, y and z axes to that reference object. We may even use curved lines like longitude and latitude instead of straight coordinate axes.

You might reasonably ask about the forces that cause a physical reference object to accelerate and gyrate as it does. But that question must be answered on a case by case basis. What interaction forces act on your chosen reference object?

If you did not use a reference object and instead conjured the coordinate system with pencil and paper, the answer is that the motion of the coordinate system came from your choice to use it.

One of the important lessons in first year physics is that one is not obliged to use a frame of reference in which the experimenter is at rest.