B Why Does a Body Experience Fictitious Force on Accelerating Objects?

[Dale]

you can also describe motion from the point of view of a fly, which is attached to the rod. And in the rotating frame you have to introduce the centrifugal force, don’t you?

This is correct. To explain the motion in the non-inertial frame you introduce the inertial forces. This is the only effect of the inertial forces.

The measurements of accelerometers or strain gauges or any other measurement device depend only on the real forces.

 

[PeroK]

Exactly, and it's always easier to understand a situation wrt. an inertial reference frame!

This homework problem was a lot easier to solve in the rotating reference frame using the centrifugal force:

https://www.physicsforums.com/threads/mass-m-sliding-without-friction-inside-a-rotating-tube.1006329

 

[jbriggs444]

Personally, I have no problem adopting a non-inertial frame and dealing with inertial forces. Just like the 99.9% of all first year physics students who are taught about the inertial force we call gravity.

 

[gleem]

I agree with @vanhees71, fictitious force is not a good term. How about apparent force instead?

 

[DA693]

Work in an inertial frame. You are standing on a carpet and someone pulls it forwards. There is a frictional force between your feet and the carpet so your feet move as well. There is no backwards force - you fall over because your feet are no longer under you.

Now work in the non-inertial frame where the carpet is stationary. In this frame, when the carpet is pulled we want it to stay stationary, but if it had a net force on it then it would accelerate. So we need to invoke an inertial force to explain why the carpet doesn't accelerate, and that inertial force accelerates everything backwards at exactly the right rate to cancel the acceleration of the carpet. But you have no other force on you so you do accelerate backwards. The important thing to realize is that inertial forces are just a way of explaining why an accelerating reference object is stationary. You can always choose a non-accelerating reference object and they disappear.

Work in an inertial frame. You are standing on a carpet and someone pulls it forwards. There is a frictional force between your feet and the carpet so your feet move as well. There is no backwards force - you fall over because your feet are no longer under you.

Now work in the non-inertial frame where the carpet is stationary. In this frame, when the carpet is pulled we want it to stay stationary, but if it had a net force on it then it would accelerate. So we need to invoke an inertial force to explain why the carpet doesn't accelerate, and that inertial force accelerates everything backwards at exactly the right rate to cancel the acceleration of the carpet. But you have no other force on you so you do accelerate backwards. The important thing to realize is that inertial forces are just a way of explaining why an accelerating reference object is stationary. You can always choose a non-accelerating reference object and they disappear.

Define "experience a force". Are you talking about your subjective interpretatio

Yr

Define "experience a force". Are you talking about your subjective interpretation?

 

[DA693]

I got that fictitious force are just for explanation when a person is in accelerating frame of reference or in non inertial frame. Alright but if you look into, then there are lots of scenario like in the video of Dr. hume ... A ball falling in an accelerating frame ok i got that... A puck sliding over the table , I got that too... But when am in the car or let's say I put a box over the table and I pulled a table or accelerates it towards myself then I am in which frame? If non inertial then it explains why box moves backwards if not then again WHY? If am in inertial frame then why it's moving backwards or it seems to be moving backwards because I am accelerating the table.

 

[jbriggs444]

I got that fictitious force are just for explanation when a person is in accelerating frame of reference or in non inertial frame. Alright but if you look into, then there are lots of scenario like in the video of Dr. hume ... A ball falling in an accelerating frame ok i got that... A puck sliding over the table , I got that too... But when am in the car or let's say I put a box over the table and I pulled a table or accelerates it towards myself then I am in which frame? If non inertial then it explains why box moves backwards if not then again WHY? If am in inertial frame then why it's moving backwards or it seems to be moving backwards because I am accelerating the table.

A frame is something that you write down with pencil and paper, not something that you climb into.

You can decide to treat your car as being at rest while objects accelerate across the seats. Or you can decide to treat your car as accelerating around a corner while objects on the seats continue to move in straight lines. Both descriptions of the situation work. Both lead to a correct prediction -- the objects and the door collide.

 

[Dale]

when am in the car or let's say I put a box over the table and I pulled a table or accelerates it towards myself then I am in which frame?

As @jbriggs444 said, you are in whichever frame you prefer to analyze. Or if you really like doing extra work you are in both frames. Or as many as you have patience to analyze the scenario in.

The reference frame is a part of the analysis, not the physics. The whole point of relativity is that you can do the analysis in any frame you like for whatever reason you want.

 

[DA693]

As @jbriggs444 said, you are in whichever frame you prefer to analyze. Or if you really like doing extra work you are in both frames. Or as many as you have patience to analyze the scenario in.

The reference frame is a part of the analysis, not the physics. The whole point of relativity is that you can do the analysis in any frame you like for whatever reason you want.

As @jbriggs444 said, you are in whichever frame you prefer to analyze. Or if you really like doing extra work you are in both frames. Or as many as you have patience to analyze the scenario in.

The reference frame is a part of the analysis, not the physics. The whole point of relativity is that you can do the analysis in any frame you like for whatever reason you want.

Sir i am actually trying to convince my brain that it is what it is but again I ask myself why that box move from stationary point of view or inertial. I have no answer or the answer is something which is not real...you all trying to explain and I am really thankful to all of you .But sorry to say I am not getting the actual point here.

 

[Dale]

why that box move from stationary point of view or inertial.

From the point of view of the inertial frame any acceleration of the box would be due to the friction with the table or whatever it is on. The box does not accelerate backwards.

 

[italicus]

@DA693
Be careful. First of all, a “reference frame “ isn’t a system of coordinates; a rf is made of physical bodies to which you refer you state: for example, in your room a rf can be represented by floor and walls. Only after you can choose coordinates, f.e. the distances from the walls. Math comes after physics.
Secondly: in your rf , inertial by hypothesis, there is a table at rest, and there is a bead at rest on the the table. Suppose there is no friction between bead and table, and push the table; then wrt to you the bead rests in its position, why? Because of the principle of inertia; note that there are two forces only on the bead,weight and table reaction, which equilibrate.
But wrt the table the bead accelerates backwards, why? Because the table is accelerating forward wrt to you , its acceleration is a vector A ; a little green man firmly sitting on the table says that a inertial force - mA arose, accelerating the bead backwards. The table is a rf for the little green man and the bead, that doesn’t move with rectilinear uniform motion wrt to YOU.

 

[vanhees71]

Sorry but this is an idea of yours. Having well in mind that the real force is the centripetal one , consider that , more o less, we live in a rotating reference frame, the one of the Earth, and say that the acceleration of gravity , in this rotating frame to which we belong, is less at the equator than at the poles. And the Coriolis force, acting on air masses which moves from the equator to North Pole , deviates them to the right, ok ? The same deviation applies to air masses that move from the NP to the equator : to the right . This causes hurricanes ( Katia...) with a counterclockwise sense of rotation , if seen from “above” , e.g. from meteorological satellites , no ? (I have simplified the real situation, because also centrifugal ( ops!) force is to be considered, and pressure variations too...but let’s be simple). So I think, ( and you can contest of course) that the description of these phenomena is as good from the inertial point of view as from the non inertial point of view.
But consider an example taken from an exercise of mechanics : a conical pendulum , made by a rod which rotates around a vertical axis on which its upper point of suspension is attached ; you can describe motion form the point of view of an external inertial observer, introducing real centripetal forces further to gravity; but you can also describe motion from the point of view of a fly, which is attached to the rod. And in the rotating frame you have to introduce the centrifugal force, don’t you?

I am doing a lot of efforts to write in a decent English !

Sure, if you want to treat phenomena on Earth where you have to take into account its rotation, it's convenient to use the non-inertial rest frame of the Earth (Foucault pendula, winds in meteorology and the like) and think in terms of inertial forces. That's what I said in one of the postings above too.

 

[A.T.]

a rf is made of physical bodies to which you refer you state:

You can use physical bodies to define a reference frame, but the reference frame is not "made of" physical bodies. For example: You can define a reference frame where some physical body undergoes a certain motion, but no physical body is actually at rest in that frame. So it does not make sense to say that the "reference frame is made of physical bodies"

 

[Dale]

a “reference frame “ isn’t a system of coordinates; a rf is made of physical bodies to which you refer you state

Please be aware that the professional scientific literature is inconsistent in this. While there are indeed authors who use the term as you describe, there are also authors who use it to mean either a coordinate system or a tetrad. So both usages are considered valid here on PF.

 

[italicus]

Please be aware that the professional scientific literature is inconsistent in this. While there are indeed authors who use the term as you describe, there are also authors who use it to mean either a coordinate system or a tetrad. So both usages are considered valid here on PF.

Well, I consider my reference authors, among which there is professor Elio Fabri, previous teacher at the University of Pisa. Ever heard? Look for him on the web.

 

[ergospherical]

 

[Ibix]

Well, I consider my reference authors, among which there is professor Elio Fabri, previous teacher at the University of Pisa. Ever heard? Look for him on the web.

Your construction is fine, but it is not the only one in wide use. Please can we not argue about which one is "better" again? It's literally an argument about whether we attach the lab to the reference frame or the reference frame to the lab.

 

[weirdoguy]

among which there is professor Elio Fabri, previous teacher at the University of Pisa. Ever heard? Look for him on the web.

Argument from authority is not a good argument. Again - stating that "a “reference frame “ isn’t a system of coordinates" is not consistent with literature, there are different definitions, and Elio Fabri won't change that.

 

[italicus]

Argument from authority is not a good argument. Again - stating that "a “reference frame “ isn’t a system of coordinates" is not consistent with literature, there are different definitions, and Elio Fabri won't change that.

I don’t agree with you. I have cited Fabri as my reference author, you are free to think differently. Which literature, please? Make some example!
It seems that I am fighting a battle against a lot of you, here , every moment…
I have simply given an example to DA693, to make him understand .
We should listen to ergo spherical: not again…

 

[Ibix]

don’t agree with you. I have cited Fabri as my reference author, you are free to think differently. Which literature, please?

Schutz defines a reference frame as a one form or a velocity in his text. Taylor and Wheeler simply define them by the properties of inertial bodies with respect to such a frame. The first is abstract, the second is closer to your definition

"Reference frame" is just words. Not everybody uses them in quite the same way and none is "righter" than the other. This is far from the only thing in physics where multiple conventions are in common use, so this shouldn't be any particular surprise.

 

[Dale]

View attachment 294480

Yes, we will not go down this path again in this thread. Further posts on this topic will be deleted as off-topic for this thread.

There are many different definitions of reference frame in the literature. If you are making a distinction, simply state the meaning you are using, but do not claim that other usages are incorrect.