I need to clarify my understanding of fictitious forces and our sensation of them. It is my understanding that fictitious forces are terms added when Newton's laws are resolved in accelerating reference frames.

And I often read that the force we expecrience upon sudden acceleration in a car say, is a fictitious force.

I am now going to present a thought experiment to consider this statement: I am standing on top of a tall, blacked-out, train by an open 'sunroof'. At the moment the train accelerates rapidly, I jump from the roof, through the 'sunroof' and I then see my self apparently accelerating backwards. A fictitious force would be required to describe my apparent acceleration from the reference frame of the train, but I do not think I would feel any force during this event. So what is the distinction between the sensed fictitious force in the above car scenario, and the unsensed one in the train scenario.

Is a fictitious force only felt when you are attached to the accelerating reference frame you are doing your physics in as it is only in this case that you actually accelerate and your inertia creates the force sensation?


1 Answer 1


Humans feel imbalanced forces. Either forces that change w.r.t or ones that are distributed unevenly. For instance, you feel a cars constant linear acceleration because the seat is pressing against your back as a point of contact, if your entire body accelerated with the car then you wouldn't feel anything.

The air resistance would be felt on your front (assuming your front is facing the direction you're moving) and thus you would absolutely be able to feel it.

A fictitious force is just a force that appears when you pick a bit of a shoddy co-ordinate frame, like a co-ordinate system on the inside of a car's wheel rather than outside on the road as it drives back. It makes it convenient in some ways, but all of a sudden you have a "fictitious force" acting outwards to account for the fact that you're no longer quite considering the rotation of the wheel. The same applies to Earth based co-ordinate systems when people choose to include a centrifugal term.

  • $\begingroup$ Thank you for your answer. It doesn't quite get at my lack of understadning I don't think, though then again I may have porrly expressed it. Queery 1: the seat pressing against your back is a real force isn't it, acting in the direction of the car's acceleration. This isn't the same as the sensation of being pushed backwards when a car accelerates forward which is what I was trying to describe. And it is this sensation that I believe people term the sensation of a fictitious force. $\endgroup$ Commented Sep 10, 2017 at 10:34
  • $\begingroup$ Queery 2: It is the distinction between the sensation of a fictitious force and our use of it to describe physics in an accelerating reference frame that confises me. In the example of the car accelerating and the feeling of being pushed backwards we would say (I believe) that the feeling is due to a fictitious force. Now, if a car weren't accelerating, but we were considering its motion in an accelerating reference frame we would have to add a fictitious force to explaion its apparent acceleration, but the person in the car certainly wouldn't experience this fictitious force. $\endgroup$ Commented Sep 10, 2017 at 10:37
  • $\begingroup$ just as the man jumping into the train wouldn't in my initial example. Hence my question of whether you have to be 'attached' to the reference frame to experience a fictitious force. I hope this all has a vague semblance of sense. $\endgroup$ Commented Sep 10, 2017 at 10:37

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