Investigating Fictitious Forces in Car Acceleration

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Discussion Overview

The discussion revolves around the behavior of fictitious forces in a car that is accelerating, particularly in relation to how smoke or a helium balloon would behave in such a scenario. Participants explore the implications of non-inertial reference frames and the interaction of various accelerations, including gravitational and fictitious forces, in both straight-line and circular motion contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that smoke behaves similarly to a helium balloon in an accelerating car, questioning whether the effects of gravity can be ignored when acceleration is significantly high.
  • Another participant argues that gravity cannot be neglected, emphasizing that the resultant acceleration is the vector sum of all forces acting on the system.
  • Some participants discuss the sensation of being pushed back in a seat during acceleration, noting that the force felt is due to the seat rather than a direct outward force.
  • There is a debate about whether the addition of vectors in a non-inertial frame is more complex than simple vector addition, with some asserting that fictitious forces still behave as vectors influencing motion.
  • One participant expresses confusion about the responses received, indicating a lack of clarity in the explanations provided by others.
  • A later reply suggests that inertial forces are defined in such a way that the addition of vectors remains straightforward, despite the presence of fictitious forces.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the effects of gravity can be ignored in high-acceleration scenarios, and there are differing views on the complexity of vector addition in non-inertial frames. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some participants express uncertainty regarding the implications of combining different accelerations and the role of fictitious forces, indicating potential limitations in their understanding of the topic.

Quadrat
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Hi,

I was sitting and thinking of the case where a smoke is rising from say, some incense inside of a car that is accelerating.
I thought that it must act the same way as a helium balloon would have. When you turn left, the smoke tends to go left, when you accelerate in a straight line the smoke goes forward etc.

I was thinking of the extreme cases where the centripetal acceleration or the acceleration in a straight line is much greater than the acceleration due to gravity if one could treat the cases as if there's no component downwards. Am I thinking of this in the wrong way? It might be more complex than just adding the two accelerations to get a resulting acceleration which leads to a centrifugal force and makes the air and smoke react in other ways than if it were inertial. Let's say in the case that the car has a constant acceleration g both forward and downward.
 
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Not sure what your question is. Can you ignore gravity as a force if your acceleration is high enough? No.

It's still the sum of the two vectors. even if one is much larger than the other.
 
DaveC426913 said:
Not sure what your question is. Can you ignore gravity as a force if your acceleration is high enough? No.

It's still the sum of the two vectors. even if one is much larger than the other.

Take the case where you are on an amusement ride going fast in a circular motion and you're experiencing the centrifugal force. I'm not talking about the philosophical aspects of it. It's not like you really care for or think of the acceleration due to gravity when you are pushed back into your seat immensely. And in the case where the acceleration due to gravity is so small in comparision to the linear or centripetal acceleration that it must be OK to neglect it altogether and think of in which way the smoke or the helium balloon will rise.
 
Quadrat said:
Take the case where you are on an amusement ride going fast in a circular motion and you're experiencing the centrifugal force. I'm not talking about the philosophical aspects of it. It's not like you really care for or think of the acceleration due to gravity when you are pushed back into your seat immensely. And in the case where the acceleration due to gravity is so small in comparision to the linear or centripetal acceleration that it must be OK to neglect it altogether and think of in which way the smoke or the helium balloon will rise.

If you were being pushed back into your seat, you would feel a force on your chest. You don't. The force you feel is the seat pushing your back inwards.
 
PeroK said:
If you were being pushed back into your seat, you would feel a force on your chest. You don't. The force you feel is the seat pushing your back inwards.

One get's the sensation of being pushed outwards in an accelerating frame of reference however. But I don't get how your reply was helping me answering the actual question.
 
Quadrat said:
One get's the sensation of being pushed outwards in an accelerating frame of reference however. But I don't get how your reply was helping me answering the actual question.

Well, I'm sorry to say, I don't really understand either of your posts. Acceleration is a vector, hence you use vector addition when you have multiple accelerations. Where's the confusion?
 
PeroK said:
Well, I'm sorry to say, I don't really understand either of your posts. Acceleration is a vector, hence you use vector addition when you have multiple accelerations. Where's the confusion?

Since it's viewed from a non-inertial reference frame I was wondering if it's more complicated than just adding the two vectors or not.
 
Quadrat said:
Since it's viewed from a non-inertial reference frame I was wondering if it's more complicated than just adding the two vectors or not.

Why should it be? And, what else would you do? Fictitious forces are still vectors influencing motion in the usual way. The difference is that you don't acually feel them (other than psychologically).
 
PeroK said:
Why should it be? And, what else would you do? Fictitious forces are still vectors influencing motion in the usual way. The difference is that you don't acually feel them (other than psychologically).

Sigh. That was why I was asking in the first place. If I had all the answers then I wouldn't be asking any questions - would I?
 
  • #10
Quadrat said:
Sigh. That was why I was asking in the first place. If I had all the answers then I wouldn't be asking any questions - would I?

Your question seems to be: if one vector is much bigger than another, then perhaps you can't just add them together, but do something complex with them instead?

I'm not sure how to answer that.
 
  • #11
Quadrat said:
Sigh. That was why I was asking in the first place. If I had all the answers then I wouldn't be asking any questions - would I?
So, did post #2 answer it?
 
  • #12
Quadrat said:
Since it's viewed from a non-inertial reference frame I was wondering if it's more complicated than just adding the two vectors or not.
Inertial forces were defined such that it's not "more complicated".
 
  • #13
DaveC426913 said:
So, did post #2 answer it?
Yes it did! I wasn't more to it than I imagined. :) The thread was more of a "did I miss something important"?
 

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