# Fictitious forces

1. Sep 13, 2015

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.

2. Sep 13, 2015

### DaveC426913

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.

3. Sep 14, 2015

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.

4. Sep 14, 2015

### PeroK

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.

5. Sep 14, 2015

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.

6. Sep 14, 2015

### PeroK

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?

7. Sep 14, 2015

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.

8. Sep 14, 2015

### PeroK

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).

9. Sep 14, 2015

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. Sep 14, 2015

### PeroK

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. Sep 14, 2015

### DaveC426913

So, did post #2 answer it?

12. Sep 14, 2015

### A.T.

Inertial forces were defined such that it's not "more complicated".

13. Sep 15, 2015

Yes it did! I wasn't more to it than I imagined. :) The thread was more of a "did I miss something important"?