# I Question based on non inertial frame of reference

1. Oct 6, 2016

### parshyaa

Suppose I am observing a object in a noninertial frame from a noninertial frame , then what will happen to the forces acting on a object with respect to both the frames, frame of reference (FOR) moving uniform with inertial FOR are themselves inertial frame , does it follow the same with a FOR moving uniform with a noninertial FOR is itself a non inertial frame.

2. Oct 6, 2016

### drvrm

its best to frame an example and convert non-inertial observers to inertial ones by using 'fictitious forces' generated out of 'non-inertial motion' and then frame a question/problem.

3. Oct 6, 2016

### parshyaa

Suppose a person A moving on a bike which is accelarating ,observes another person B moving on another bike which is also accelarting(with the same accelaration and in same direction) , when person A ask a person B , hey there are you feeling a force , what will he say , here a(AB){accelaration of person A w.r.t B} is 0 , and what will happen if a person standing on ground says , hey there are you feeling a force ,in this case a≠0, then what will be his answer ? , yes you are right we can add fictitious force and make a non-inertial frame of a person A as a inertial frame and as second person is moving uniform w.r.t person A(as a(AB) =0) his frame of reference will also become inertial frame. You mean a noninertial frame moving uniform with a non inertial frame is same as inertial frame moving w.r.t inertial frame after adding fictitious force to any frame .

4. Oct 6, 2016

### A.T.

Fictitious forces cannot be "felt", so the answer about feeling a force is frame independent: Yes, B feels a force forward from the bike. In A's rest frame there is also a fictitious force backwards on B, so coordinate acceleration of B is 0 in that frame.

5. Oct 6, 2016

### parshyaa

Ok then he will give same answer to both the person that yes I am feeling a fictisious force(suppose he is a physicist) , how can you say that force is independent of F.O.R , I belive that yes it is independent of F.O.R, but how can you show this , as F depends on a , and a depends on F.O.R, therefore we can say that F depends on frame of reference , I know its wrong but how can you show that F is independent of F.O.R

6. Oct 6, 2016

### A.T.

The real (interaction) forces are frame independent. The fictitious (inertial) forces are frame dependent.

7. Oct 6, 2016

### parshyaa

[QUOhow c"A.T., post: 5585983, member: 85613"]The real (interaction) forces are frame independent. The fictitious (inertial) forces are frame dependent.[/QUOTE]
Ok, how can you say that real forces are frame independent , there is no gravitational force in rocket in deep space ,but it is there on earth , it means it depends on FOR

8. Oct 6, 2016

### A.T.

Per definition.

A frame of reference is not a location.

9. Oct 6, 2016

### Staff: Mentor

To echo what @A.T. said, real forces are frame independent. They represent some interaction between two objects according to the third law. Fictitious forces only exist in non-inertial frames, and they do not have a corresponding 3rd law pair, also fictitious forces are always proportional to mass and cannot be detected by accelerometers.

Don't worry about gravity at this point. It is complicated, and handled differently by different theories. Stick to bicycles or cars or rotating disks, for now.

10. Oct 6, 2016

### parshyaa

Thanks A.T. and dale

11. Oct 6, 2016

### Khashishi

To answer parshyaa's original question, yes. If frame A is non-inertial, and frame B is moving with uniform velocity with respect to frame A, then frame B is also non-inertial.

Proof is by reversing the statement. Suppose frame B is inertial. If B is moving with uniform velocity wrt A, then A is moving with uniform velocity wrt B.
Uniform velocity is an inertial motion, so it follows that A is inertial. So, for A to be non-inertial, B cannot be inertial.

12. Oct 6, 2016

### parshyaa

Yo I got it , we can also prove it by taking two frame of reference S and S'(S is a non inertial and S' is moving uniform with frame S) , suppose there is a particle 'P' in frame S' , then aPS' =aPS - aS'S, as aS'S is zero , therefore aPS' = aPS , as frame one is non inertial and aPS do not follow newtons first law , therefore aPS' also not follows newtons first law , therefore it is also a non inertial frame, now question arised what if person B is moving nonuniformly with respect to A

13. Oct 9, 2016

### parshyaa

• A frame moving uniform with a inertial frame is itself a inertial frame.
• A frame moving uniform with a non inertial frame is itself a non inertial frame.
• A frame accelerating w.r.t inertial frame is a non inertial frame of reference.
Now question arises, what will happen to a frame accelerating w.r.t a non inertial frame, will it be noninertial or inertial

14. Oct 9, 2016

### parshyaa

• Newton's first and second law acts in both inertial and non inertial frame of reference if we add pseudo forces, but newton's third law acts only in inertial frame.
Is this statement correct?

15. Oct 9, 2016

### Staff: Mentor

Yes.

16. Oct 9, 2016

### parshyaa

Yo mean it will be non-inertial, whats the reason , is it , if we make the first non inertial frame as inertial by adding psuedo forces, and since second frame is accelarting, then as frame accelarting with inertial frame are noninertial, therefore frame accelarting w.r.t non inertial frame will become non inertial

17. Oct 9, 2016

### Staff: Mentor

No, I mean "yes it will be non inertial or inertial."

There is not enough information given.

Last edited: Oct 9, 2016
18. Oct 9, 2016

### Staff: Mentor

I would not say it this way. The third law applies to real forces, both in inertial and in non inertial frames. Fictitious forces violate the third law, and fictitious forces show up only in non inertial frames. But real forces show up in all frames and obey Newton's laws.