Questions about understanding circular motion & the forces involved

[nrqed]

Why would you ask such a question?

Because I am trying to understand your point of view. And I asked several questions before the one you are referring to. But you did not answer any of them and then you ask me why I would ask such a question. So I guess you prefer not to answer any of them and therefore there is no way to continue the discussion.

 

[jbriggs444]

I expect the student would have an understanding of Newton's law of motion and gravity from high school so I don't think you are going to get away with telling them that gravity disappears 100 miles above the earth. But it is precisely because there are no inertial pseudo forces required to make Newton's laws of motion work in a free-falling frame of reference that the principle of equivalence was postulated. It is based on Newtonian physics.

This amounts to a claim that free-falling reference frames are inertial (an eminently reasonable claim). But post #26 claims that they are non-inertial. You cannot have it both ways. Which is it?

 

[Andrew Mason]

This amounts to a claim that free-falling reference frames are inertial (an eminently reasonable claim).

It is not inertial if you apply Newtonian physics. But for purposes of applying Newton's laws of motion, it is locally equivalent to an inertial frame.

But post #26 claims that they are non-inertial. You cannot have it both ways. Which is it?

Why can it not be a non-inertial frame (as analysed Newtonian physics) that is locally indistinguishable from an inertial reference frame (as analysed in Newtonian physics)?

AM

 

[jbriggs444]

Why can it not be a non-inertial frame (as analysed Newtonian physics) that is locally indistinguishable from an inertial reference frame (as analysed in Newtonian physics)?

It can be. But you need to pick one and not keep jumping back and forth, willy nilly without notice as has been your habit in this thread.

 

[Andrew Mason]

It can be. But you need to pick one and not keep jumping back and forth, willy nilly without notice as has been your habit in this thread.

I have always been talking about the frame of reference of a body being accelerated only by gravity and the fact that inertial forces are not needed in order to apply Newton's laws. Read my posts 6, 10, 13, 15,18, 22, 31 for example. What other scenario do you think I have been referring to?

In post 4 the OP stated:

• "What I understand is: In the situation of in an accelerating car, if I put a scale between the seat and my back, a non-zero number would pop up. This is analogous to just standing on a scale right? You would have a "gravity" towards the seat. And this perceived gravity is just the normal force accelerating you. So in short, you'll always perceive gravity in the opposite direction of the net force on you?"

My comment in post 6 was to clarify that if the force accelerating you is gravity, you don't feel that push back into the seat.

• "Gravity is different than all other forces. There is no "inertial force effect" when the acceleration is provided by gravity. When an astronaut orbits the Earth he/she feels no centrifugal (outward) effect - no sensation of centrifugal force."

I am at a complete loss as to why you would think my comment would be controversial.

AM

 

[A.T.]

Why can it not be a non-inertial frame (as analysed Newtonian physics) that is locally indistinguishable from an inertial reference frame (as analysed in Newtonian physics)?

The key word here is "local", while Newtonian inertial frames extend to infinity. The observational local indistinguishability can be matched by different models. And if "Newtonian physics" includes the Newtonian interpretation of gravity, then the non-inertial free falling frame involves an inertial force that opposes gravity.