Question

Non inertial frame of reference

Now we all know that accelerated frame of reference is one where newton's laws don't hold good (and u need to consider the inertial force to make it work)

My question is, how do you know (in practice) if u are in inertial or non inertial frame of reference. Again i always thought the answer is simple, if u are in uniform motion, u don't feel anything, but if accelerating u feel the inertial push

but imagine u were accelerating towards a planet due to gravity and nothing else. Would you feel that inertial push? I don't think so

Answer 1

Cause if it did, we wouldn't feel weightless in free fall. There is some fundamental flaw in my understanding here

i feel that in this case, each part of my body experiences the acceleration at the same time, and so i don't feel my inertial kicking in (unlike when u are in a bus which suddenly brakes or starts)

so if i am right, its not possible to distinguish between accelerated and inertial frames of references ? when u are under the influence of an invisible force field? or am i missing something?

Answer 2

@AllTehMaffs @Vincent-Lyon.Fr @JamesJ

Answer 3

I do not know what the 'standard' answer to that question is.
Accelerated motion and gravity are undistinguishable.
Only the knowledge of a nearby mass can allow you choose what the status of your frame is.

An elevator in free-fall can either be considered as an inertial frame of reference if you ignore Earth's presence, or as a non inertial frame of reference, if you take Earth's gravity into account and you must add an inertial force to justify why objects are floating in the cabin.

Answer 4

well i was just thinking about the fact that, velocity is relative and ACCELERATION IS ABSOLUTE.. right?

but if acceleration is absolute, in the case that we are refereing to , if we can't distinguish if we are being accelerated or not.. how can we say whether or not something is accelerating? :P


so imagine, i am racing towards this planet (and nothing else in the universe)

how do i know its ME who is accelerating towards the planet, and not the planet racing towards me?

Answer 5

"or as a non inertial frame of reference, if you take Earth's gravity into account"
in that.. how do i know its ME who is accelerating, and not the earth accelerating towards me?

Answer 6

"well i was just thinking about the fact that, velocity is relative and ACCELERATION IS ABSOLUTE.. right?"

True if you do not play with gravitational fields, like cancelling them by a free-fall motion.

The real answer lies in General Relativity. In this sense, Newton's vacuum tube is the first ever experiment of General Relativity ;-)

Answer 7

darn.. why do we have to go to general relativity,

i know a lil about it, it considers gravity not as a force, but a mere distortion of space time.. somehow that magically solves the problem eh? :P

Answer 8

Yes, Einstein shows cannot distinguish between acceleration and gravitation.
However, you can see whether or not you or the mass is accelerating. E.., red shift.

Answer 9

what if there is NO source of light?

i don't want distinction between a and g,
i want distinction between gravitation and NO ACCELERATION :P

Answer 10

You want an experiment you can do in a closed environment that can distinguish between no acceleration and accelerating under gravitation, in free fall? Looking outside the box, you can use any method that lets you compare your speed with other objects, as your relative velocities will change over time. Stuck in the box, I'm not sure what would work.

Answer 11

i am fine with looking outside the box.. even looking outside the box, i would see other things accelerating towards me right?.. how do i know its me who is accelerating?

Answer 12

As you look around, you will see objects ahead of you accelerating toward you and objects behind you accelerating away, while objects to either side will be moving
faster and faster sideways. The simplest explanation will be that it is you who is accelerating uniformly rather than all of them who are accelerating in ways to fool you!

Like adding more epicycles to geocentric model of universe. Too complex. Occam's razor.

Answer 13

Isn't it the same if u were moving with uniform speed..?

and still we say speed is relative.. ? so instead of accelerating, if you were moving with uniform speed and u looked outside and saw things going away on one side, and coming towards you.. we would still say that they are moving relative to you right? and do the physics..

why not here then? :D

Answer 14

Yes, Mashy, nice Question and we do not feel it because all parts of our body feels the same acceleration unlike a bus. But this is in case of earth becuase of atmosphere, we reach a terminal velocity and don't and hence as we are not accelerating, we will not feel anything but in another planet, you will surely have the feeling of continuosly being pulled towards you.

Answer 15

yea well m not talking about atm and terminal speed :P..