# What are different inertial frame of references?

I'm 11 and I'm trying understand what Special Relativity is about. How many different inertial frames of reference do we have and what are they?? I know what it means.

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Orodruin
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Inertial frames are essentially coordinate systems in space-time, i.e., ways of assigning time and space coordinates to different events (or in other words, specifying when and where something occurs).

Inertial frames are chosen such that the laws of physics hold in their simplest form and all inertial frames are in constant uniform motion with respect to each other. There is an infinite number of different inertial frames and each of them is perfectly valid for describing physics.

HallsofIvy
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You are in an "inertial frame of reference" as long as there are exterior force on you and you are moving, relative to some point, at a constant velocity. Any two frames of reference, moving at constant velocity relative to each other, are "different inertial frames of reference".

phinds
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And just to add to the above, you are NOT in an inertial frame when you are accelerating.

SO ... let's say you are on a train and it is pulling out of the station and picking up speed. This is not an inertial frame of reference. Then the train settles down to a constant speed relative to the tracks that it is on. Now you are in an inertial frame of reference. You can assign the coordinates of that frame to be anywhere on the train.

You are in an "inertial frame of reference" as long as there are exterior force on you and you are moving, relative to some point, at a constant velocity. Any two frames of reference, moving at constant velocity relative to each other, are "different inertial frames of reference".
I am afraid you are missing something in your formulation. Or maybe you have too much.
What force are you thinking about?

HallsofIvy
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Yes, thank you! I missed the word "no"!

"You are in an "inertial frame of reference" as long as there are NO exterior forces on you and you are moving, relative to some point, at a constant velocity. Any two frames of reference, moving at constant velocity relative to each other, are "different inertial frames of reference".

I don't think the force requirement is necessary, as stated. You can have many forces acting of you and still moving with constant velocity.
It is the net force which has to be zero in order to have constant velocity in an inertial frame.

I don't think the force requirement is necessary, as stated. You can have many forces acting of you and still moving with constant velocity.
It is the net force which has to be zero in order to have constant velocity in an inertial frame.
Probably what he meant.

Mister T
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I'm 11 and I'm trying understand what Special Relativity is about. How many different inertial frames of reference do we have and what are they?? I know what it means.
If you want to understand inertial frames of reference I suggest you look at what's called Galilean relativity. You can easily find his account of being in the hold of a ship that moves with constant speed in a straight line.

That's the basis for what Einstein called his first postulate.

Many people skip this step and then get confused when they try to understand special relativity and the effect that the second postulate has on a theory of relativity.

• vanhees71