All right, we've been talking about frames of reference and

I have a couple more things to do with them.

First, I want to introduce the term inertial frames of reference.

Where does that come from?

You've probably heard the term inertia, it actually comes from the Latin word inert,

which can mean things like lazy.

And how does that apply here?

Well, it actually goes back to Newton's, Isaac Newton's First Law of Motion,

where essentially, he stated that any object that is at rest or in motion,

in constant velocity motion, it's inertia is resistance to it's change of state.

In other words, if it's at rest and you need to put it into motion.

The more inertia it has, the harder it is to change it from rest to some velocity.

Or if it's traveling with constant velocity, the difficulty you have in terms

of actually changing it to a different velocity.

So that's where the term inertia comes from, at least in physics terms.

So we talk about an inertial term of reference.

For our purposes, what this will mean,

as we've been talking about Alice's frame of reference, Bob's frame of reference.

An inertial frame of reference is a reference frame that's either at rest or

in constant velocity motion, that there's no acceleration or

anything else going on like that.

And the special theory of relativity deals with inertial frames of reference.

Later on Einstein, about ten years after the Special Theory of Relativity

was able to put the final touches on his so called General Theory of Relativity

that dealt with non-inertial frames of reference.

And that could be something where you're accelerating.

But if you imagine, think about just being in a car.

Even as you go around a curve you're sort of thrown to the outside.

That's a non inertial frame of reference,

there are some acceleration involved in that circular motion or that turning.

If you're going straight along in a car at constant velocity,

you don't feel yourself push back in the seat or thrown to the side.

So that's one piece of evidence that you're in a non-intertial

frame of reference if you feel that acceleration,

if you feel yourself being pushed back in the seat.

Or if you feel yourself going around a curve, thrown to the side, and

so on and so forth.

Or actually, a gravitational field is a non-intertial frame of reference.

But that's beyond our course, that's the general theory of relativity,

as I mentioned.

So In this course, anytime we talk about a frame of reference

we will assume it is an inertial frame of reference, unless otherwise specified.

So again, inertial frame of reference just means it's either at rest or

in constant velocity motion.

So we talk about, one frame of reference,

maybe Alice's versus Bob's frame of reference and

we know they're inertial if the motion between them is just constant velocity.

There's no acceleration going on.

So inertial frames of reference.

Other thing we want to mention here is combining velocities.

Now, everything we're doing here is actually before,

pre Einstein, so we haven't gotten into that yet.

So this is combining velocities in what we would say the low velocity limit when

velocity are low enough not near the speed of light then everything we've said so

far applies.

Again we'll see how it changes when we get to Einstein's principles and

his two key principles we'll be getting to, and

then the implications of those as he worked them out.

But, so combining velocities, let's just do a little example here.

Let's say we've been talking about Bob and Alice.

Let's go back to Bob's spaceship here.

So here's his Spaceship, here with his cockpit.

And he's going, Alice is observing him.

So maybe he is going by with some velocity v.

Now remember, for Bob, as far as he's concerned, he's not moving.

He has his whole grid or lattice of clocks all synchronized with each other.

And so he can measure events in his frame of reference.

He's an inertial frame of reference.

Because as far as he's concerned, he is at rest there.

And he can tell that because he doesn't feel any acceleration going on.

So he has his lattice of clocks there.

And we're going to imagine here that he has a spaceship such that

if something goes wrong, he has an escape pod.

And so we'll just put the, not a very fancy spaceship here,

but we'll assume the escape pod is just that spherical object there.

And so if necessary he can get in that escape pod or

put something else in that escape pod and have it shoot off from his spaceship.

And so we're going to assume that if he does that

it will shoot off from his spaceship at a velocity v.

So know what's going on here.

From Alice's perspective, from her frame of reference, Bob is traveling by

at velocity v, but he's also shooting off his escape pod at velocity v.

So let's backup to Bob's perspective.

From Bob's perspective he's just sitting there in his cockpit.

He actually maybe sees Alice going, receding in the background from him.

But he assumes that she is in motion, not himself.

He assumes he's at rest.

He can see all his lattice of imaginary clocks, as it were.

And so he shoots off his escape pod at velocity v.

So he sees that escape pod going away from him at velocity v.

And he can measure that on his clocks and his measuring apparatus there.