- [Instructor] What I'd like to do in this video
is talk about the notion of a frame of reference
and this is an introductory video.
In future videos, we'll go into a lot more depth,
but a frame of reference is really the idea
it's a point of view from which you are measuring things
and as we'll see, many of the quantities that
we might measure in physics, like velocity or displacement,
they could be different depending on our point of view,
depending on which frame of reference we are measuring from
and to get this an intuitive grasp of it,
I'm going to draw the exact same scenario
from three different frames of reference.
There's the first one,
this is the second one,
and this is the third one.
So in this first frame of reference, this first scenario,
we're gonna talk about the frame of reference of the ground.
So if you are a stationary observer on the ground,
so you could imagine this is you here
and you're the person doing the measuring
of let's say we want to measure velocities.
So from your point of view,
since you're stationary relative to the ground,
what does the ground's velocity look like?
Well, you and the ground appear to be stationary,
appear to not be moving.
Now, what if you take out your instruments
for measuring velocity or you see a change in,
you see what the displacement is over a certain time
for the plane and the car
and you're able to see okay, look,
this plane has a velocity to the right
of 250 meters per second,
250 meters per second,
and let's say this car
that is moving quite fast by car standards
is moving to the left at 50 meters per second.
So this should be 1/5 of that length.
So let me draw a little bit.
So let's say this is moving to the left
at 50 meters per second.
Well, none of this seems crazy.
You might be able to go outside next to the highway and see,
well 50 meters per second would be quite fast,
but anyway, you could observe this type of thing happening
and it seems completely reasonably.
But what if we were to change our frame of reference,
change the point of view from which we are measuring things.
So let's take the frame of reference of the car.
Well in this frame of reference,
let's say you're sitting in this car
and I don't recommend you doing this while driving,
let's say someone else is driving
or it's an autonomous vehicle of some kind
and you take out your physics instruments
with the stopwatch and you see what the displacement is
of the ground and the plane over, say, a second
and you are able to first say, from your point of view,
you're like well the car is stationary,
the car has a velocity of zero,
the car is stationary
and from your point of view,
you would actually measure the ground to be moving.
You would see the trees move past you to the right,
or behind you if you're moving to the left,
and so from your point of view
the ground would actually look like it's moving
in this direction, in that direction,
at 50 meters per second.
It would look like it's moving behind you
or in this case, the way we're looking at it,
to the right at 50 meters per second.
Now, what would the plane look like?
Well, the plane not only would it look like
it's moving to the right at 250 meters per second,
not only would it be just that 250 meters per second,
but relative to you it'd look like it's going even faster
'cause you're going past it,
you are going to the left from the stationary,
from the ground's point of view at 50 meters per second.
So the plane, to you,
is gonna look like it's going 250 plus 50 meters per second.
So the vector would look like this
and so it would look like it's going to the right at 300,
let me write that in that orange color,
at 300 meters per second.
Now, what about from the point of view of the plane?
What if we're talking about the plane's frame of reference?
Why don't you pause this video
and think about what the velocities would be
of the plane, the car, and the ground
from the plane's point of view.
All right, now let's work through this together.
So now, we're sitting in the plane
and once again we shouldn't be flying the plane,
we're letting someone else do that,
we have our physics instruments out
and we're trying to measure the velocities
of these other things from my frame of reference.
Well, the plane, first of all,
is going to appear to be stationary
and that might seem counterintuitive,
but if you've ever sat in a plane,
especially when there's no turbulence
and the plane is already at altitude
and it's not taking off or landing,
oftentimes if you close your eyes
you don't know if you are moving.
In fact, if you close all of the windows,
it feels like you are in a stationary object,
that you might as well be in a house.
So from the plane's point of view, you feel like,
or from your point of view in the plane,
it feels like the plane is stationary.
Now the ground, however,
looks like it's moving quite quickly.
It'll look like it's moving past you
at 250 meters per second.
Whoops, try and draw a straight line.
At 200...
At 250.
Sometimes my tools act funny.
So, at 250 meters per second to the left.
And the car, well it's moving to the left even faster.
It's going to be moving to the left
50 meters per second faster than the ground is.
So the car is gonna look not like it's just going
50 meters per second,
it's gonna look like it's going 50 meters
plus another 250 meters per second
for a total of 300 meters per second to the left.
So this gives you an appreciation
for what frames of references are.
You can view it for this introductory video
as a point of view
from which you're making your measurements.
Now, it's tempting for a lot of folks to say
well there must be one correct frame of reference
and a lot of times in our everyday world
you might say well this, maybe this is the correct
frame of reference and these are just,
we're just imagining this or this is just the mistake
and the reason why we do that is because
we're using the frame of reference
of this big, giant thing called the earth,
but it actually turns out
that none of these frames of reference
are more valid than the other ones,
that they are all equivalent,
that they are all valid frames of reference,
not, I shouldn't say they're equivalent,
we're obviously getting different measurements from them,
but they're all, from a physics point of view,
equally valid frames of reference.