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- [Instructor] What I'd
like to do in this video

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is talk about the notion
of a frame of reference

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and this is an introductory video.

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In future videos, we'll
go into a lot more depth,

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but a frame of reference
is really the idea

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it's a point of view from
which you are measuring things

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and as we'll see, many
of the quantities that

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we might measure in physics,
like velocity or displacement,

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they could be different
depending on our point of view,

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depending on which frame of
reference we are measuring from

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and to get this an intuitive grasp of it,

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I'm going to draw the exact same scenario

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from three different frames of reference.

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There's the first one,

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this is the second one,

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and this is the third one.

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So in this first frame of
reference, this first scenario,

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we're gonna talk about the frame
of reference of the ground.

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So if you are a stationary
observer on the ground,

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so you could imagine this is you here

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and you're the person doing the measuring

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of let's say we want
to measure velocities.

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So from your point of view,

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since you're stationary
relative to the ground,

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what does the ground's velocity look like?

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Well, you and the ground
appear to be stationary,

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appear to not be moving.

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Now, what if you take out your instruments

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for measuring velocity
or you see a change in,

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you see what the displacement
is over a certain time

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for the plane and the car

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and you're able to see okay, look,

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this plane has a velocity to the right

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of 250 meters per second,

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250 meters per second,

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and let's say this car

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that is moving quite fast by car standards

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is moving to the left
at 50 meters per second.

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So this should be 1/5 of that length.

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So let me draw a little bit.

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So let's say this is moving to the left

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at 50 meters per second.

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Well, none of this seems crazy.

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You might be able to go outside
next to the highway and see,

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well 50 meters per second
would be quite fast,

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but anyway, you could observe
this type of thing happening

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and it seems completely reasonably.

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But what if we were to change
our frame of reference,

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change the point of view from
which we are measuring things.

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So let's take the frame
of reference of the car.

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Well in this frame of reference,

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let's say you're sitting in this car

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and I don't recommend you
doing this while driving,

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let's say someone else is driving

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or it's an autonomous vehicle of some kind

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and you take out your physics instruments

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with the stopwatch and you
see what the displacement is

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of the ground and the
plane over, say, a second

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and you are able to first
say, from your point of view,

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you're like well the car is stationary,

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the car has a velocity of zero,

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the car is stationary

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and from your point of view,

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you would actually measure
the ground to be moving.

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You would see the trees
move past you to the right,

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or behind you if you're
moving to the left,

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and so from your point of view

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the ground would actually
look like it's moving

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in this direction, in that direction,

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at 50 meters per second.

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It would look like it's moving behind you

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or in this case, the
way we're looking at it,

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to the right at 50 meters per second.

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Now, what would the plane look like?

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Well, the plane not
only would it look like

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it's moving to the right
at 250 meters per second,

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not only would it be just
that 250 meters per second,

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but relative to you it'd look
like it's going even faster

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'cause you're going past it,

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you are going to the
left from the stationary,

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from the ground's point of
view at 50 meters per second.

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So the plane, to you,

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is gonna look like it's going
250 plus 50 meters per second.

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So the vector would look like this

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and so it would look like it's
going to the right at 300,

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let me write that in that orange color,

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at 300 meters per second.

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Now, what about from the
point of view of the plane?

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What if we're talking about
the plane's frame of reference?

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Why don't you pause this video

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and think about what
the velocities would be

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of the plane, the car, and the ground

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from the plane's point of view.

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All right, now let's work
through this together.

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So now, we're sitting in the plane

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and once again we shouldn't
be flying the plane,

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we're letting someone else do that,

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we have our physics instruments out

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and we're trying to measure the velocities

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of these other things from
my frame of reference.

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Well, the plane, first of all,

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is going to appear to be stationary

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and that might seem counterintuitive,

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but if you've ever sat in a plane,

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especially when there's no turbulence

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and the plane is already at altitude

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and it's not taking off or landing,

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oftentimes if you close your eyes

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you don't know if you are moving.

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In fact, if you close all of the windows,

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it feels like you are
in a stationary object,

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that you might as well be in a house.

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So from the plane's point
of view, you feel like,

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or from your point of view in the plane,

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it feels like the plane is stationary.

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Now the ground, however,

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looks like it's moving quite quickly.

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It'll look like it's moving past you

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at 250 meters per second.

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Whoops, try and draw a straight line.

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At 200...

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At 250.

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Sometimes my tools act funny.

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So, at 250 meters per second to the left.

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And the car, well it's moving
to the left even faster.

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It's going to be moving to the left

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50 meters per second
faster than the ground is.

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So the car is gonna look
not like it's just going

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50 meters per second,

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it's gonna look like it's going 50 meters

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plus another 250 meters per second

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for a total of 300 meters
per second to the left.

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So this gives you an appreciation

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for what frames of references are.

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You can view it for
this introductory video

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as a point of view

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from which you're making
your measurements.

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Now, it's tempting for
a lot of folks to say

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well there must be one
correct frame of reference

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and a lot of times in our everyday world

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you might say well this,
maybe this is the correct

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frame of reference and these are just,

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we're just imagining this
or this is just the mistake

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and the reason why we do that is because

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we're using the frame of reference

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of this big, giant thing called the earth,

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but it actually turns out

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that none of these frames of reference

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are more valid than the other ones,

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that they are all equivalent,

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that they are all valid
frames of reference,

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not, I shouldn't say they're equivalent,

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we're obviously getting
different measurements from them,

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but they're all, from a
physics point of view,

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equally valid frames of reference.