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Let's say I have
something moving
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with a constant velocity
of five meters per second.
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And we're just assuming
it's moving to the right,
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just to give us a direction,
because this is a vector
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quantity, so it's moving in
that direction right over there.
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And let me plot its
velocity against time.
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So this is my velocity.
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So I'm actually
going to only plot
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the magnitude of the
velocity, and you
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can specify that like this.
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So this is the magnitude
of the velocity.
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And then on this axis
I'm going to plot time.
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So we have a constant velocity
of five meters per second.
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So its magnitude is
five meters per second.
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And it's constant.
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It's not changing.
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As the seconds tick away the
velocity does not change.
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So it's just moving
five meters per second.
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Now, my question to you
is how far does this thing
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travel after five seconds?
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So after five seconds-- so
this is one second, two second,
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three seconds, four seconds,
five seconds, right over here.
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So how far did this thing
travel after five seconds?
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Well, we could think
about it two ways.
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One, we know that velocity
is equal to displacement over
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change in time.
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And displacement is
just change in position
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over change in time.
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Or another way to
think about it--
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If you multiply
both sides by change
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in time-- you get velocity
times change in time,
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is equal to displacement.
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So what was of the
displacement over here?
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Well, I know what
the velocity is--
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it's five meters per second.
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That's the velocity,
let me color-code this.
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That is the velocity.
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And we know what the change in
time is, it is five seconds.
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And so you get the seconds
cancel out the seconds,
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you get five times five-- 25
meters-- is equal to 25 meters.
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And that's pretty
straightforward.
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But the slightly more
interesting thing
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is that's exactly the area under
this rectangle right over here.
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What I'm going to show
you in this video,
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that is in general,
if you plot velocity,
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the magnitude of velocity.
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So you could say
speed to versus time.
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Or let me just stay
with the magnitude
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of the velocity versus time.
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The area under
that curve is going
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to be the distance traveled,
because, or the displacement.
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Because displacement is
just the velocity times
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the change in time.
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So if you just take out a
rectangle right over there.
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So let me draw a
slightly different one
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where the velocity is changing.
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So let me draw a situation
where you have a constant
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acceleration .
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The acceleration
over here is going
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to be one meter per
second, per second.
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So one meter per
second, squared.
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And let me draw the
same type of graph,
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although this is going to
look a little different now.
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So this is my velocity axis.
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I'll give myself a
little bit more space.
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So this is my velocity axis.
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I'm just going to draw the
magnitude of the velocity,
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and this right over
here is my time axis.
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So this is time.
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And let me mark
some stuff off here.
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So one, two, three, four, five,
six, seven, eight, nine, ten.
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And one, two, three, four, five,
six, seven, eight, nine, ten.
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And the magnitude
of velocity is going
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to be measured in
meters per second.
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And the time is going to
be measured in seconds.
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So my initial
velocity, or I could
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say the magnitude of
my initial velocity--
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so just my initial
speed, you could say,
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this is just a
fancy way of saying
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my initial speed is zero.
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So my initial speed is zero.
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So after one second
what's going to happen?
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After one second I'm going
one meter per second faster.
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So now I'm going one
meter per second.
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After two seconds,
whats happened?
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Well now I'm going another meter
per second faster than that.
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After another second--
if I go forward in time,
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if change in time is
one second, then I'm
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going a second faster than that.
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And if you remember the idea of
the slope from your algebra one
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class, that's exactly
what the acceleration
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is in this diagram
right over here.
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The acceleration, we
know that acceleration
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is equal to change in
velocity over change in time.
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Over here change in time
is along the x-axis.
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So this right over here
is a change in time.
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And this right over here
is a change in velocity.
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When we plot velocity or
the magnitude of velocity
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relative to time, the slope of
that line is the acceleration.
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And since we're assuming the
acceleration is constant,
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we have a constant slope.
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So we have just a line here.
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We don't have a curve.
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Now what I want to do is
think about a situation.
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Let's say that we accelerate it
one meter per second squared.
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And we do it for--
so the change in time
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is going to be five seconds.
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And my question to you is
how far have we traveled?
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Which is a slightly more
interesting question
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than what we've
been asking so far.
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So we start off with an
initial velocity of zero.
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And then for five
seconds we accelerate
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it one meter per second squared.
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So one, two, three, four, five.
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So this is where we go.
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This is where we are.
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So after five seconds,
we know our velocity.
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Our velocity is now
five meters per second.
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But how far have we traveled?
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So we could think about
it a little bit visually.
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We could say, look, we could try
to draw rectangles over here.
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Maybe right over here,
we have the velocity
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of one meter per second.
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So if I say one meter per
second times the second,
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that'll give me a
little bit of distance.
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And then the next one I have
a little bit more of distance,
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calculated the same way.
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I could keep drawing
these rectangles here,
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but then you're like, wait,
those rectangles are missing,
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because I wasn't for
the whole second,
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I wasn't only going
one meter per second.
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I kept accelerating.
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So I actually, I should maybe
split up the rectangles.
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I could split up the
rectangles even more.
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So maybe I go every half second.
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So on this half-second I
was going at this velocity.
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And I go that velocity
for a half-second.
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Velocity times the time would
give me the displacement.
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And I do it for the
next half second.
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Same exact idea here.
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Gives me the displacement.
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So on and so forth.
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But I think what you see as
you're getting-- is the more
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accurate-- the smaller
the rectangles,
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you try to make here, the closer
you're going to get to the area
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under this curve.
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And just like the
situation here.
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This area under
the curve is going
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to be the distance traveled.
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And lucky for us, this is
just going to be a triangle,
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and we know how to figure
out the area for triangle.
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So the area of a triangle
is equal to one half
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times base times height.
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Which hopefully
makes sense to you,
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because if you just
multiply base times height,
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you get the area for
the entire rectangle,
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and the triangle is
exactly half of that.
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So the distance traveled
in this situation,
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or I should say
the displacement,
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just because we want to make
sure we're focused on vectors.
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The displacement
here is going to be--
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or I should say the magnitude
of the displacement,
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maybe, which is the same
thing as the distance,
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is going to be one
half times the base,
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which is five seconds,
times the height,
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which is five meters per second.
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Times five meters.
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Let me do that in another color.
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Five meters per second.
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The seconds cancel
out with the seconds.
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And we're left with one half
times five times five meters.
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So it's one half times 25,
which is equal to 12.5 meters.
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And so there's an interesting
thing here, well one,
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there's a couple of
interesting things.
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Hopefully you'll realize that
if you're plotting velocity
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versus time, the
area under the curve,
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given a certain amount
of time, tells you
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how far you have traveled.
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The other interesting thing
is that the slope of the curve
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tells you your acceleration.
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What's the slope over here?
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Well, It's completely flat.
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And that's because the
velocity isn't changing.
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So in this situation, we
have a constant acceleration.
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The magnitude of that
acceleration is exactly zero.
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Our velocity is not changing.
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Here we have an acceleration of
one meter per second squared,
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and that's why the slope of this
line right over here is one.
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The other interesting
thing, is, if even
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if you have constant
acceleration,
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you could still figure
out the distance
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by just taking the area
under the curve like this.
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We were able to
figure out there we
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were able to get 12.5 meters.
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The last thing I want to
introduce you to-- actually,
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let me just do it
until next video,
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and I'll introduce you to
the idea of average velocity.
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Now that we feel
comfortable with the idea,
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that the distance
you traveled is
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the area under the
velocity versus time curve.