Main content

## 4th grade

### Unit 12: Lesson 2

Measuring angles# Measuring angles using a protractor 2

CCSS.Math:

Sal measures several angles with a protractor. Created by Sal Khan.

## Video transcript

This is the video for
the measuring angles module because, clearly, at the
time that I'm doing this video, there is no video for the
measuring angles module. And this is a
pretty neat module. This was made by Omar Rizwan,
one of our amazing high school interns that we had
this past summer. This is the summer of 2011. And what it really is, is
it makes you measure angles. And he made this really
cool protractor tool here so that you actually
use this protractor to measure the angles there. And so the trick here is you
would actually measure it the way you would
measure any angle using an actual physical protractor. You'd want to put the center
of the protractor right at the vertex of where
the two lines intersect. You can view it as the
vertex of the angle. And then you'd want
to rotate it so that, preferably, this edge,
this edge at 0 degrees, is at one of these sides. So let's do it so that
this edge right over here is right along this line. So let me rotate it. So then-- I've got to rotate
it a little bit further, maybe one more. No, that's too far. So that looks about right. And then if you
look at it this way, you can see that the angle-- and
I don't have my Pen tool here. I'm just using my
regular web browser-- if you look at the
angle here, you see that the other line
goes to 130 degrees. So this angle that we
need to measure here is 130 degrees, assuming
you can read sideways. So that is 130 degrees. Let me check my answer. Very good, I got it right. It would have been
embarrassing if I didn't. Let's do the next question. I'll do a couple of
examples like this. So once again, let us put the
center of the protractor right at the vertex right over there. And let's get this
0 degrees side to be on one of these sides
so that this angle will be within the protractor. So let me rotate it this way. And this really is pretty cool
what Omar did with this module. So let's see. Let's do it one more time. That's too far. And so that looks about right. And then you can see that
the angle right over here, if we look at where the
other line points to, it is 40 degrees. Check answer-- very good. Let's do another one. This is fun. So let's get our protractor
right over there. And you don't always have
to do it in that same order. You could rotate it first
so that the 0 degrees is-- and what you want to do is
you want to rotate the 0 degrees to one of the sides
so that the angle is still within the protractor. So let's rotate it around. So if you did it like
that-- so you don't always have to do it in
that same order. Although I think it's
easier to rotate it when you have the
center of the protractor at the vertex of the angle. So we have to rotate
it a little bit more. So 0 degrees is this line. And then as we go
further and further up, I guess, since this
is on its side, it looks like this other
line gets us to 150 degrees. And hopefully you're noticing
that the higher the degrees, the more open this angle is. And so this one right
over here is 150 degrees. And so let's do that-- 150. Let's do one more. Now let me show
you what not to do. So what not to do
is-- so you could put the center right over there. And you might say,
OK, let me make the 0 go right over on
this side, right over here. So if you did that, notice
your angle would not be within the protractor. So you won't be
able to measure it. And what you're attempting to
do is measure this outer angle over here, which is
an angle, but that's not the angle that this question
is asking us to measure. This little arc over
here is telling us that that's the angle
that we need to measure. So that arc has to be
within the protractor. So let's rotate this
protractor a little bit more. I overdid it. And so this looks like
this is 0 degrees, and then this right
over here is 60 degrees. 60 degrees-- we got
that one right, too. So hopefully that helps
you with this module. It's kind of fun.