Video Transcript
Measuring Angles
In this video, we’re going to learn
how to measure angles in degrees using a protractor and estimate the sizes of
angles. We’re going to learn how to
identify and measure straight angles, obtuse angles, right angles, and acute
angles.
These children are making angles
with their arms. We’re not going to measure angles
with our arms; we’re going to use a protractor. We can use a protractor to measure
angles up to 180 degrees. We can see from the outside of the
protractor that the scale begins at zero and goes around the protractor in tens all
the way up to 180. Did you notice the protractor is a
semicircle? If we were to place two protractors
together like so, we’d make a complete circle. A complete turn within a circle
from the center point is 360 degrees, and so half a turn is 180 degrees.
Straight angles measure 180
degrees. We can fit two right angles into
this angle. Right angles measure 90
degrees. Angles which measure less than 90
degrees are called acute angles. We call angles that measure more
than 90 degrees but less than 180 degrees obtuse angles.
What’s the measure of this
angle? Here’s the vertex or the point
where our two lines meet. And we’re measuring the amount of
turn between these two rays or arms, as they’re sometimes called. We lined our protractor up. We can start counting around the
protractor from zero. 10, 20, 30, 40. So, the measure of this angle is 40
degrees. Now that we’ve learned how to
measure angles using a protractor and how to identify different types of angles,
let’s apply what we’ve learned by answering some questions.
Jacob and Mia are measuring
angles. Which of the two is correct, Jacob
or Mia? Jacob says, “The measure of the
angle is 70 degrees.” And Mia says, “The measure of the
angle is 110 degrees.”
Let’s look more closely at the
protractors and see who’s correct. Both children are measuring the
same angle, so we know that one of them has made a mistake. It looks like Jacob started
measuring here at zero. 10, 20, 30, 40, 50, 60, 70. This is why he thinks the measure
of the angle is 70 degrees. But we’re not trying to find the
measure of this angle. It looks like Jacob started in the
wrong place. There are two scales on our
protractor. It looks like Mia started at zero
on this scale, on the opposite side of the protractor. And she’s counted round to 110. 10, 20, 30, 40, 50, 60, 70, 80, 90,
100, 110. The measure of the given angle is
110 degrees. So, Mia is correct.
Choose the angle measure that best
approximates the measure of angle 𝐴𝐵𝐶. 90 degrees, 120 degrees, or 58
degrees.
Let’s think about what we know
about angles. To help us approximate or make a
sensible guess about the measure of angle 𝐴𝐵𝐶. We know that straight angles
measure 180 degrees, and we know that right angles measure 90 degrees. We can see that angle 𝐴𝐵𝐶
measures less than 90 degrees. So, we can eliminate our first
possible answer.
We also know that angles which
measure more than 90 degrees are called obtuse angles. Our second possible answer is 120
degrees. This would be an obtuse angle. And we already know that the given
angle measures less than 90 degrees. If the measure of angle 𝐴𝐵𝐶 is
less than 90 degrees, we know it’s an acute angle. 58 degrees is less than 90
degrees. Angle 𝐴𝐵𝐶 is an acute angle. So, the angle measure that best
approximates the measure of angle 𝐴𝐵𝐶 is 58 degrees.
We used our knowledge of the
different types of angles to help us find the best approximate for the measure of
angle 𝐴𝐵𝐶.
Is an angle of 140 degrees obtuse,
acute, or a right angle?
Let’s use the protractor to help us
draw an angle which measures 140 degrees. We’ve drawn our vertex on our first
ray, we’ve have lined it up with zero on the protractor, and we can use the scale to
count around to 140. Now, we’re going to draw our second
ray. The angle measures 140 degrees. We can see the measure of the angle
is greater than 90 degrees and less than 180 degrees. So, the given angle is an obtuse
angle.
We drew the angle on the protractor
and used the protractor to help us work out that an angle which measures 140 degrees
is an obtuse angle. It measures more than 90 degrees
and less than 180 degrees.
The measure of an acute angle is
more than 90 degrees, less than 90 degrees, or 90 degrees.
This is a right angle. The measure of this angle is 90
degrees. This helps us to eliminate one of
our possible answers. An acute angle is not 90
degrees. A right angle measures 90
degrees. An angle which measures less than
90 degrees is called an acute angle. And an angle which measures more
than 90 degrees is called an obtuse angle. So, we can say the measure of an
acute angle is less than 90 degrees.
Acute angles measure less than 90
degrees, right angles measure 90 degrees, and obtuse angles measure more than 90
degrees and less than 180 degrees. The measure of an acute angle is
less than 90 degrees.
What have we learned in this
video? We have learned how to measure
angles in degrees using a protractor. We also learned how to estimate the
sizes of angles.
