In this video we discuss how to use a protractor to measure and draw angles. We go through examples using the left and right side of the protractor to measure and draw.
Transcript/notes
When first learning angle measurements we use a protractor, which you see here. A protractor is a half circle, called a semi-circle. Each one of these tick marks on the outer edge of the protractor represent 1 degree.
The point of origin for drawing or measuring angles is this little whole in the center here. There is kind of an imaginary line here at the bottom of the protractor that goes from 0 degrees on the left to 180 degrees on the right and passes through the origin or hole, this is called the baseline.
These outer numbers with increments of 10 degrees here on the protractor are angle measurements for angles we are measuring from the left side of the protractor.
So, for example, this would be a 60 degree angle that I have drawn on the screen.
And these inner numbers are also angle measurements, but from the right side of the protractor. And here is a 60 degree angle from the right side of the protractor.
So, these 60 degree angles have the same 60 degree measurement.
We use protractors to measure angles and to draw angles.
For instance, here is an angle and let’s say we need to measure it. We can measure this angle from the right or left side of the protractor. I will do it from the left side first. We start by lining up the vertex of the angle with the center hole or origin of the protractor, and also lining up one of the sides of the angle with the baseline of the protractor, which is the 0 degree mark from the left side. From here, we look at where the other side of the angle lines up with the outer tick marks on the protractor. You can see that this side of the angle lines up with the tick mark at 40 degrees, so this is a 40 degree angle.
And here is the same angle, but measuring its degrees from the right side of the protractor. Same method, line up the vertex with the hole, then the side of the angle with the baseline on the right side of the protractor, which is 0 degrees from the right side, and measuring from the inner tick marks, we get the same measurement of 40 degrees.
Now for drawing angles. Let’s say that we need to draw a 100 degree angle. We will start from the left side of the protractor.
We first draw a dot on the paper in the center hole of the protractor. Next we draw a mark on the paper at the 0 degree mark on the outside numbers of the protractor. Then we find the 100 degree tick mark on the outside numbers of the protractor and put a mark on the paper at that spot. Now we can use the straight edge of the protractor and draw a line from the 0 degree mark to the dot we drew in, and then use the straight edge to draw a line from the 100 degree mark to the dot, and we have our 100 degree angle.
Now for drawing the same angle measurement from the right side. We use the same process, draw a dot in the center hole, then make a mark for 0 degrees on the right side from the baseline, and then find the 100 degree mark from the inner tick marks and mark that spot on the paper. Then use the straight edge to draw lines from both marks to the dot or vertex and we have a 100 degree angle.
And that is the basics of how to use a protractor to measure and draw angles.
Timestamps
0:00 The parts of a protractor
1:02 Measure angles from the left
1:29 Measure angles from the right
1:51 Draw angles from the left side
2:41 Draw angles from the right side