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How to use a protractor

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Subido el 22 de septiembre de 2013 por Samuel E.

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This video shows how to use a protractor.

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Hi, today we're going to be measuring angles using a protractor. 00:00:00
Sometimes we use a full-size protractor and sometimes we use a half protractor. 00:00:05
The half protractor is much more common, so our goal is to learn to use that. 00:00:09
So first we have to talk about what an angle is. 00:00:14
An angle is formed by two rays, or line segments, and do you remember 00:00:17
what the end point is called? That's right, it's called a vertex. 00:00:20
That's where the lines, or line segments, or rays meet. 00:00:25
There are three types of angles, three you've learned in the past. 00:00:28
A cute angle is smaller than 90 degrees, a right angle is exactly 90 degrees, 00:00:34
and an obtuse angle is greater than 90 degrees, but less than 180 degrees. 00:00:40
In fourth grade, we learn about a new type of angle called a reflex angle. 00:00:45
A reflex angle is greater than 180 degrees. 00:00:49
The first thing you need to do when measuring angles is estimate the measure of the angle. 00:00:54
This one should be pretty easy. This one is a 90 degree angle. What about this one? 00:00:59
You can use what you know about angles on clocks to help you figure out that 00:01:08
this one is a 30 degree angle. This one is a little harder. We know that it's 00:01:12
greater than 90 degrees but less than 180 degrees. We could estimate this one. 00:01:20
What would you guess? If you guessed close to 150 degrees you did a great job. 00:01:25
What about this one? 120 degrees. You had to know that the measure the angle I was 00:01:31
measuring was this inside angle and not the outside angle. We'll look at that 00:01:40
later when we look at showing a directional arrow. So you can use a full 00:01:45
protractor to help you measure angles. A full protractor starts at zero and 00:01:52
measures all the way around. Here's 180 degrees, 270 and all the way back up to 00:01:58
360. There are 360 degrees in a circle and a protractor is just a circle. You 00:02:06
can also use a regular protractor to measure angles. When you take a look at 00:02:13
this one, notice that there are two sets of numbers. One number on the inside has 00:02:17
a zero starting on the right. You can measure up to 90 degrees and then angles 00:02:23
greater than 90 degrees on this side all the way to 180. On the outside, you have a zero 00:02:28
on the left, and use the outside numbers to measure this way, all the way up to 90, and 00:02:34
then continue on all the way up to 180. What you need to know when measuring angles is 00:02:40
where the baseline is. The baseline will always line up to zero degrees on the protractor. 00:02:47
You also need to know where the vertex of the angle is, because that must line up with 00:02:53
center of the protractor. So where is the baseline? Our angles will have directional 00:02:58
arrows as you can see on these two angles. The baseline is where the directional arrows 00:03:05
begin. So on this one, the baseline is this line where the directional arrow begins. And 00:03:11
on this one, can you tell where the baseline is? Yep, it's this line segment where the 00:03:18
directional arrow begins. So we will measure this angle on the right from here around this 00:03:23
way. So how do you use your protractor to measure the angle? Well the first thing 00:03:29
you need to do is place the dot on your protractor over the vertex of the angle 00:03:36
and line zero degrees to the baseline. The second line array tells how many 00:03:41
degrees the angle is open and that's how we measure the angle. So let's try 00:03:47
measuring. The first thing we'll do here is open up our protractor. This is a full 00:03:53
circle protractor. The first thing I want to do is put the middle of the 00:04:03
protractor right on the vertex of my angle and then I want to line up my 00:04:06
baseline to zero. Now that I have my baseline lined up to zero I can look at 00:04:12
my other line and see that it's not quite long enough to go to the other 00:04:23
side of the protractor. What we do then is we can I could make the protractor 00:04:29
smaller but on paper you could just extend the line and now I can clearly see that this is a 25 00:04:33
degree angle that makes sense to me because I know that a 90 degree angle is a right angle and this 00:04:41
one is much smaller less than half the size of a 90 degree angle so the measure of this angle is 00:04:48
25 degrees well what about this one I'm going to line up the vertex of the angle with the center 00:04:53
of my protractor and then I'm going to turn my protractor so zero degrees lines up with 00:05:02
the baseline and I'm going to use the line on the right to show my baseline to zero degrees. 00:05:10
Now the angle I'm measuring is this angle. I know that that angle is greater than ninety 00:05:18
degrees so when I look at my other line, actually it's a ray, I see that there are two numbers 00:05:24
it could be. Is that angle between 230 degrees and 00:05:31
240 degrees like the top numbers say? Or between 120 00:05:34
and 130 degrees? Well my reasoning tells me that it's probably 00:05:38
between 120 and 130 degrees because it's 00:05:43
greater than a 90 degree angle, but less than a 180 degree 00:05:46
angle. So when I count up here, I see that that is a 00:05:50
126 degree angle. 00:05:54
So I can write on my line 126 degrees. 00:05:57
I have to put a little dot there to show degrees. 00:06:09
Now we can also use a half-circle protractor to do the same thing. 00:06:13
So I'm going to switch over here to a half-protractor. 00:06:17
Now you can see that there are two sets of numbers. 00:06:21
You have to use your reasoning before you start using the protractor. 00:06:26
You have to look at the angle and know that it's less than a 90-degree angle. 00:06:29
So the measurement has to be less than 90 degrees. 00:06:33
So I'm going to move my protractor down so that the vertex is right at that cross. 00:06:35
And then I need to line up the baseline with 0 degrees. 00:06:42
Now when I look at this other line segment, I can see that it's between 20 and 30 degrees. 00:06:47
I know it's not 150 to 160. That just wouldn't make sense. 00:06:53
And so when I count the little degrees in between 20 and 30, 00:06:58
I can see that that angle is a 26 degree angle. 00:07:02
When I use my half protractor on this one, I need to use my reasoning again. 00:07:06
I know that this angle is greater than 90 degrees. 00:07:11
So when I put my protractor on this baseline at 0, I see 0 is on the inside. 00:07:14
So I'm going to use the inside numbers as I count up. 00:07:22
And I see it's between 120 and 130 degrees. 00:07:26
degrees, but it's closest to 126 degrees. That makes sense. So why don't you go ahead 00:07:28
and try measuring some angles on your own. 00:07:36
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Idioma/s:
en
Etiquetas:
EducaMadrid
Autor/es:
mrsgratzclass
Subido por:
Samuel E.
Licencia:
Reconocimiento - No comercial - Compartir igual
Visualizaciones:
75
Fecha:
22 de septiembre de 2013 - 20:04
Visibilidad:
Público
Centro:
IES JOAQUIN ARAUJO
Duración:
07′ 43″
Relación de aspecto:
16:10 El estándar usado por los portátiles de 15,4" y algunos otros, es ancho como el 16:9.
Resolución:
1152x720 píxeles
Tamaño:
31.69 MBytes

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