Learn Acute, Obtuse & Right Angles and Measure Angles with a Protractor - [5-9-7] - Free Educational videos for Students in K-12

Learn Acute, Obtuse & Right Angles and Measure Angles with a Protractor - [5-9-7] - By Math and Science

Transcript


00:00	Hello . Welcome back . The title of this lesson
00:02	is called measuring angles . This is part one .
00:05	So in the last lesson we've learned what a right
00:07	angle is , what an acute angle is and also
00:10	what an obtuse angle is . Remember it's all related
00:13	to if you're smaller or larger than 90 degrees or
00:16	if you're exactly equal to 90 degrees in this lesson
00:18	, we're going to actually measure the angle measure between
00:22	to raise . We're gonna measure it with an instrument
00:23	called a protractor . So let's say that I'm taking
00:27	a look at this angle on the board . What
00:30	I'm really doing when I try to find the angle
00:32	measure here is I'm trying to basically figure out how
00:34	open these rays are . If this uh ray here
00:37	I j we're actually not here . If it were
00:40	really close to this one here , then we would
00:42	say this is a very small angle because it's not
00:44	very open and the more and more and more open
00:47	it gets the larger the angle measure . If you
00:50	get two straight up and down , it's a 90
00:51	degree angle measure . And if you go past 90
00:54	degrees , then you get over here actually straight across
00:56	is called 180 degrees . So basically this is small
01:00	angles getting bigger , bigger , bigger , bigger ,
01:02	bigger , bigger , bigger as we go like this
01:03	, the more open the rays are , the larger
01:06	the angle . Now , if you want to measure
01:07	this angle here , we have something called a protractor
01:10	. What you do is you put the corner of
01:12	the angle , call it the vertex right there and
01:15	then we line one of the raise up here and
01:17	then the other ray kind of extends through the measurement
01:20	device and we read it here . Now we have
01:22	a couple of different number scale . I know you
01:24	can't really read this , I'm gonna show you it's
01:26	gonna blow it up here . But basically what you
01:28	do is you read the position of this right off
01:31	of the numbers that are here . Now again ,
01:32	I know it's almost impossible for you to see that
01:34	. So what I've done here is I've redrawn it
01:37	down below . We have the same angle here .
01:39	Here's H . Here's I and here's jay and we
01:41	re drawn it down with my protractor on top of
01:44	it . And so you can read the angles a
01:46	little bit clip more clearly . Now in order to
01:48	solve the problem , we want to give three things
01:50	. I want to give the name of the angle
01:52	, The measurement of the angle in degrees and also
01:54	the classification . In other words , we want to
01:56	write down if it's an acute angle and two single
01:59	or a 90° right angle . All right . First
02:03	things first , let's take a look at this .
02:04	What is the name of this angle ? So we
02:06	have a church here . We have I here and
02:08	we have jay here . So I'm gonna call this
02:09	angle H I J . Now it's important for you
02:14	to know when you name angles , you can name
02:17	an H I . J . Or you can name
02:19	it J . H . That's totally fine . You
02:21	can name it in any order you want . But
02:24	the I hear the center . We call it the
02:26	vertex of the angle has to be in the middle
02:28	. So either H I J or J I H
02:31	. But I has to be in the middle .
02:33	Now let's skip over the measurement to the classification .
02:36	If you notice that if you were to go straight
02:38	up and down from here , the number here goes
02:40	right through 90 degrees . So if the angle were
02:42	up and down , then it would be a 90
02:44	degree right angle and it would be called a right
02:46	angle . This is larger than that . You can
02:49	see the numbers are larger here in black . So
02:51	is this an acute or obtuse angle ? This is
02:54	an obtuse angle . And we know in some twos
02:58	just because we know that it's bigger than 90 degrees
03:00	. That's it . Now , the only thing we
03:02	have left to do is write down the actual measurement
03:04	. If you look at the numbers here , 0
03:06	10 2030 40 50 60 70 80 90 then 100
03:11	. But this ray is right between 100 and 100
03:14	and 10 and may be hard for you to see
03:16	there . But this is exactly between 101 110 which
03:19	means it's gonna be exactly 105 . So the angle
03:23	measure here is 105 and for degrees we put a
03:27	little circle here . The little circle means we're measuring
03:29	degrees 105 degrees . Now , you might say ,
03:33	what do these numbers in blue here ? That's because
03:36	basically you have to read whichever scale makes sense .
03:39	You know , this angle is bigger than 90°. .
03:41	So , you know , you have to read the
03:42	number off that's bigger than 90°. . This other number
03:46	between here's 70 and here's 80 . Between 70 and
03:49	80 would be 75 . Basically this inner scale ,
03:53	the blue scale is telling me how many degrees from
03:55	this . zero over here . So basically this angle
03:58	here is 105 degrees uh from this ray over here
04:03	. But from this one over here , if I
04:05	were measuring from this from a ray over here ,
04:07	it would actually be 75 degrees . So , the
04:10	blues scale measures from the right , the right zero
04:13	point here , this angle measure . And then the
04:16	black scale measures from this . But the simple way
04:19	to do it is when you're looking at the angle
04:21	, you know if it's acute or obtuse . So
04:23	you just you read the correct number off the outside
04:26	as we have done here . And I'll show you
04:27	as we go into the next couple of problems .
04:30	All right . So let's take a look at the
04:32	next problem . All right . So here we have
04:35	uh , an angle J K L . Right .
04:39	And I can name it J K L . Or
04:41	I can name it L K J . But K
04:44	has to be in the middle . Right . Now
04:46	. If I wanted to measure , I would put
04:47	a protractor on top and I would line it up
04:49	and I would read the position of this race .
04:51	So that's what I've done down here . So you
04:53	can read it more easily . This is the exact
04:55	same angle the vertex K . Goes in the center
04:58	. And then I need to basically read the position
05:00	of this ray right here . Now how would I
05:01	call this angle J K . L . You put
05:05	a little angle symbol . This symbol means angle J
05:09	. K . L . Or if I wanted to
05:11	I could call it angle L . K . J
05:14	. Alright . Now is this acute or obtuse ?
05:17	Well straight up and down here would be 90°. .
05:19	This is larger than that . So just like last
05:21	time . This is also obtuse . Now what is
05:26	the measurement of this guy ? We have to look
05:28	and see what numbers we see . Well here is
05:31	130 degrees in black . Exactly . And my other
05:35	choices 50 degrees . Of course it can't be 50
05:37	because that would be an acute angle . This is
05:39	an obtuse angle has to be larger than 90 .
05:42	So we have to write down 130 degrees . And
05:46	the little degree bubble means that you're measuring degrees Now
05:49	. What would be this 50 degree angle measure ?
05:51	That would be if I wanted to know the angle
05:52	from here to this zero point would be 50 degrees
05:55	. That's why the blue numbers are there . So
05:57	we ignore that and we write down 130 degrees .
06:00	And of course it's an obtuse angle . All right
06:04	, let's move on to the next uh problem here
06:07	, we have an angle here . What would we
06:09	name this angle ? We could call it P .
06:10	Q . R . Or we could call it R
06:13	. Q . P . Either way is fine .
06:15	So we put our protractor on top , which is
06:17	what we have here . And we're gonna call this
06:19	angle P . Q . R . P . Q
06:23	. R . I call it P . Q .
06:25	R . You can call it angle R . Q
06:27	. P . If you like . Now is this
06:29	acute or obtuse ? Well , if you think about
06:31	it , this is one of the raise A 90
06:33	degree ray would be going straight up and down here
06:36	. And this is way smaller than that . So
06:38	this is way smaller than 90 degrees . So this
06:40	is a cute . So now we have to measure
06:44	the angle here . What choices do we have here
06:46	is 10 and blue And 20 right in the middle
06:49	is 15°. . Right ? What is my other choice
06:52	? Here's 161 170 . So 165 but 165 is
06:57	way bigger than 90 . That's obtuse . We know
07:00	this is not enough to single . It's very small
07:02	angle . It has to be 15 degrees right between
07:04	10 and 20 . So we write down 15 degrees
07:09	and that's the final answer . So angle P .
07:10	Q . R 15 degrees is an acute angle .
07:15	All right , here's the next problem . We have
07:17	this angle here . We put our protractor on top
07:19	. First of all . How would we name this
07:21	angle ? We could either call it angle X ,
07:23	Y . Z . Or we could call it angle
07:25	Z , Y . X . And so for this
07:28	one , I'm gonna call it angle symbol X .
07:31	Y . Z . But again , you can go
07:33	the other way . Why has to be in the
07:34	middle as you see it here . Now what is
07:37	the measurement here measure from this ray up to this
07:40	one ? What are the numbers that goes right through
07:42	90 on the black and it goes through 90 on
07:44	the blue ? Why is that ? Because the blue
07:46	is measuring the angles from here and that's 90 degrees
07:50	. And the black is measuring the angles if I
07:51	was coming from the other side and that makes sense
07:54	because 90 degrees is straight up and down . So
07:56	it should be 90 this way in 90 this way
07:59	. And that also tells us that if you have
08:01	an angle go all the way over flat to the
08:03	other side , 90 plus 90 is 180 that's why
08:06	you have 100 and 80 in blue and also 180
08:09	blue . But this angle goes through 90 . So
08:11	we call it 90 degrees . What kind of angle
08:15	is a 90 degree angle ? We call that a
08:17	right angle . Any angle that goes up and down
08:21	like that , 90 degrees . We call it a
08:23	right angle . Alright next problem . What do we
08:28	call this angle we have , we can call it
08:29	angle abc or we could call it angle C .
08:32	B . A . But either way be has to
08:34	be in the middle . So we call it angle
08:37	A . B . C . Call it A .
08:40	B . C . Now , is this an acute
08:42	angle or an obtuse angle ? The right angle would
08:44	be straight up here to 90°. . This is larger
08:47	than that is way larger . So this has to
08:49	be an obtuse angle . Now what angle measure is
08:54	it ? As we go over here and read we
08:56	have two choices . We can choose 20° or 160°.
09:00	. It has to be 160 degrees because it has
09:04	it's an obtuse angle , it's bigger than 90 .
09:06	So we have to read 160 . That's the angle
09:09	measures from this side . What is the 20 degree
09:11	means ? That means that would be the angle from
09:13	here . Down to here . That would be a
09:15	20 degree and a very small angle from here to
09:17	here . This is 100 and 60 degree angle measure
09:20	. So it's obtuse angle and here is our last
09:24	problem . What angle do we have here ? We
09:26	can call it D E . F . Or we
09:28	can call it angle F . E . D .
09:30	You can call it either one you want . I'm
09:32	gonna call it angle D . E . F .
09:36	And then we're gonna ask ourselves is this obtuse or
09:38	acute ? Well , a right angle would be here
09:41	but this is larger than that , so it has
09:43	to be obtuse . And then we ask ourselves ,
09:48	what numbers are we going to read as we look
09:50	here ? This is 2030 . This is 40 ,
09:52	so right between 30 and 40 , this is 35°
09:56	, But it can't be 35° because this isn't up
09:59	to singles . So what's this reading here ? 1
10:01	51 40 . Right between 1 41 50 is 1
10:04	45 . So it has to be 1 45 degrees
10:08	. So what we're saying is the angle measure between
10:10	this ray and this rate is 145 . But if
10:13	I were instead measuring from here down , it would
10:15	be actually this other reading which is 35° there .
10:19	So this is 145°. . So that is how we
10:23	measure angles . We look at one ray and we
10:25	use the protractor to read the degree markings off to
10:28	the other way . We just have to be a
10:30	little careful about which scale do we read ? And
10:33	that's why we have to learn what acute and obtuse
10:35	angles are , what a 90 degree angle is .
10:37	So you know how to read the protractor . I'd
10:39	like you to practice all of these yourself and then
10:41	follow me on to the next lesson . We'll get
10:42	a little more practice with measuring angles

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