Lesson video

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Hello everybody.

My name is Mr. and welcome to today's lesson about reading angles within a protractor.

Now before we start, you will need a pen, a piece of paper or something to write on and you also need a protractor.

Also please try find a quiet place around the house.

Somewhere that you're not going to be disturbed and don't forget to move away any sort of distractions.

For example, put your mobile phone on silent or move it away completely.

We're going to start by looking how a protractor works.

we'll then look at reading a protractor.

We'll then look at how we measure angles to 10 degrees then to 5 degrees and finally to 1 degree so we got good level of accuracy.

And at the end as always it's quiz time.

I've mentioned already you need a pencil, a piece of paper, a protractor for today's lesson.

The words that we will be using today are right angle, acute, obtuse and reflex angles.

We will be talking about a protractor, the baseline of a protractor and the crosshairs of a protractor.

In order to access this lesson, you will need to understand some basics, facts and some information about angles.

You need to understand that an angle is the point where two lines meet.

So if I look at these two lines here, ail see that they meet at a point.

If I look at these two lines here, I see that they don't meet at a point.

So the first one is an angle.

The second does not form an angle.

Secondly, we are going to talk about the word 'turn' which means to rotate around a point.

If I have a point, I am going to turn around that point.

I can do a full turn which is 360 degrees, a half turn which is 180 degrees or a quarter turn which is 90 degrees.

Let me talk about a quarter turn, 90 degrees.

We call this a right angle and we mark a right angle with a small square.

You need to understand there're different types of angles.

An acute angle is any angle measuring less than 90 degrees.

A right angle is exactly 90 degrees.

An obtuse angle is between 90 and 180 degrees.

A straight line is 180 degrees and a reflex angle is between 180 degrees and 360 degrees.

Pause the video and when you have a think press play to carry on.

This is called a protractor and it's used to measure angles, specifically the size of an angle.

How much turn there is at that angle.

First thing is the baseline is the line found across the bottom and that's where we measure our angle from.

Be careful.

Some people measure their angles from here and that's incorrect.

The reason it's incorrect is because if you follow that line along you can see that you are still some degrees out from the starting point which is zero.

So your angle will never ever be correct.

So that's called the baseline.

Second thing you need to know is the centre over here.

This is called the crosshairs.

This is where you measure the vertex of the angle, the corner or the point of the angle from.

You need to understand that angles are measured from either side.

So I can measure an angle from here because I've got a zero here and I can measure from here.

But I need to understand the difference between those.

So if my angle starts from the left hand side, and I've got it measurement on zero, I am measuring around this way.

If however my angle is lined with zero here, I am measuring around this way.

I should actually say I am measuring around here which is the inside coz I can see it starts from zero, 10,20,30,40,50.

I also need to know that I can measure angles and degrees.

An angle is the size of turn.

I know that if I measure my angle from this side, I'm reading around, clockwise because I'm starting from the zero point here.

I'm going 10,20,30,40,50, et cetera.

However, if my angle is lined up along this side, I'm starting from this zero on the inside and I'm counting around on the inside.

I'm going 10,20,30,40,50 et cetera.

If I am measuring angles to the nearest 10 degrees, I need to make sure number one; my baseline is lined up.

Number two, my crosshair are exactly lined up.

Now, I am measuring an angle from the zero mark here all the way around here.

I can see that I'm counting 10,20,30,40,50 so I've turned 50 degrees.

I also notice it's an acute angle because it's less than 90 degrees.

I can label my angle if I need to.

There is also another number here of 130.

So why is not 130? Actually it's because I got 130 by counting all the way around this way.

Actually my angle doesn't start from here.

My angle doesn't start from the zero on the right hand side.

It starts from the zero on the left hand side.

Okay.

I'm going to draw another angle.

See if you can work out what this angle measures.

Pause the video.

I give you five seconds.

Okay.

This angle is 30 degrees.

I can see I'm starting at zero and counting around 10,20,30 degrees.

So I can label this as 30 degrees.

Next I need to think about, what if I have an obtuse angle.

An angle above 90 degrees.

My 90 degrees is here so I am looking for an angle above that.

I'm going to start at the baseline.

Line up my angle with zero and I'm going to draw an angle to here.

This time I'm starting, I'm going all the way around the outside.

I can see that I've counted 10,20,30, 110, 120.

I can see this angle is 120 degrees.

So far we've looked at measuring on the outside and we've looked at measuring angles to the nearest 10 degrees.

Let's now change that to the nearest five degrees.

Again, I'm going to draw myself an angle and I'm going to, this time go to here.

So I am starting at zero and I'm counting on 10,20,30.

Now if I look at the gaps between here I can count there are 10 gaps and I can count like fives along.

But actually there is a quick way to do this because if you look in between each of the lines, there a little half way point.

This shows that this is five degrees.

Rather than counting on one, two, three, four, five, I can actually just look at the middle of that and think I've already moved five degrees along.

So I know that this angle is 35 degrees.

Do the same with an obtuse angle.

Again I will line up a baseline with my angle and this time I'm going to go to here.

You can see my line is a little bit out because it's hand-drawn but normally you'll be measuring lines which are drawn with a ruler.

Okay, so I'm going to start a baseline and start from zero and I'm counting on 10,20,30 all the way to 90,100, hundred and.

ah, that's 110 degrees on half of it between 100 and 110 degrees which is 105 degrees.

Again you can see my angles are just a little bit out but that's because it's hand-drawn.

But, I mean, this is my 105 degrees.

My final skill is to the nearest one degree.

I'm going to apologise because it's really difficult to draw to the nearest one degree but I will try.

Okay.

This angle here, I'm going to start from my zero and count around 10 ,20,30,40,50.

Now I can see, I can take a half jump to 55 and I've got one more gap to go.

So I can see this is 50 degrees, the angle.

So 56 degrees the angle.

Okay.

We're now to look at how you measure using the right hand side of the protractor.

First thing you need to know is you will always see angles lined up this side.

Previously, we worked on angles on the other side, this time you will be measuring from this way.

Couple of things to remember.

Number one, the line at the bottom is lined with the baseline and we can see it is.

Number two, the angle starts at the crosshairs which is the angle on the centre and it does.

This time we go around, but we can either count this way but that takes us to 130 and I know that this angle is not 130 degrees because I can see that the angle is acute.

It's less than a right angle.

It's less than 90 degrees.

So actually I need to count on the inside.

I'm going 10,20,30,40,50.

So I can see that this angle is 50 degrees.

Now, it's up to you how you prefer to count.

Once you are starting with this you might prefer to counting on the inside and in tens.

Once you are confident, you might feel okay counting on the outside and just knowing that it's not 130 degrees instead it's 50 degrees.

It's the same again.

If this time we will draw an obtuse angle.

So I can see that my angle is all this 10.

So I can start at zero and I can count on 10,20,30,40,50,60,70,80,90, 100, 110 degrees.

So I know that this angle is 110 degrees.

Okay, an angle for you to try.

Can you take a moment and work out what this angle is? I give you five seconds.

Fantastic! This is an acute angle and it's 40 degrees.

Next one.

Can you tell me what this angle is? Again, five seconds.

You should have identified this as 130 degrees coz you are starting at zero degrees and you are turning anti clockwise all the way round to 130 degrees.

Quick check see, can you tell me what this angle is? Five seconds.

Okay, a bit of a tricky question here.

Some people might have been thinking that they are still counting from zero here but actually my angle starts here.

So I'm going back to the scale I used a moment ago and I'm counting all the way around here.

So I know that this angle here, is 70 degrees.

I can double check that because I know its an acute angle.

It's less than a right angle.

It's less than 90 degrees and I can see that clearly.

Okay, before we go on to practise reading angles from a protractor you also need to understand a little bit about angle notation.

That's how people show angles.

You can see here we've got lots of examples of how an angle can be shown.

I'm going to do the first angle for you.

We are talking about angle BAJ.

So B starts from here and it goes to A and then I need to find J.

I don't need to draw a line all the way along because actually I can see that the letter J is pointing to an angle and I can see that I've got to 20 degrees and then I'm going one step, two steps further along.

I know that angle BAJ is 22 degrees.

Ail do the second one as well but only because it's going from the opposite side.

This time we talk about angle CAN.

So letter C to A to N, and I can see its an acute angle again and I can see it lines nicely on the line and I can count along and go 10,20 degrees.

Okay, close the video.

Can you finish up the others? Hope you've got them well.

From a protractor, there are 20 angles here.

Can you look at reading the measurements of the protractor.

Pause the video or manually press play.