Lesson video

Hello everyone, my name is Mr. Kelsall and welcome to today's lesson about acute and obtuse angles.

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

Please also find yourself a quiet place somewhere that you won't be disturbed and ideally remove away any sorts of distractions, for example, put your phone on silent or move it away completely.

Now, pause the video, and then when you're ready, let's begin.

So today's lesson is all about how to recognise acute and obtuse angles.

And in order to do that, we need to understand what is an acute angle and what is an obtuse angle.

We then need to look at these angles in shapes.

Then we'll look at how to draw acute and obtuse angles.

And finally, we'll take our quiz.

As I've mentioned, you'll need a pen piece of paper and some graph paper or square paper, and the star words, the words were going to be using today.

We'll talk about angle and turn and right angle, we'll talk about greater than, smaller than, acute and obtuse.

So just to revise what we should know at the moment, we know that an angle is the point where two lines meet.

If we look at the example in the bottom left hand corner, we can see we've got angle where two lines meet.

We've also got two lines which don't meet, so they don't form an angle.

The word turn means to rotate around the point.

And you've heard the word turn around.

A full turn is 360 degrees, a half turn is 180 degrees.

And the quarter turn is 90 degrees.

90 degrees is also known as a right angle.

We normally mark a right angle, as little square where the angle is.

So, take a moment and just look at all these angles on the page.

What do you notice about all of them? I'll give you five seconds to think.

Well, I'm hoping you would have compared all of these to our right angle.

And you would have noticed that every single one of these is less than 90 degrees and that's right.

And these are all called acute tangles.

An acute triangle, is an angle measuring less than 90 degrees.

Can you say that? Say it out loud.

So we've got this out here.

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

Now, if we know what an acute angle is, take a moment, have a look at all the angles on this page and see what you notice about these.

I'll give you five seconds.

Fantastic, all of these angles measure between 90 degrees and 180 degrees.

Again, can you say that? So you understand what the definition of an obtuse angle is.

They would make, an obtuse angle is an angle measuring between 90 and 180 degrees.

So have a look at these two angles on the page.

Have a look at the first angle, is it acute or obtuse? I'm going to give you the sentence to say, the angle is doubled up because it is doubled up on a right angle.

So you could say the angle is acute because it is smaller than a right angle.

Or you could say the angle is obtuse because this is bigger than a right angle.

So take a moment.

Say out loud what you think on number one is.

You should have said that the first angle is acute because this is less than 90 degrees.

You should have said that the angle is acute because this smaller than 90 degrees.

You should have said the angle is acute because this is smaller than a right angle.

Repeat the same exercise with angle number two, I'll give you five seconds.

Fantastic, you should have said the angle is obtuse because it is bigger than a right angle.

Or the angle is obtuse because it's is greater than a right angle.

Okay, a lot more angles on the page now.

Pause the video and ask yourself, are these angles acute or obtuse? Use the sentence and go a is doubled up because it is double up than a right angle.

Okay, can you say these with me? Angle a is acute, because It's less than a right angle.

Angle b is obtuse because this is greater than a right angle.

Angle c is acute because it's less than the right angle.

Did you notice on line angle c that there are two lines, one line is really long, and the other line is really short.

It doesn't matter how long the lines are because we're only interested in how much turn down the angle has.

Look at angle b.

Angle b is it cute, because it is less than a right angle.

Angle e is abit tricky, isn't it? So if we look carefully at angle e, we can see that actually that's 90 degree.

That's a right angle.

So is a right angle acute or obtuse? Actually it's neither because the definition of acute angle is an angle measuring less than 90 degrees, and the definition of an obtuse angle, is an angle measuring between 90 and 180 degrees.

So actually a right angle is just a right angle.

It is neither acute or obtuse.

Okay, look angle f.

You'll probably realise that you don't have to say anything on angle f because angle f is not an angle.

The two lines don't meet and therefore it doesn't form an angle, So it's neither acute nor obtuse, and it's not actually an angle.

And finally angle g.

Well, most people think that angle g is an obtuse angle, but actually angle g is a straight line because it has exactly 180 degrees, so it's not obtuse, it's a straight line.

Okay, let's move on to our developing learning stage.

This time we've got angles within angle and we've got angles within shapes.

But we can apply exactly the same principle.

Pause the video, try and complete the sentences.

Okay, we should have said that angle a, is acute because it is less than a right angle.

Now, if you look at this angle a, I'm only interested in the two lines, either side of the letter a, and this is a cute, this part is less than 90 degrees.

However, angle b, I'm only interested in the two lines between angle b, the amount of turn between those two lines.

So angle b is obtuse because it's is greater than a right angle, it's more than a right angle.

Now let's look at the triangle.

Angle a is obtuse because this is greater than a right angle angle b and c are both acute because they are less than a right angle.

Okay, question here.

Have a look at these shapes.

There are five shapes.

Have a look at every single angle, and can you label and work out whether they are acute obtuse or a right angle.

Pause the video now, press play when you're done.

So how did you get solving that? Well, the first shape is a square.

I have one, two, three, four right angles.

The second shape is a hexagon that has one right angle here, one right angle here, and then one, two, three, four obtuse angles.

The third shape is a triangle and in the corner is a right triangle.

And the other two angles are both acute angles.

The fourth shape is a trapezium and in the trapezium the bottom two angles are above acute angles.

Whereas the top two angles are above obtuse angles.

And finally, we've got the arrow of the lightning bolt, and a little bit tricky here, but we have one acute, two acute, three acute four acute, five acute, six acute angles.

We also have in the top corner here, something which looks like a right angle, but it's just a tiny little bit bigger than a right angle, so it makes it an obtuse angle.

We also have some other angles, as well.

For example, this angle here is bigger than 180 degrees.

So we call it a reflex angle and you'll learn all this in future lessons.

But we have one, two, three, four reflex angles.

Okay, have a look around the room.

Where else can you find acute tangles? And where else can you find obtuse? Pause the video, take a moment to think, and then press play.

When I look around my room, I find a lot of right angles, but I really struggle to find the acute angles and obtuse angles.

So I'm almost got to make some of these when I open in my book and I turn the page, I turn it just a little bit, so it's an acute angle.

I keep on turning it, so it becomes an obtuse angle.

The same way I open my door, I open my door and it starts off as an acute angle, but the more I open it, the bigger the angle gets.

And I can stop at 90 degrees, which is a right angle, or I can carry on and open it a little bit further and make it an obtuse angle.

Okay, now it's time for your independent task.

On your graph paper, can you try and draw these shapes.

Some shapes you'll be able to draw and draw quite easily.

What are the shapes you might struggle drawing? If you struggle drawing them, think why you struggle drawing them.

Pause the video, press play When you're done.

Hopefully, you managed to successfully complete that task.

If you'd like to please ask your parents or carer to share your work on Twitter, tagging @OakNational and #LearnwithOak.

Also don't forget to complete the quiz.

So that brings to the end of today's lessons on acute and obtuse angles.

A really big, well done, for a fantastic learning that you've achieved.

But before you finish, could you just think back about the most important parts of today's lesson.

You should have understood the acute angle is an angle measuring less than 90 degrees.

An obtuse angle is an angle measuring between 90 degrees and 180 degrees.

You should be able to identify acute and obtuse angles, and you should be able to identify them in shapes as well.

Well, I guess all that is left for me to say is thank you very much.

Take care and enjoy the rest of your learning today.