Measuring reflex angles
Source of all images : Mr Kelsall or Oak National Academy

What is the size of this reflex angle?

Which angle is this protractor measuring?

How many degrees is this angle?

Reading angles on a protractor part 3

﻿Hello, everyone. My name is Mr. Kelsall and welcome to today's lesson about reading angles on a protractor. Now, before we start, you will need a pen, a piece of paper and a projector. Also, please try and find a quiet place around the house. Or somewhere that you're not going to get disturbed. And don't forget to remove any sort of distractions. For example, put your mobile phone on silent or put it away completely. When you're ready, pause the video. I know you must be. So today's lesson is all about drawing angles with a protractor. Specifically acute and obtuse angles, and we'll look at reflex angles in the future. So we're going to start by looking at those obtuse and acute angles and how to draw them. We'll look at common mistakes and then we'll look at how to draw multiple angles. And at the end we've got a quiz as usual. Today you'll need a pencil, piece of graph paper and a protractor. Our star words are right angle, acute, obtuse and reflex angles. We'll use a protractor looking at the baseline and it's crosshairs. And we'll also talk about the vertex, which is the corner of the angle. You'll need to know that an angle is where two lines meet. A turn refers to rotating around a point. We have a full turn, which is 360 degrees. A half turn, 180 degrees and a quarter turn 90 degrees, which is a right angle. You'll need to know an acute angle is less than 90 degrees. A right angle is 90 degrees exactly. An obtuse angle is between 90 and 180. A straight line is 180 degrees. And a reflex angle is between 180 and 360 degrees. We'll talk about the shapes, quadrilateral, pentagon, hexagon as four, five, and six sided shapes. And you will also need to know how to read a protractor from both sides. Being able to measure to the nearest 10 degrees, five degree and one degree. So, a bit of revision before we start this lesson. Have a look at the picture on the screen. And can you estimate the size of the angle? Press pause, and when you're ready, press play to continue. So if we look at these angles, one by one, I've got acute, acute, obtuse, acute, acute, obtuse, acute, acute, obtuse, right angle. Acute, acute, acute, acute, acute, acute, obtuse, acute, obtuse, acute, obtuse, acute, right angle. Acute, obtuse, obtuse, acute. I can then start investigating the angles and the centre, which are created by lines. And I know that this is acute, and this is acute. And they're actually equal. This is obtuse, this is obtuse. And again, they're equal. This one is done like this, acute, acute, equal, obtuse, obtuse, equal. If I look at the lines here, they look very similar to right angles. I could get my angle, my right angle measure, and measure them. But I'm going to say they're right angles. So, right angle, right angle, right angle, right angle. This leaves these angles, acute, acute, obtuse, acute, acute, acute. And this one here acute, acute, acute, acute, acute, acute. And I think that's all the angles in there. So to start on your learning, we're looking at drawing an angle. Specifically, we're looking at sketching these angles and then checking them with a protractor. When I say sketch, I mean, I'm going to do the first one. I'm going to sketch a right angle. I know, or I think that's about 90 degrees. Pause the video, and do the other angles. Okay. So, to my 90 degree right angle. Well, actually, I'm going to extend my right angle. I'm going to add another right angle there. And I know that, that is 180 degrees. So, 180 degrees is two right angles together or a straight line. My next one is a 45 degree angle. Well, if I use facts that I know. I know right angle is 90 degrees, half the right angle is 45 degrees. Now 135 degrees. Well, if I break this number down, I can see this, made up of 90 degrees which is a right angle, and 45 degrees which is half a right angle. Okay. So let's try it. Well, there's my right angle. There's my other right angle. And I'm going to do half a right angle. So this is 90, this is 45. So these two angles here, should add up to 135 degrees. I've now got some other angles. 40 degree, well. That is a right angle. That's half of my right angle. So 40 degrees is just below. It's just a little bit smaller than half a right angle. And that means 50 degrees, I've got my half a right angle. 50 degrees is just a little bit above. Now. I'm looking at my 10 degrees. So I'm starting to think here, well, I'll start on a right angle again. I know that's 45 degrees. So 10 degrees split that into four and a bit. So 10 degrees is probably about that much. Well, 80 degrees if I start with my 90 degrees, and do 10 degrees off. That's about 80 degrees. I'm now looking at 100 degrees. So that is a 90 degrees, 10 degrees more, is about that. And finally, I'm looking at a straight line, which is 180 degrees. 10 degrees less is 170. Let me briefly check these angles. Okay. I'm happy with 180 degrees. A 45 degrees, I'm happy with. My 135 degrees crosshairs, baseline. That's 140 degrees. So I'm a little bit out, but it's within five degrees. That's pretty good for a hand sketch drawing. This angle here was 40 degrees. And I will rotate my protractor round a little bit. So zero, okay. I can see it's going just over the 40 degrees, but happy with it. This one, just a little bit above, it's about 58 degrees. But again, it's not bad given it's hand-drawn. So we said that was the 50 degree sketch. Now I've gone onto my 10 degrees. So I can see my 10 degrees, is just over 10 degrees, but it's pretty good. This is looking at 90 degrees. So crosshair, baseline. 90 degrees is there. You can see this angle just over 80 degrees. But it is relatively good for a hand sketch. Crosshair, baseline all the way round 90. Just over a 100. Good start. And finally, 170 . all the way around to the 170. Now I've deliberately drawn these as a sketch because I want you to start thinking about how big and angle is. Can I work out what size an angle is before I even need to draw it accurately. Our next task is you, can you now draw these angles? When I say draw them, I mean draw them with a protractor. So I'm going to do my first one. I have done must starting line. I've got my crosshairs, baseline and I'm measuring on the inside all the way to 90 degrees. I put a little line there. And this time I'm going to join up my vertex and the mark. And that is 90 degrees. I then check it straight away with a protractor. Crosshairs, baseline. I can see its 90 degrees. And I can label it with a small square. I can say it's 90 degrees. Can you do the same with the other angles? I'm not going to draw all these angles out. I just want to focus on one or two. Let's try drawing 50 degrees. This time I'm going to put my angle, my protractor, my line above an angle. That's going to be where my vertex meets. So I'm going to manipulate my protractor and line up the crosshairs exactly. My baseline isn't lined up yet. So I need to rotate it a little bit until it's lined up. Let me just zoom in a bit so you can see this a little bit more accurately. So crosshairs. I'll just moved off. This point here. crosshairs, rotate it for the baseline, adjust a little bit. Crosshair is lined up. Baseline lined up. Crosshair and baseline. If you want to then hold it in place you can do. Be careful you don't smudge a little bit because you'll see it goes out. So crosshairs, baseline, hold it and I'm measuring round from the inside because my zero is here. 50 degrees, 10, 20, 30, 40, 50. I can't mark it on there. So I have to follow this line along and make my mark there. Be very careful. Your mark needs to be exact. Paint needs to go, not like this, not like this not over here. Definitely not like that. It needs to go like this. I'm then going to use my protractors a ruler, a pencil on my point. I'm going to adjust my line until it's exactly lined up. And then I' am going to draw my line. That should be a 50 degree angle. I'll check it. I'm going to check it this way actually. Crosshairs, baseline. Crosshairs, baseline 10, 20, 30, 40, spot-on 50 degrees. Just realised you can't see that. So I'll zoom out a little bit. So I'm going to adjust this crosshairs, baseline. Count round 10, 20, 30, 40, 50 degrees. So that brings us to the develop learning section of today. For your next task. Draw a straight line and then draw these two angles on it 45 degrees and 70 degrees. What is the remaining angle? Now, yours might look differently than mine because your line will be a different size than mine. I'm going to take my protractor, and I'm the line, Crosshairs, baseline, I'm going to measure 45 degrees. 10, 20, 30, 40, and that's my halfway point at 45 degrees. And I'm going to join it up with a light line and just extend it. I know that's a 45 degree angle. Crosshairs, baseline. Hold this time I'm doing 70 degrees. My angle starts there. 10 20, 30, 40, 50, 60, 70. A little mark. Join up from the vertex to that. Now I know that's 70 degrees. And my question is how big is this remaining angle? Crosshair, baseline. I'm reading from the inside 10, 20. 30, 40, 50, 60, 65 and one more. I suppose 65 degrees. Isn't it? Now, your angle, your, your triangle might look a little bit bigger. Mine look a little bit smaller, but the angles will still be 45, 70, and 65. Let's continue with this. Again, draw a straight line and draw these two angles on it. This time you've got an obtuse angle to work with. Pause the video, and when you're ready, press play to continue. So this time I'm going to draw quite a long line. On one side, I'm going to measure Crosshairs, baseline. And now I'm measuring 65 degrees. I'm going to draw quite a long line. I noticed this have gone over my writing, but at this moment, that's not a big worry too much. Crosshairs, baseline and my angle is 105 degrees. 90, 100, 105 degrees. Okay. What do you notice? These lines aren't going to meet for a long, long, long time. You know that. So I need to understand that actually, having a long line at that point, won't help me. So what I'm going to do is, am going to, just draw a little bit further along. I'm going to go a really short distance along. Let's say to above here. And I'm going to draw this angle again here. So, crosshairs, baseline. And I'm going from zero all the way to 100, 105 degrees. I'm joining my angle from there, to there. Now I can see it's almost joined. So I need to extend this line as well. Okay. Now I can see it. I do have a small angle there. So this angle is 65 degrees. This angle is 105 degrees. What is the remaining angle? So, crosshairs, baseline. And I'm going 10 degrees along. Now you might find that you've drawn a long line to start with and your angles don't meet for a long, long time. So think about, can you draw a shorter line and can you try get to angles, your lines to meet there. Now we're moving on to drawing angles around a point. And we were given three angles and we'd like to know what is the fourth angle? So when it says around a point, what does it mean? I'm just going to start off by drawing a line. And this is my point, my end. My first angle is 35 degrees. So crosshairs, baseline. 10, 20, 30, 35 degrees. That line join up my two points. Label my angle, 35 degrees. And then drawing an angle of 115 degrees. Now some people will start from here and go around to 115. But actually, I'm drawing from this line here. So crosshairs, baseline. Crosshairs, baseline. Going all the way around to 10 20, 30, 40, 50,,60, 70, 80, 90, 100. I'll just move it up a little bit. 90, a 100, 110, 115. I'm going to join up a line and label that as 115 degrees. My third angle. I'm going to start here. Cross hairs, baseline. And then measuring around 65 degrees. 60, 65 degrees. From the point at the centre join up. Another 65 degrees. And my final angle, I now need to measure, to check. So I'm thinking crosshairs, baseline. We're going all the way round to 140, 144 just less than 145 degrees. And what's quite interesting is we can remember the angles around the point should add up to 360 degrees. Bear in mind it might be one or two degrees out. So 145 and 35, plus 50, 60, 70, 80. Oh, that's interesting. And that is a straight line. So that's 180 degrees on that straight line there. This should be a straight line as well. Shouldn't it? So these should add up to 180 degrees, 115 add five so 120, add 60 is 180. So I've got two straight lines and two straight lines add to 360 degrees. That's a really, really good way of checking. Bear in mind, you want be one degree or two degrees out. For example, this angle here, it was about 144, maybe a little bit over. So I rounded up to 145. That might be quite usual for you. Okay. Can you draw this shape? It's 35 degrees and 65 degrees. What's the remaining angle? So here's the shape that I've drawn. I've got 35 degrees, 65 degrees and I'm just going to measure the remaining angle. And I can see that is 80 degrees. Let's check these add up to 180 degrees. 65, 70, 80, 90, 100 180 degrees And angles in a triangle addd up to 180 degrees. Now we move on to our independent task. Can you draw these triangles and quadrilaterals? Pause the video and when you're ready, press play to continue. You can see I've completed all three triangles, I'm now measuring the final angle of my third triangle to see it makes sense. And it does. So I'm going to draw these two quadrilaterals quickly, and then I'm going to slow down and talk you through the third quadrilateral. This last shape is a little bit tricky cause I've got a reflex angle of 200 degrees. So, I'm going to start, or I'm going to draw through the lines first. So, I've got a line. I'm going to look at my first angle zero. I've got my crosshairs lined up, baseline line up. I'm going to do 55 degrees. Join that up. Nice long, my line. Drawn that one. From this side, I'm going to do 65 degrees. 60, 65. Okay. After this one, now I need to do a 40 degree angle. Well, let's go from, let's go from here. Okay. Somewhere, I'll need to do a 200 degree angle, and from looking at this point here, Let's say I'm going from there. I know that, that bit is 180 degrees. So I need to add on a 20. So my crosshairs, baseline I'm adding on 20 degrees. Now I can say at this point, here is my 220 degrees. Here is my 40 degrees. Here is my 65 degrees. And that there was my 55 degrees. But, actually if I look at my shape, it goes, So this one here, if I measure this angle, and look at that. It looks a lot the same. So, let's check and see if this is actually 55 degrees. Crosshairs, baseline. 50, 55 degrees. So I've now drawn my quadrilateral and I've got all the angles correct. Congratulations on completing your task. If you'd like to, please ask your parent or carer to share you with on Twitter tagging @OakNational and also hashtag #LearnwithOak. And before we go, please complete the quiz. So that brings us to the end of today's lessons on reading angles on a protractor. A really, really big, well done for all the fantastic learning that you've achieved. Now, before you go, perhaps, just take a moment to review your notes and try and find the most important thing from your learning today. And well over and up, all that is left for me is to say, thank you very much. And enjoy your learning for the rest of the day.

Draw a triangle with these angles (45°, 55°). What is the size of the missing angle

Draw a triangle with these angles (95°, 105°). What is the size of the missing angle

Draw a quadrilateral with these angles (45°, 55°, 120°). What is the size of the missing angle?

Draw angles with a protractor part 1

In this lesson, we will learn how to use a protractor to draw acute and obtuse angles as well as identify common mistakes.

Maths

Y5 Maths

Key Stage 2

Year 5

Draw angles with a protractor 1

Angles

Geometry

THEME

UNIT

Understanding angles

Recognise right angles

Recognise acute and obtuse angles

Angles within a shape

Compare and order acute, obtuse and reflex angles

Reading angles on a protractor (Part 1)

Reading angles on a protractor (Part 2)

Reading angles on a protractor Part 3

Draw angles with a protractor (Part 2)

Calculating angles on a line or around a point

Calculating angles within a shape 1

Calculating angles within a shape (Part 2)

Calculating angles within a shape 3

Revision and Angle fluency facts

What does an angle measure?

How many angles does this shape have

Which shapes have a right angle (tick 2)

Which angles are obtuse (tick 2)

How big is angle B?

What is the approximate size of an internal angle of this regular pentagon?

What is the size of the missing angle?

Which is the smallest angle?

A numberline starting at 0 and ending at 100 is divided into 10 equal parts. Each equal part represents:

0.7 of a container is filled with water.  How much of the container can still be filled?

Lesson overview:Draw angles with a protractor 1

Core Content

Angles: