Reading angles on a protractor (Part 2)

Key Stage 2

Maths

Angles

﻿Hello, 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 protractor. Also, please try and find a quiet place somewhere around the house, 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, and then let's begin. Today's lesson is all about reading angles on a protractor. And to start with, we're going to look at all the common mistakes that people make. We're then going to look at how to measure multiple angles, so one angle after another angle. And then we're going to look at measuring angles within a shape. And after that, it's quiz time! As I've mentioned, you will need a pen, a piece of paper, and a protractor. And our star words for today are: Right angle, acute, obtuse, and reflex angle. We'll be talking about a protractor, its baseline, as well as its crosshairs. And we'll use the term vertex, or vertices, meaning a point or points. So a little bit of revision: You know that an angle is the point where two lines meet. You know turn means rotate around a point. You know we've got a full turn, which is 360 degrees. A half turn, which is 180 degrees. And a quarter turn, which is 90 degrees. We call a quarter turn a right angle. You know that an acute angle is less than 90 degrees, and right angle is exactly 90 degrees, and obtuse angle is between 90 and 180 degrees, and a straight line is 180 degrees. A reflex angle is 180 degrees to 360 degrees. The shapes we'll be talking about are a quadrilateral, a four-sided shape. A pentagon, a five-sided shape. A hexagon, a six-sided shape. And we'll also remind ourselves how to use a protractor, we can measure from both sides, and we can measure to the nearest 10 degrees, 5 degrees, and 1 degree. Now take a moment, pause the video, and just remind yourself what are the parts of a protractor, and explain how you read a protractor. You should have said that you know the baseline is this line right along the bottom. And you know the crosshairs are this point, here. If your angle starts on the left hand side, you start at 0 and you measure around this way. However, if your angle starts on the right hand side, you start at the 0 on the inside, and you measure around this way. Over the next few slides, we're going to look at how we can identify common mistakes, and how we can avoid those, as well. Our new learning for today: We're going to start by looking at some common problems that people make. First thing, I'm going to show you how to use a protractor. And we need to remind ourselves that we've got the baseline, and the crosshairs. Second thing, we're then going to look at what mistakes people commonly make. Okay, if I've got an angle, and I draw myself a little angle, and I'm going to measure this angle. First thing, I'm going to put my baseline on the 0. If it's a little bit out, it's not going to measure an angle. So I've got to make sure my baseline is lined up on the 0. And then I'm going to move my crosshairs over, until it meets the vertex of the angle, exactly. And at the same time I'm going to double check my baseline. My baseline is still on 0, my crosshairs are in the right place. And that means that I can read the angle. Now, remember, am I reading the inside angle, or the outside angle? Well, I'm starting from 0, so I'm reading the inside angle. I go 10, 20, and I don't have any little gaps here, so I'm going to go on over to here, and I'm going to count 1, 2, 3, 4, 5 gaps. So I know that this angle is 25 degrees. Now you notice that I measured my angle from the right hand side. If I wanted it to measure from the other side, I can quite easily do this. Again, I'll spin my protractor around, and I'm going to line up my baseline, my baseline's here, with the 0. So I keep looking, I think all right, baseline is there. Find the 0. But my vertex isn't quite lined up with the crosshairs, so I'm going to move it along until the vertex is lined up with the crosshairs. But look now, I'm slightly out. So I just need to adjust a little bit, and I'll double check the crosshairs' lined up with the vertex, baseline on 0. This time I'm using the inside of the angle, and I'm going to count 10, 20, 1, 2, 3, 4, 5. So I can see it's 25 degrees. Now, you might think that it's easier to move the protractor to measure your angle. You might also think that it's easier to move your piece of paper to measure the angle. So if I put it this way, I can line up my crosshairs, I can line up my baseline, and I can see it's a lot easier when I move my paper. But you need to be able to know all of these skills. So you need to be able to move the protractor and read it accurately, or if you want, move the piece of paper and read it accurately. So that's how we use a protractor. Let's have a look at some common mistakes that people make. Number one, I'm going to line up my protractor on this angle. My baseline is accurate, but look here, my crosshairs is not at the vertex. This is a common mistake that people make. And I'll show you. My angle is 0, and I'm measuring around, 10, 20, 30, 31, 32, 33, 34 degrees. If I move my crosshairs to the exact correct place, I've got my baseline at 0, crosshairs lined up, I can see the angle is 10, 20, 1, 2, 3, 4, 5, 25 degrees. Not that 34 degrees we had before. So the first error is crosshairs at the vertex. The second error that people make, sometimes they actually put the crosshairs here, at the very edge of the angle, rather than at the vertex where the angles meet. Where the lines meet. So be aware of that one, crosshairs at the edge. Your next one, we've discussed it already, is lining up the baseline. So some people, to get the vertex in the correct place, and after they put the vertex down, the baseline isn't lined up to 0. And you can see the angle is incorrect. If I move it along, I've got a baseline lined up at 0. My vertex meets the crosshairs, I can measure the angle accurately. So be careful about lining up your baseline. A very common mistake is some people will very accurately line up your vertex, line up your baseline, double check vertex, baseline, great. Did you notice that my hands moved a little bit? And now, I'm slightly out. Some people will line it up correctly, then take their arms off, and when they put it back, move the protractor a little bit. And you can see, again, not only is the baseline out, the vertex is out. So be careful of lining up the protractor, and then moving it. Another common mistake is reading from the wrong side. Now if I just look at this angle, I know it's an acute angle. I know it's less than 90 degrees, I don't need to think about that. I'm going to measure it from this side, this time. I'm going to line up the vertex with the crosshairs. My baseline is on 0. Ooh, do you see it moved a little bit? I'll just put it back. Crosshairs with the vertex, baseline at 0. And I'm going to count. Right, that's 155 degrees. Can you see I've read from the wrong side? I've started at the outside, when actually my 0 is on the inside. So I should be reading at 0, and measuring 10, 20. Then use the little ones here, 1, 2, 3, 4, 5. If you actually notice, there's a line in between, there. So I don't need to count 1, 2, 3, 4, 5. I can just look at the 5, part way along. That's reading from the wrong side. Your next one is rounding up or down from 5. I'll do this angle, this time. Do you notice I moved my protractor around? Crosshairs, baseline. Okay, now some people will go, 10, 20, 5. Some people will look at the 30, and they'll go back 5. But sometimes, they look at the 30 and they think it's forwards 5. So they think this angle is 35 degrees. But we know that's incorrect, because 35 degrees would be there. I can see it's 10, 20, 25. So be careful not just to read and add on 5. And the final one is extending the line. Now, if I look at this angle here, I've got a little bit of a problem. I'm going to line up my crosshairs, line up my baseline, but I can't see where the angle goes. So perhaps a simple way to do this is just extend the line. Now be careful when you extend it, don't draw it incorrectly. Make sure that your line is perfectly lined up. Your line is perfectly lined up. And now, we can start measuring it. So I've got crosshairs at the vertex, baseline, and I'm going to measure. Which side do I start from? Do I start from the inside, or the outside? Well it seems high, doesn't it? It comes after the 0, so 10, 20, and I'm going to count along 25, 26, 27, 28, 29 degrees. So just be careful if you're extending the line, extend it accurately. As we continue with our new learning, we need to look at how we can measure multiple angles. So this time, I've got three lines, and two angles. I can see one angle is obtuse, and I can see one angle is acute. So if I start by looking at the obtuse angle, to begin with, I'm going to put my crosshairs on the vertex, and this time the vertex is where the two lines meet. I'm going to line up my baseline. I'm just going to put my finger on, so it stays in place. And I know that, as I'm reading around, I'm starting from 0 on the left and I'm reading all the way around to, that's a nice easy one, 150. Be careful, some people think it's 30, but this obtuse angle is 150 degrees. I'm going to write it on, and I'm going to put the little angle notation around it. If I wanted to, I could measure it using this side. So my crosshairs lines up with the vertex, the baseline is on 0. And this time I'm reading around from the inside. From naught, past 90, all the way to 150. My second angle is an acute angle. I'm going to go crosshairs, baseline. Oh, it's 150. That's because I'm reading from the outside, I need to remind myself to read from the inside. So 0, 10, 20, 30. So I know that angle is 30 degrees. Write it on, label it. Angle number two, let's try this one. This time I've not got a straight line, so it might be a little trickier. But I'm still going to do the same process. Crosshairs to the vertex, baseline. Crosshairs, vertex, baseline. I know it's an acute angle, so I'm thinking it's less than 90 degrees. I'm on 0, I'm reading out from the outside. And I got all the way around, and it's just after 80 degrees. It's about 81 degrees. 81 degrees, I'll mark it on. I can check it from the other side, as well. Same process. Crosshairs, vertex, baseline. This time, my 0's here, so I'm reading from the inside. 10, 20, 30, 40, 50, 60, 70, 80... I'll go onto the outside, and I can see it's 80... Between 81 and 82. So we'll say 81. Angle number two is obtuse. Do you know what I've noticed? That, actually, the lines are just a little bit too short. So I'm extend these slightly. Okay? So crosshairs, baseline. I'm reading from the inside. All the way around, so 110, that's 115, and I come up to here, 117 degrees. Just going to zoom in a bit so you can see clearly on the next one. This time I'm going to measure from the other way around. So I've got my crosshairs, baseline. I'm going to move around so baseline 0 is on the outside. So I'm reading all the way around, 110, 115, 116, 17, 18, 19. Oh, that's a little bit out. So I've made a mistake, there. And if I look back here, I can see my crosshairs aren't lined up perfectly. So I'm going to adjust it slightly. Crosshairs, vertex, baseline. Read it again. 115, 116, 117, 118 degrees. So I've got one which I've measured 118, one which I've measure 117. I'm happy with that, because I'm within 1 degree, and that's an acceptable tolerance. Okay. Can you measure and work out this other angle? Let's double check it. Crosshairs, baseline, I'm reading from the inside. It's an obtuse angle, I'm reading all the way around to 160. And that looks like 161. Let's check it this way. Crosshairs, baseline, going all the way around. On the outside, this time. 160, and I can see it's 1. Move that along a little bit. 160, 161. Okay, can you complete the rest of these angles, yourself? Just bear in mind, if you print this piece of paper out, it might be slightly out with the angles. So one way to check, is these two angles should add up to 180 degrees. These ones should add up to 360. These ones should add up to 360. And these ones to 180. So our third skill for today is to measure multiple angles. And we're going to measure the angles inside these shapes. And we're also going to think about what the interior angles of a triangle add up to. So, same skill as previous. I'll just zoom in a little bit so you can see. Okay, I'm going to start with this angle in the corner. Crosshairs, baseline, my 0's on the outside, so I'm reading along all the way to 40, 1, 2, 3, 4. This one is 44 degrees. Check this one, here. So crosshairs, baseline. This time I've reading from the inside, because my 0 is on the inside. 10, 20, 30, 40, and I can either count 45, 46, 47, 48, 49, or I can go all the way to 50 and count back 1, to 49. So I'm thinking that's 49 degrees. And finally, I've got this angle in the corner, which looks like a right angle, doesn't it? Let's see if it is a right angle. So I've got crosshairs, baseline. 0 on the outside, so I'm going all the way around to 80. And I can count up 85, 86, 87, 88, 89. Or I could go all the way to 90, and go back one. So hang on, that's 89 degrees. Okay, so angles in a triangle. We've got an 89 degree angle, we've got a 49 degree angle, and we've got a 44 degree angle. What do they add up to? 9, 18, 19, 20, 21, 22. 8, 12, 13, 14, 15, 16, 17, 18. So I know that my triangle adds up to 182 degrees. Now, angles on a triangle actually add up to 180 degrees. But mine's a little bit out, because you've noticed I use quite thick lines, here, so that you can see this clearly. And that means, when I've got thick lines, I might be out one degree on this angle, 1 degree on this angle, 1 degree on this angle. So I might be out a little bit. But normally, angles in a triangle add up to 180 degrees. Can you complete the other two triangles? Obviously you can check them by adding up the angles, and see what they add up to. We'll develop this learning a little bit more. This time, can you measure the angles inside the shapes? What do the interior angles of a quadrilateral add up to? Pause the video, and when you're ready, press Play to continue. Okay a little trick with quadrilaterals, and we'll look at these later on, I'm just going to draw a line to create two triangles. And if I've got one triangle which has 180 degrees, and another triangle with 180 degrees, that means angles inside a quadrilateral should add up to 360 degrees. Check yours, they should add up to 360 degrees. Now it's time for your independent task. Pause the video, and when you've done this, press Play. Congratulations on completing your task. If you'd like to, please ask your parent or carer to share your work on Twitter, tagging @OakNational, and #LearnwithOak. Now before we go, please complete the quiz. So that brings us to the end of today's lessons on reading angles in a protractor. A really really big well done for all the fantastic learning that you've achieved. Now before we go, perhaps, just take a moment to review your notes and try and find the most important thing from your learning today. And well, other than that, all that's left is for me to say thank you very much, and enjoy your learning for the rest of the day.

Reading angles on a protractor (Part 2)

Reading angles on a protractor (Part 2)

Slide deck

Lesson details

Key learning points

Licence

Video

Worksheet

Starter quiz

5 Questions

Exit quiz

4 Questions

Lesson appears in

UnitMaths / Angles

Maths