#DidYouKnow: A circle is the most symmetric because it has infinite lines of symmetry (diameters). Same goes for a sphere. To learn more about Symmetry, enroll in our full course now: infinitylearn.com/cbse-fullcourse?RU-vidDME&S-ZmpJfw&Comment To watch more Geometry videos, click here: bit.ly/GeometryPart1_DMYT
I'm stupid, so this might sound dumb, but wouldn't a circle just have a really large number of lines of symmetry instead of Infinite? I mean if there was 1 line for every Planck length around the circle then it would just be a really big number. Idk, I'm confused
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Let's draw an Isosceles triangle. It's a triangle in which two of the sides are equal to each other. And we had seen in the previous video, that it has a vertical axis of Symmetry. If this part is flipped to the other side, we see that the two parts match exactly. And that's why we say that this shape is symmetrical. This is called reflection symmetry. Why is it called that? Let's see. If we take one part and keep it against a mirror, we will get the original shape. The reflection of one part completes the shape. But wait, what is the other kind of symmetry? Do we have another kind? To know the answer here's another figure for you. This figure is made up of six squares to be precise. Now I want you to tell me if this shape has reflection symmetry or not. Can we draw a line through it, such that the two parts formed match exactly with each other? If we try out different lines we realize that no such line can be drawn. This shape has no reflection symmetry, but what the shape has is 'rotational symmetry'. Yes ! 'Rotational symmetry'. What does that mean? As the name suggests, let's try rotating the figure about its center point. And what does rotating about a center point mean? Take the example of this triangle. If we rotated about its center point, it will rotate like this. If we rotate it about this point, it will rotate like this. And if we rotated about one of its vertices it will rotate like this. The shape will rotate differently depending on the point around which it is rotated. Now let's come back to our figure. We rotate the figure completely around the center point once, and see how many times it looks exactly like the original one. Rotating it completely, means rotating it by 360 degrees. Let's have a counter on the right which counts the number of times the rotated figure looks like the original. Let's start! Now the figure rests at zero degrees. After rotating it by 90 degrees. We get this shape. It's not the same as the original. The counter is still at zero. Okay, so let's rotate the original shape by 180 degrees. And we see that it looks exactly like the original shape. It fits in perfectly. We see an increment of one on the counter. We continue rotating it till we finish one complete rotation. Rotating it by 270 degrees gives us the shape which is not the same as the original. And rotating it by 360 degrees gives us the original shape back . The counter changes to two. What does this two tell us? It tells us that when this figure is rotated completely by 360 degrees, the rotated image looks exactly like the original image twice. Once at 180 degrees, and another at 360 degrees. So we say that this shape has rotational symmetry of order two. To recap, when do we say that a shape has rotational symmetry? Okay, this is long. So I want you to listen to it carefully. A shape has rotational symmetry if, it looks exactly like the original shape, a number of times when rotated about the center point by 360 degrees. Here, the number of times it looks like the original is 2. So we say that this shape has rotational symmetry of order 2. Is it easy to find the order of rotational symmetry? Let me give you a few shapes, and why don't you try finding their order of rotational symmetry. First, an oval looks like this. Next a square, an equilateral triangle and a circle. Each of them has rotational symmetry, but we need to find the order. We begin with the oval shape and start rotating it. Let's see how many times the rotated image looks like the original. Once and twice. We saw that it looks like the original shape two times after the complete rotation. Its order of rotational symmetry is two. In a similar way why don't you try finding the order of rotational symmetry, for these three shapes? Let's start rotating the square now about its centre point. 90 degrees, 180 degrees, 270 degrees and 360 degrees. Clearly, the order of rotational symmetry for a square is 4. Now for the equilateral triangle. 120 degrees at 240 degrees, And at 360 degrees. Three times in one complete rotation, the order is three. And now we come to the circle. What do you think will be the answer here? No matter how we rotate the circle, it will always match the original shape . It will have rotational symmetry of order Infinity. So remember, a shape has rotational symmetry if it looks exactly like the original shape. A number of times when rotated about the center point by 360 degrees.
Too good... The book I was reading didn't had clear explanation of point symmetry... Even the term rotational symmetry was not used... This video helped me a lot... Thanks ❤
This lesson was a lifesaver. We had a substitute teacher in class the day we were meant to be learning this, and I didn't understand it at all. He just gave us worksheets. Thank you so much!
Great lesson! You speak so clearly and the animations along with the sound effects are very effective. Thank you for sharing this content! Keep up the great work!
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Omg I 'am speechless. It took me months to cover the topic symmetry (the chapter is like 50 pages in the book) and u explained it all in 7 mins. Thx a lot 😇😇😇😇😇👌👌👌
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Those are 3-Dimensional shapes. Perhaps you could request a lesson, though I think it's the same concept. If you are still stuck, you might learn that not all shapes have rotational symmetry - that it is so unique it never repeats itself.
Nice video. One of the common confusions with rotational symmetry is that each shape will match itself after a rotation of 360 degrees. Should we consider each shape to be exhibiting rotational symmetry then?
For regular shapes, where all sides and internal angles are congruent such as equilateral triangles, squares, and regular pentagons, the order of rotational symmetry is equal to the number of the reflective axes of symmetry which in turn is equal to the number of sides the shape has.
Rotational symmetry of order 1 means it only looks the same after it goes 360 degrees. Such an object is rotationally asymmetric. Order 0 isn't possible.
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