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## Rotations

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**Objectives**Be able to rotate a shape by any angle about a centre of rotation. Keywords: Rotation, centre of rotation, clockwise, anti-clockwise**Describing a rotation**A rotation occurs when an object is turned around a fixed point. To describe a rotation we need to know three things: • The angle of the rotation. For example, ½ turn = 180° ¼ turn = 90° ¾ turn = 270° • The direction of the rotation. For example, clockwise or anticlockwise. • The centre of rotation. This is the fixed point about which an object moves.**Determining the direction of a rotation**Sometimes the direction of the rotation is not given. If this is the case then we use the following rules: A positive rotation is an anticlockwise rotation. A negative rotation is an clockwise rotation. For example, A rotation of 60° = an anticlockwise rotation of 60° A rotation of –90° = an clockwise rotation of 90° Explain why a rotation of 120° is equivalent to a rotation of –240°.**Rotation**Which of the following are examples of rotation in real life? • Opening a door? • Walking up stairs? • Riding on a Ferris wheel? • Bending your arm? • Opening your mouth? • Opening a drawer? Can you suggest any other examples?**Rotating shapes**If we rotate triangle ABC 90° clockwise about point O the following image is produced: B object 90° A A’ image B’ C C’ O A is mapped onto A’, B is mapped onto B’ and C is mapped onto C’. The image triangle A’B’C’ is congruent to triangle ABC.**Rotating shapes**The centre of rotation can also be inside the shape. For example, 90° O Rotating this shape 90° anticlockwise about point O produces the following image.**Rotation**X**A**A A B B C C B B B C C C A A y Rotate triangle ABC 90° clockwise about the origin. Rotate triangle ABC 180° anti-clockwise about the origin. 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 2 2 X 0 x -6 -5 -4 -3 -2 -1 1 2 3 4 5 6 2 2 2 2**y**9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 x**Inverse rotations**The inverse of a rotation maps the image that has been rotated back onto the original object. For example, the following shape is rotated 90° clockwise about point O. 90° O What is the inverse of this rotation? Either, a 90° rotation anticlockwise, or a 270° rotation clockwise.**Inverse rotations**The inverse of any rotation is either • A rotation of the same size, about the same point, but in the opposite direction, or • A rotation in the same direction, about the same point, but such that the two rotations have a sum of 360°. What is the inverse of a –70° rotation? Either, a 70° rotation, or a –290° rotation.**Rotations on a coordinate grid**The vertices of a triangle lie on the points A(2, 6), B(7, 3) and C(4, –1). A(2, 6) 7 6 5 B(7, 3) 4 3 C’(–4, 1) 2 Rotate the triangle 180° clockwise about the origin and label each point on the image. 1 –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 –1 –2 C(4, –1) –3 –4 What do you notice about each point and its image? B’(–7, –3) –5 –6 –7 A’(–2, –6)**Rotations on a coordinate grid**A(–6, 7) The vertices of a triangle lie on the points A(–6, 7), B(2, 4) and C(–4, 4). 7 B(2, 4) 6 5 C(–4, 4) 4 3 2 B’(–4, 2) Rotate the triangle 90° anticlockwise about the origin and label each point in the image. 1 –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 –1 –2 –3 –4 What do you notice about each point and its image? –5 C’(–4, –4) –6 A’(–7, –6) –7**Finding the centre of rotation**Find the point about which A is rotated onto its image A’. A A A’ • Draw lines from any two vertices to their images. • Mark on the mid-point of each line. • Draw perpendicular lines from each of the mid-points. • The point where these lines meet is the centre of rotation.**Finding the angle of rotation**Find the angle of rotation from A to its image A’. A A A’ 126° This is the angle of rotation • Join one vertex and its image to the centre of rotation. • Use a protractor to measure the angle of rotation.**Rotation - Summary**When the centre of the rotation is the origin Rotation at 90°: P(x, y) P’(-y, x) Rotation at 180°: P(x, y) P’(-x, -y) Rotation at 270°: P(x, y) P’(y, -x)