Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Rotational Symmetry occurs when a shape looks exactly the same as its original position after being rotated by a certain angle around its center. If you can't tell that the shape has been moved, it has rotational symmetry.
A Half Turn ( turn) is a rotation of 180^\\circ, which is equivalent to turning an object upside down. For example, if you rotate the letter 'H' or 'S' by 180^\\circ, they look exactly the same as their original form.
A Quarter Turn ( turn) is a rotation of 90^\\circ. This is like the movement of a clock hand from to . A square looks the same after a quarter turn, but a horizontal rectangle will look like a vertical rectangle after the same turn.
The Center of Rotation is the fixed point around which a shape turns. Imagine a pin stuck in the middle of a paper windmill; the pin is the center, and the blades rotate around it. The shape of the blades looks the same after specific turns depending on the number of blades.
One-Third () and One-Sixth () Turns: A turn corresponds to 120^\\circ and is commonly seen in equilateral triangles. A turn corresponds to 60^\\circ and is common in regular hexagons, which look identical after every 60^\\circ rotation.
Symmetry in Digits and Letters: Some numbers and letters look the same after a half turn. The digits and look identical when turned upside down, while the letters and also retain their appearance after a 180^\\circ rotation.
Visualizing Changes: To check if a shape looks the same after a turn, imagine the shape inside a circle. A turn moves a point on the top to the right side, while a turn moves a point from the top to the very bottom.
📐Formulae
\\text{Angle of Turn} = \\text{Fraction of Turn} \\times 360^\\circ
\\text{Quarter Turn} = \\frac{1}{4} \\times 360^\\circ = 90^\\circ
\\text{Half Turn} = \\frac{1}{2} \\times 360^\\circ = 180^\\circ
\\text{One-Third Turn} = \\frac{1}{3} \\times 360^\\circ = 120^\\circ
\\text{One-Sixth Turn} = \\frac{1}{6} \\times 360^\\circ = 60^\\circ
💡Examples
Problem 1:
Which of these English words looks the same after a half turn: ZOOM, MOW, SWIMS, SIS?
Solution:
- Analyze 'ZOOM': After a half turn, 'Z' stays 'Z', but 'M' becomes 'W' and 'O' stays 'O'. It becomes 'WOOZ' (reversed). Not the same.
- Analyze 'MOW': After a half turn, 'M' becomes 'W', 'O' stays 'O', and 'W' becomes 'M'. It reads 'W-O-M' (upside down). Not the same.
- Analyze 'SWIMS': After a half turn, 'S' stays 'S', 'W' becomes 'M', 'I' stays 'I', 'M' becomes 'W', and 'S' stays 'S'. Upside down, it still reads 'SWIMS'.
- Analyze 'SIS': After a half turn, 'S' stays 'S' and 'I' stays 'I'. It still reads 'SIS'.
Explanation:
We test for rotational symmetry by rotating the word 180^\\circ. 'SWIMS' and 'SIS' remain readable and identical because their individual letters either stay the same or transform into the other letters of the word in a way that preserves the spelling.
Problem 2:
A fan has 3 blades spaced equally. After what fraction of a turn will the fan look exactly the same?
Solution:
Step 1: A full circle is 360^\\circ. Step 2: Since the fan has 3 identical blades spaced equally, we divide the full turn by the number of blades: \\frac{360^\\circ}{3} = 120^\\circ. Step 3: Convert 120^\\circ back into a fraction of a turn: \\frac{120^\\circ}{360^\\circ} = \\frac{1}{3}.
Explanation:
For any object with identical parts arranged symmetrically around a center, it will look the same after a turn.