Proving Circle Symmetry through Rotation: Understanding the Equations

In summary, the conversation is about proving the symmetry of a circle by showing that x² + y² = a² holds when the circle rotates. The formulae x = x'cosθ - y'sinθ and y = x'sinθ + y'cosθ are used to prove this, but the origin of these equations is unclear. It is suggested to drop perpendiculars from the x' and y' axes to the x and y axes, respectively, and use properties of similar and right-angled triangles to find x in terms of x' and y'. The conversation also includes discussions about diagrams and browser settings.
  • #1
raintrek
75
0

Homework Statement


http://tng.trekcore.com/1.JPG

I'm trying to prove that the circle is symmetrical by showing that x² + y² = a² holds when the circle rotates.

I know that this is proved given the following two formulae:
x = x'cosθ - y'sinθ
y = x'sinθ + y'cosθ

but I don't know where those two equations have come from based on my diagram. Help!
 
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  • #2
Drop a perpendicular from where the x' axis cuts the circle to the x-axis and another perp from where the y' axis cuts the circle to the y-axis. Use some properties of similar triangles and right angled triangles.
 
  • #3
I've got the x'cosθ part of the expression for x, but I just cannot see how the -y'sinθ is found...

http://tng.trekcore.com/1.GIF
 
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  • #4
My mistake for giving a hasty answer. Sorry.

Take a point P:(x,y) in the x-y system. Now draw x' and y' axes, rotated by some theta. If you drop perps from P on the x-axis and the x' axis, the first perp cuts the x-axis at a dist x from O and the 2nd perp cuts the x'-axis at a dist x' from O. Now, find x in terms of x' and y', using elementary geometry.
 
  • #5
Just out of curiosity, why was this posted under "Introductory Physics"?
 
  • #6
OK, i think I'm almost there,

I have the x'cosθ term, and I know I need to minus the purple section, which I trust is y'sinθ -- but I can't seem to show that it is, lol, it's the last stumbling block

http://tng.trekcore.com/2.GIF
 
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  • #7
(HallsofIvy has asked you a question. I am also curious.)

Have you drawn the diagram as I said in my 2nd post? You can show us, if possible.
 
  • #8
EDIT: ignore
 
  • #9
Hi raintrek,

I'm not able to see the pictures you posted initially. Have you removed them, or is something wrong with my browser settings? Please answer asap.
 

1. What is the definition of symmetry in a circle?

The symmetry of a circle refers to its ability to be divided into two equal halves that are mirror images of each other. In other words, any line drawn through the center of a circle will divide it into two congruent semicircles.

2. How can it be proven that a circle has rotational symmetry?

A circle has rotational symmetry because it can be rotated by any angle around its center and still maintain its original shape. This can be proven by using a protractor to measure the angle of rotation and observing that the circle remains unchanged.

3. What is the significance of the center point in a circle's symmetry?

The center point is the point of intersection for all lines of symmetry in a circle. It is significant because it acts as the rotational axis for the circle's symmetry, and any rotation around this point will preserve the circle's shape.

4. Can a circle have more than one line of symmetry?

Yes, a circle can have an infinite number of lines of symmetry. This is because any line passing through the center of a circle will divide it into two congruent halves, creating a line of symmetry. Additionally, any line that is perpendicular to a line of symmetry will also be a line of symmetry for the circle.

5. How is the symmetry of a circle used in real-world applications?

The symmetry of a circle is used in various real-world applications, such as in architecture, engineering, and design. For example, the symmetry of a circle is often utilized in the construction of circular buildings, bridges, and other structures. It is also commonly used in the design of circular logos, patterns, and artwork.

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