What is perhaps the most seen shape

These tasks will require a computer, access to the web, paper and pencil. Read this section. On answer sheet draw and label the parts on the circle diameter, radius, chord, arc, etc. Below the circle are the words radius, chord, diameter, arc, and semi-circle. Click on each one at a time this will help you identify the parts of a circle. Chord does not work but you can find out about it in the reading.

Go to the bottom of the page and click NEXT. You might know if you found one, but if you haven't found one, you haven't proved that they don't exist," said David Kinderlehrer, Alumni Professor of Mathematical Sciences. While nature might be out of our control, shouldn't it at least be possible to draw or make a perfect circle? For a circle to be perfect, we would need to measure an infinite number of points around the circle's circumference to know for sure.

Each point would need to be precise from the particle level to the molecular level, whether the circle is stationary or in motion, which makes determining perfection a tricky feat. Much like Schrodinger's cat's suspended existence, the answer is not clear cut — there are all kinds of possibilities. In this vein, maybe a circle in nature is perfect, maybe it isn't, but our ways of knowing are limited by the constraints of our physical senses.

What we do know is that perfect circles abound in mathematics where lines and points are safe from the finite restrictions and forces of the material world. Man-made circles — that may or may not be perfect — abound around CMU's Pittsburgh campus. Have you seen all of the items and locations highlighted below? A diameter is a special type of chord, which passes through the center of the circle. As with any segments, two chords are congruent if their lengths are equal.

A portion of a circle is called an arc. Arcs are measured in degrees, like angles, and are classified in a similar way. There are minor arcs, major arcs, and semicircles. A minor arc measures between 0 and degrees. A major arc is between and degrees.

Arcs also have length. Imagine placing a string along a portion of a circle, then picking it up and stretching it out, and measuring it. That is arc length. The sum of all the lengths of the non-overlapping arcs of a circle is called the circumference of that circle. This is the distance around the circle. Two circles that have the same center are called concentric circles. The circles on a bull's-eye target are concentric.