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What’s the equator? Geography and the Earth

By |2018-04-12T08:53:24+00:00August 20th, 2017|Physics|

This globe shows the equator running around the widest part of the Earth. The equator is an imaginary line running around the widest part of the Earth, halfway between the North Pole and the South Pole. So if you are north of the Equator, you're in the Northern Hemisphere. And if you're south of [...]

Area of a sphere – Volume of a sphere

By |2017-07-29T13:10:50+00:00July 29th, 2017|Math|

This is a sphere. A sphere is a three-dimensional circle. Or you could say that a sphere is the set of all the points that are at the same distance from the center of the sphere. In nature, centrifugal force and gravity tend to make a lot of things into spheres: soap bubbles, for instance, atoms, and planets. The radius of a sphere [...]

What is a radius? Radius of a circle

By |2018-05-10T10:38:52+00:00July 29th, 2017|Math|

What is a radius? The geometry of a circle What is a radius? The radius of a circle is the distance from the center point to the edge of the circle. It's the same distance anywhere on the circle, because the circle has radial symmetry. So it doesn't matter where you measure the radius on the circle, [...]

What is pi? Simple geometry – Math, circles, circumference, diameter

By |2018-04-23T16:01:03+00:00July 29th, 2017|Math|

What is pi? The geometry of a circle What is pi? Pi is a number that is just a little bigger than 3.14. It is the number you get if you divide the circumference of any circle by its diameter. It's the same for all circles. You can approximate pi for yourself by taking some circular things like the tops [...]

What is a cylinder? Simple geometry

By |2017-07-29T09:33:37+00:00July 29th, 2017|Math|

This is a cylinder. What can you find around your house that's a cylinder? A cylinder is a circle moved through space to make a solid form. The ends are circles that are in parallel planes, and the sides of the cylinder are the line segments that connect the circumferences of the two circles. Because the circular ends run parallel to [...]

Circumference of a circle – Geometry

By |2017-07-28T08:49:28+00:00July 28th, 2017|Math|

The geometry of a circle The circumference of a circle is the distance all the way around the circle. It's the same as the perimeter, but a special word that we only use for the perimeter of a circle. The circumference is always equal to twice the radius of the circle times π (pi), or 2πr. Dinner [...]

Proof for the area of a circle

By |2018-03-14T12:32:23+00:00July 28th, 2017|Math|

Divide a circle up into sections like an orange We know it's true that A=πr-squared is the area of a circle, but how can we prove that it is always true for every possible circle? Here's how Euclid did it. Start by dividing a circle into sixteen sections like an orange. We know that together they add up to the [...]

Area of a circle – Geometry

By |2017-07-28T00:00:26+00:00July 28th, 2017|Math|

The geometry of a circle The area of a circle is the flat surface inside its circumference. The area is always equal to the radius squared, multiplied by π (pi). Here's a proof that this is always true for any circle. Learn by doing: figure out the area of some circles around you. Radius Diameter Circumference of [...]

Greek mathematics – Geometry and Proofs – Ancient Greece

By |2018-04-22T10:26:50+00:00July 18th, 2017|Greeks, Math|

Greek math: An Egyptian papyrus from about 100 AD which is a piece of one of Euclid's books Greek math: geometry not algebra Because people in ancient Greece had only very clumsy ways of writing down numbers, they didn't like algebra. They found it very hard to write down equations or number problems. Instead, Greek mathematicians were [...]