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Good Illustration, Horrible Illustration

March 10, 2009

A friend recently asked me if a wheel moves faster at its circumference than at the center. He recognized that a point on the outside of the circle has much further to go in order to make a complete revolution than a point closer to the middle, but the idea of different points on a solid object moving at different speeds annoyed him.

Who can blame him? It’s annoying.

Anyhow, I decided to explain it like this:

Imagine you’re in space, looking down on the North Pole. If you’re high enough in space, the Earth will look like a disc, and North America will be visible at the edge. If you watch for 24 hours, you’ll see the disc make one complete revolution. In that time, any given spot on North America will have moved close to 20,000 miles to get back to its starting point; meanwhile, the North Pole will barely have moved at all.

The actual numbers came from here:
The speed at which the earth spins varies upon your latitudinal location on the planet. If you’re standing at the north pole, the speed is almost zero but at the equator, where the circumference of the earth is greatest, the speed is about 1,038 miles per hour (1,670 kph). The mid-latitudes of the U.S. and Europe speed along at 700 to 900 mph (1125 to 1450 kph).

So there you go, my Creepy friend, a wheel turns faster at its edge than in the center.

May physics interest your family for many Creepy generations to come,


PS – Give Jonesy a snausage for me.

One Comment leave one →
  1. drmath permalink
    March 15, 2009 12:04 am

    Good explanation! But of course, the question about the wheel all depends on what you mean by “moving faster.” If the wheel is rolling along the ground, then after any number of complete rotations, a point on the rim of the wheel will be the same distance from its starting point as the center is from its starting point. Therefore, the average velocity for the two points over that period of time is the same. However, the point on the rim has a greater average speed; it just spends a lot of the time moving backwards (relative to the center).

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