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Equation of the Day #4: Measurement Standards

Posted by Akano Toa of Electricity , in Math/Physics Nov 25 2012 · 121 views

Standard Meter Pendulum
If you're building something and want to tell other people how to build it, it's useful to show the dimensions of said something (how big it is) relative to other things that people are familiar with. However, there are very few things in this world that are exactly the same size as other similar things (e.g. not all apples weigh the same or have the same volume). So, some smart people once upon a time decided to make standards of measurement for various properties of matter (which I think we can all agree was a smart decision). I wanted to talk about one of these today: the meter.

The word meter (or metre for those who live across the pond/in Canada) comes from the word for "measure" in Greek/Latin (e.g. speedometers measure speed, pedometers measure steps, &c.), but the meter I'm talking about is the International System (SI) unit of distance. The original definition of the meter was one ten-millionth of the distance from the Earth's equator to the North Pole at sea level (not through the Earth). The first person to measure the circumference of the Earth was the Greek mathematician/astronomer/geographer Eratosthenes (and he was accurate to within 2% of today's known value) circa 240 B.C., so this value was readily calculable in 1791 when this standard was accepted.

In 1668, an alternative standard for the meter was suggested. The meter was suggested to be the length a pendulum needed to be to have a half-period of one second; in other words, the time it took for the pendulum to sweep its full arc from one side to the other had to be one second. The full period of a pendulum is

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So, when L = 1 m and T = 2 sec, we get what the acceleration due to gravity, g, should be in meters per second per second (according to this standard of the meter). It turns out that g = pi2 meters per second per second, which is about 9.8696 m/s2. This is very close to the current value, g = 9.80665 m/s2 which are both fairly close to 10. In fact, for quick approximations, physicists will use a g value of ten to get a close guess as to the order of magnitude of some situation.

So, you may be wondering, why is it different nowadays? Well, among a few other changes in the standard meter including using a platinum-iridium alloy bar, we have a new definition of the meter: the speed of light. Since the speed of light in a vacuum is a universal constant (meaning it is the same no matter where you are in the universe, unlike the acceleration due to gravity at a point in space), they decided to make the distance light travels in one second a set number of meters and adjust the meter accordingly. Since the speed of light is 299,792,458 meters per second exactly, this means that we have defined the meter as the distance light travels in 1/299,792,458th of a second.

This is all nice, but it's not a very intuitive number to work with. After all, we humans like multiples of ten (due to having ten fingers and ten toes), so why not make a length measurement of the distance light travels in one billionth (1/1,000,000,000th) of a second (a.k.a. nanosecond)? That seems a bit more intuitive, don't you think? It turns out that a light-nanosecond is about 11.8 inches, or about 1.6% off of the current definition of a foot. In fact, one physicist, David Mermin, suggests redefining the foot to the "phoot," or one light-nanosecond, since it's based off of a universal constant while the current foot is based off the meter by some odd, nonsensical ratio.

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Fairy Knight
Nov 25 2012 12:23 PM
I just realized that I really HAVE to follow your blog. Not only but certainly also because of entries like this one. :3

Also:
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Akano Toa of Electricity
Nov 25 2012 01:39 PM
Yay! My first blog approval!

I need to make an approval sidebar now...or later. :P

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Seriously, we need to make the switch to metric and get it over with. Almost all English standards are now nonsensical.
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Akano Toa of Electricity
Nov 25 2012 07:14 PM
I think we should keep Fahrenheit for weather temperature, though, since it's a good gauge of "on a scale from 0 to 100, how hot will I feel if I step outside?" Celsius is fine for use in cooking and anything else, though, since it makes sense to compare those things to the boiling/freezing point of water, but since Fahrenheit is based off of human body temperature, it works well for how warm it is outside.

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Fairy Knight
Nov 26 2012 11:14 AM
Why don't we all just switch to Kelvin and never have to deal with negative temperatures ever again? :3
Other than that, I kinda like Celsius for temperatures. ._.
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Akano Toa of Electricity
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Name: Akano
Real Name: Forever Shrouded in Mystery :P
Age: 25
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Likes: Science, Math, LEGO, Bionicle, Ponies, Comics, Yellow, Voice Acting
Notable Facts: One of the few Comic Veterans still around
Has been a LEGO fan since ~1996
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Twitter: @akanotoe

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