A Pledge.

At this point, I pledge to you internets, that I will teach my children to completely own unit conversions. Not metric to english junk, I’m talking unit analysis to bring your inch^3/second into Tons/Hr.

I think it is the single most important skill for engineers. I think it even beats out basic differentiation by 5×10^-14 Parsecs.

fun fact: what is a parsec? it stands for parallax of one second of an arc. Parallax is the act of seeing something in two different spots because you’re moving: perspective shift. So if you’re driving in your car, and you took two rapid photographs of a tree… once you’d downloaded your photographs you’d see that the tree was in two different spots. Parallax is an astronomical method of using this basic theory to try and figure out how far away we are from celestial objects. So, we could look at some distant star from Earth now… and then again in 6 months. Since the Earth has moved around the sun we can figure out how far away that object is – presuming we know how far we’ve gone and where the relative change in position is of the star. The actual parallax is half of the distance that the star has appeared to move.

So that explains the first half. The “one second of one arc” (or arcsecond) is just 1/60th of an arc minute. An arcminute is 1/60th of a degree. There are 360 degrees in a circle. That means that one arcsecond is 1/1,296,000 of a circle. So, a parsec is the distance that a star would be away from the earth for its parallax to equal 1/1,296,000 of a circle.

This comes out to be about 3.26 light years. Neat eh?

7 thoughts on “A Pledge.

  • 4/30/2008 at 7:09 pm

    cubic feet/sec of what???? Air? Lead? Brains? each of those would have a different Tons/hour conversions.

  • 5/1/2008 at 8:44 am

    For this particular thought it was steel. Density = 0.284 lb/inch^3

    But the part I’m referring to is the unit analysis which is something like:

    inch^3/sec = X(tons)/hr * (inch^3)/0.284 lb * (2000lbs)/(1 ton) * 1(hr)/3600(seconds) = 1.956 * X (tons/hr)

  • 5/1/2008 at 2:42 pm

    Mike D, I wholeheartedly agree with you on this. The day that I mastered the “factor-label” method of arithmetic that my 7th grade science teach forced upon us, I felt like the most powerful man on earth. He had us consistently figuring out how many gallons of water were required to get a metric ton of it, and how many cubic inches that would be.

  • 5/1/2008 at 4:05 pm

    Another great/terrible thing: When you put “5×10^-14 Parsecs” in the google toolbar, it drops down the “suggestions” with the conversion to kilometers. And I can see what you were going for (only off by 64 meters) which isn’t bad for one significant figure.


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