Friday, September 12, 2008

Big Numbers

Working on my first physics lab report tonight, based on measurements taken in class on Tuesday. Most of the information was already there, and it was left for me to work out the density of three objects. Since the three objects in question were an aluminum rod, a brass sphere and a small block of indeterminate wood, the equations varied.

It's important to the rest of this story that the exact type of wood was unknown.

Firstly, the equation for density is mass divided by volume. Mass was determined in the lab, by simply weighing the things. The volume is a little tricker, as each shape has its own equation. The block was the easiest, with the volume found by multiplying the length by the width by the height. The cylinder gets a little crazier, with the volume found by squaring the radius, multiplying it by pi, then multiplying it by the length. And the sphere rolls all the way to crazytown, with the volume determined by cubing the radius, multiplying by pi and multiplying that by 4/3, or 1.333.

Our measurements (I was in a group of four) and my calculations weren't so bad for the sphere and rod. The percent error against the accepted density was small, less than 3% in both cases, attributable to the fact that both rod and sphere had big holes in them.

Of course, the wood density was off by over 300%. So I'm going to triple-check my math tomorrow (the first and most obvious culprit), and then bring in the professionals (Jim and my dad), and then just assume that the percent error was because the only two types of wood with accepted densities listed in the lab book's appendix are oak and pine, and that this piece of wood was neither. Oak and pine also have pretty wide densities, with oak being around .75 g/cm3 and pine being about .43 g/cm3.

Mind you, even if the wood were special Jupiter-bred dwarf star petrified ebony, it wouldn't account for the fact that I ended up with a density roughly three times greater than the accepted density of either stated woods. The reason they picked those two to state is that they're pretty far apart, as wood density goes.

The professor said right at the beginning of class that we can't simply attribute error to 'human error.' But does that mean that I can't blame the idiot lab mate sitting across from me, who sighed and said 'whateverrrr,' a lot? Because I'm pretty sure it's his fault. He's a living and breathing human error if ever I met one.


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