Science In Real Life

Some of you, dear readers, a rare few sporks in a drawer full of forks and spoons, grew up watching a television show called Bill Nye the Science Guy. I certainly did. Those of you who did so will almost certainly remember the introductory theme song, which among other things made the following two statements:

1) BILL BILL BILL BILL BILL BILL
2) Inertia is a property of matter.

That second statement is our topic today here on Science In Real Life. I trust the first one needs no further explanation. Inertia is indeed a property of matter – of all ordinary matter, so far as we know, and indeed of energy as well. Conspicuously absent from that inclusion is dark matter, and the even more mysterious dark energy. Those are essays unto themselves, but suffice it to say we don’t know for sure what properties those substances may have.

Inertia tends to come up in common usage in a manner befitting the original root of the word, to describe how thoroughly and completely something or someone is not moving. The inability of Congress to pass, well, any reform at all might be described as inertia. Historically, the question of objects and motion had many different proposed solutions, notably Aristotle’s assertion that objects only move under a continued impetus, and naturally come to rest without it. This assertion is, of course, wrong. Sorry, Ari.

It wasn’t until Kepler, Galileo, and Newton did their thing that the modern notion of the principle of inertia was born: objects maintain a constant velocity unless acted upon by an external force. Note that velocity is a vector quantity, so just like your epic winning move in a game of Battleship you have to provide two pieces of information – in this case speed and direction. In other words, turning an object counts as acceleration (a change in velocity). Great, you say, but what is inertia? As a quantity, it is the extent to which an object resists changes in momentum. The more inertia your lazy co-worker has, the harder it is to get him to finish his half of the presentation.
This is starting to sound like Newton’s second law, and with good reason. Take a gander at this highly accurate diagram:

F=ma
(Not to scale.)

What we have here is Newton’s 2nd: force equals mass times acceleration. The more force you apply, the more acceleration you produce. But this process is modulated by the object’s mass. If you have a ridiculously large mass, like a star, you could apply PHENOMENAL COSMIC FORCE and get back itty bitty acceleration. This proportionality is why physicists today think of mass as a measure of inertia. Now, I’m going to tell you all a secret. You have to promise not to spread this around, okay? Okay. There are actually two kinds of mass.

One kind is called inertial mass; it is the type we have just been discussing and for any given object it is measured by applying Newton’s 2nd law. The other is called gravitational mass, and it represents how much an object is affected by a gravitational field. The question of their equivalence has long been hovering in the background of physics research. Einstein’s theory of General Relativity, which forms the foundation for current understanding of gravity, assumes they are exactly equivalent, and so far all the experiments conducted have not proved Albert wrong. But we have no theoretical or a priori justification for concluding that they are the same except that we would really rather like them to be. All of this touches further on the question of just what mass IS to begin with, which is what the Large Hadron Collider attempts to shed light on by searching for the Higgs boson. That’s too deep to go into right now; suffice it to say that it is hoped that by confirming or disproving the Higgs boson, a better understanding of how inertia and gravity are related will be there for the taking.

There is one final concept to mention regarding inertia: inertial and non-inertial frames. Just like no one realized they had always been speaking in prose until someone invented poetry, no one knew they had been in frames all this time until classical mechanics came along (Newton and a few others). To take the analogy further, imagine there are an infinite number of languages and each one represents a different constant velocity. Everyone speaking the same language is in one inertial frame, and they all observe the same rules of grammar (laws of physics). If we observe someone speaking a different language, all we have to do is translate back to our inertial language-frame to get the same rules of grammar again.

Acceleration, then, is represented by someone speaking in verse, where for the purposes of this analogy we imagine that poetry has the magical quality of continuously and smoothly moving through different languages. To keep up with it would require an infinite and infinitely fast sequence of translations, and so this is not an inertial language-frame. Obviously if someone is poetcelerating, they are not observing the same rules of grammar physics – extraneous words (also known as fictitious forces) show up that we have to remember are artifacts of introducing meter and rhyme to the system. And you thought “poetry in motion” was just a figure of speech.