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When asked to cite a very basic law of physics, many people reply with something that they
might call "the law of inertia": An object at rest tends to stay at rest; and an object
in motion tends to stay in motion.
This would-be law of physics suffers from some BIG problems: Its first half seems OBVIOUS;
its second half seems OBVIOUSLY WRONG; and the whole thing sounds imprecise. TENDS TO?!
This is supposed to be a law of PHYSICS. There shouldn't be any "tends tos" in physics: either
it does or it doesn't!
We'll get to the REAL law of inertia soon. And there won't be any "tends tos" in it!
But, for now, it WILL help to look at this term "inertia," and to understand why it's
often described as a "tendency."
Inertia IS a "tendency" to resist changes in motion. Are there DEGREES of inertia? Yes:
Some things have a greater tendency to resist changes in motion than others. Of course!
A full suitcase resists changes in motion more than an empty suitcase does. Just try
wiggling them! It's much easier to wiggle an empty suitcase than a full suitcase. Likewise,
a leaf may dance in the breeze, but a stone certainly doesn't.
The leaf and the stone have different amounts of inertia, as do the two suitcases. We can
make this more accurate. We can even MEASURE inertia. The measure of an object's inertia
is called its MASS. That's what mass IS: the numerical (or "quantitative") indicator of
inertia -- the measure of an object's tendency to resist changes in motion. The more mass
something has, the more it resists changes in motion. A stone has more mass than a leaf;
a full suitcase has more mass than an empty suitcase.
Notice that SIZE has nothing to do with this. The two suitcases may have the same size,
but the full one has much more mass, and more inertia. The stone may be smaller than the
leaf, but it has more mass, and more inertia.
We're not talking about WEIGHT, either. Sure: a full suitcase weighs more than an empty
suitcase. But you can tell that without wiggling either. (We'll explore "weight" later on;
it's NOT the same as mass. Knowing THIS is very important.) The wiggle test is the key
to understanding mass, and to understanding inertia. If you want to know how much mass
something has, wiggle it -- try to change its state of motion.
Back in RP2, the second taproot episode of Radical Physics, we encountered two VERY deep
ideas that have no explanation. We "took them for granted." These are the ideas of distance
and time. Now we have a third fundamental idea to take for granted, namely "mass." Mass
is every bit as fundamental and mysterious as distance and time.
Even though distance and time are mysterious, we know how to MEASURE them. That is crucial.
Distance is measured with a ruler or meter stick, and its agreed-upon unit of measurement
is the meter. Time is measured with a clock or stopwatch, and its agreed-upon unit is
the second. Since mass is as deep and important as distance and time, we'd better learn how
to measure mass, and we'd better have a unit for mass. The unit of measurement for mass
is easy; it's the kilogram. Around the world, people agree on that. What we don't have is
a conceptually clear and practical way to measure mass.
Remember: the key is the wiggle test. We need a device that systematically wiggles an object.
Then, that object's resistance to wiggling -- its resistance to change in motion -- can
tell us the object's mass -- can tell us its inertia.
To measure mass, we need something like a paint shaker. Astronauts have such a device
for measuring their bone and muscle loss while floating for long periods in space. But we
need something smaller (and cheaper!) that I can show you here in the gamma sector of
the Milky Way. Such a device to measure mass would be called a Massometer. I would load
an unknown mass into its "keeper;" I would nudge the keeper; and, because of the attached
spring, it would then vibrate back and forth. The Massometer would time the rate of vibration,
and read out the object's mass. High-mass objects would vibrate slowly; low mass objects
would vibrate quickly.
What a useful instrument this would be! And what a great teaching tool! But, as of mid-2012,
no company on Earth manufactures a Massometer. Wonderfest, the educational nonprofit corporation
that produces Radical Physics, is ready to share credit for -- and benefits from -- Massometer
usefulness, but no science equipment company has stepped forward to build and market it.
Distance, time, and mass are called the three fundamental physical DIMENSIONS. There is
just one other fundamental dimension that we will need to "take for granted" in the
huge arena of classical physics. But we won't need that HERE in Radical Physics until around
episode 100, RP100! Until then, we are studying the incredibly rich territory of classical
MECHANICS. And every new concept that we encounter in classical mechanics will be based upon
these three fundamental physical dimensions: distance, time, and mass.
We began this episode with a bad law of physics: a poor rendition of the law of inertia. Now
that we understand inertia -- and its measurement as MASS -- we are prepared to grasp the REAL
law of inertia -- the first
law of nature!