Mechanics 1: Linear Kinematics and Calculus - 3D Math Primer for Graphics and Game Development - page 483

The first two are shattered by relatively, and the second two by quan-
tum mechanics. Thankfully, these two subjects are not necessary for video
games, because your authors do not have more than a pedestrian under-
standing of them.
We will begin our foray into the field of mechanics by learning about
kinematics, which is the study of the equations that describe the motion of
a particle in various simple but commonplace situations. When studying
kinematics, we are not concerned with the causes of motion—that is the
subject of dynamics, which will be covered in Chapter 12. For now, “ours
is not to question why,” ours is just to do the math to get equations that
predict the position, velocity, and acceleration of the particle at any given
time t, or die. Well, forget about the last part anyway.
Because we are treating our objects as particles and tracking their po-
sition only, we will not consider their orientation or rotational effects until
Chapter 12. When rotation is ignored, all of the ideas of linear kinematics
extend into 3D in a straightforward way, and so for now we will be limit-
ing ourselves to 2D (and 1D). This is convenient, since the authors do not
know how to design those little origami-like things that lay flat and then
pop up when you open the topic, and the publisher wouldn't let us even
if we were compulsive enough to learn how to do it. Later we'll see why
treating objects as particles is perfectly justifiable.

11.2 Basic Quantities and Units

Mechanics is concerned with the relationship among three fundamental
quantities in nature: length, time, and mass. Length is a quantity you are
no doubt familiar with; we measure length using units such as centimeters,
inches, meters, feet, kilometers, miles, and astronomical units. 2 Time is
another quantity we are very comfortable with measuring, in fact most of
us probably learned how to read a clock before we learned how to measure
distances. 3 The units used to measure time are the familiar second, minute,
day, week, fortnight, 4 and so on. The month and the year are often not
good units to use for time because different months and years have different
durations.

2 An astronomical unit is equal to the average distance between the Earth and the
sun, approximately 150 million kilometers or 93 million miles. That's a big number, but
I wouldn't say it's astronomical.
3 In fact, it's even easier now. Only dinosaurs like the authors know how to read an
analog clock that has hands.
4 OK, maybe that one's not so familiar. It is to one of the authors, but that's because
he's British.