The motion of an object does not change unless an unbalanced force acts on it.
Thus a tennis ball travels in a straight line with unchanged speed when there is no force of gravity or air friction or strings of a tennis racket or ... well only when its sitting "still" is there no unbalanced force. When the ball is still, and there is no unbalanced force, it stays still -- it's motion does not change. Let's think about the hockey puck. (In lab you will use air tables which balance the downward force of gravity with the upward stream of air.) For perfect ice and a slowly moving puck (so that air friction is of little consequence) the motion does seem unchanged: the puck travels in a straight line at a constant rate. The moon? Gravity is the unbalanced force. The diver?
The assertion that an object's motion is unchanged unless an unbalanced force acts on it, generally called Newton's first law, is so basic, so powerful that it is called a law of nature. Laws of nature are never broken. To observe any phenomena contradicting any assertion is to label it a common observation, not a law.
As students of the physics of motion our goals are a description of motion and laws of physics that generalize motion. We begin with observations that require quantifying the motion of objects.
1.4 The Quantities of Motion
Physical quantities are, of course necessary, to describe motion as well as all other aspects studied in physics. Many of these physical quantities are familiar to us such as length, time, velocity, and perhaps mass. Many other fundamental quantities appear to be combinations of these familiar quantities. For example momentum is written as the product of mass and velocity, but momentum is a crucial fundamental quantity, perhaps more so than velocity.
Let's begin with the familiar example of speed or velocity. After this sentence we will use only the term velocity which means speed in a certain direction. Velocity is of course the rate of change of position which we write as
The arrow over 
reminds us that this quantity includes the
direction in which the object is moving. 
is the
change in position which also includes the direction. After all, moving
to your left one meter is completely different from moving one meter straight
up and so on. 
or 
is the interval of time during which the
change in position of the object, 
takes place.
Thus 
is really the average of the rate of change of position
during the interval 
.
It is as essential to
state the direction of motion as it is the magnitude or the units used.
Physical quantities which have a direction as well as magnitude go by
the name vector. We will deal with such quantities in one, two, and
briefly in three
dimensions. It is good practice to ask, always, if a physical quantity is a vector quantity. If so, remind
yourself by writing it with an arrow as in 
. Manipulation of vectors
will be discussed later. For the present time, I present a table of
The Quantites of Motion and
the international accepted units (SI or systeme international units).
1.5 Time
Time is characterized by its passage. The dictionary definition refers to the
duration of an action or an interval. Time passes as you read from the beginning of this
sentence to the end. It takes time to cover distances. Thus the important physical
quantity describes the passage of time or a time interval. We label this as
or 
.
Time and keeping time provide a perfect example of the interplay of technology and science. It has been useful, even essential to measure the passage of time since the dawn of civilization...knowing when the seasons will change, when the dark will come, how much daylight and how much night preoccupied early man. Today it's the start of the next class or meeting, computer "clock" speeds, the Global Positioning system, and the rate of pulsar revolution. Time is practically measured from millions of years to femtoseconds [10^(-15) seconds, 28 orders of magnitude!]. The progress of the modern science of motion has relied on even more precise clocks, capable of measuring ever smaller time intervals with greater stability. GPS works with atomic clocks on board an array of satellites (originally established for missile guidance). By measuring the propogation time delay of radio signals from four of the GPS satellites, the time and position of any receiver can be determined with an absolute accuracy of 20 meters! (The system is intentionally degraded by the military. It is actually capable of providing accuracy of 10 feet!). Atomic clocks are so good that the meter is now defined in terms of the distance light travels in 1/ 299,792,458 s in a vacuum. One second is the length of time it takes Cs-133 to undergo 9,192,631,770 vibrations. There's no need for a standard one meter long stick for comparison to all others.
The appropriate time interval for the modern study of motion is the second. The first clocks capable of measuring time in second-like intervals were pendulum clocks. Our hero Galileo was the first to note the isochronas pendulum -- the observation that the period of a swinging church chandelier seemed independent of the amplitude of the swings. Galileo, in church, timed the swings with his pulse ... not terribly reliable, but certainly worthy of the observation step early in the Process. Galileo was right only in the approximation that the angle of the swing is small as we will see later.
For now, we will learn just how inappropriate a person's pulse is as a time keeping device. (Imagine how excited Galileo must have become as he realized his discovery. It surely woke him up!) As we try to determine the distribution of pulse rates among those in a classroom, note the width of the distribution as well as the average. In fact, the current, true definition of the second and of universal time (depending on position, motion etc.) also relies on a distribution of clocks. In the U.S., the U.S. Naval Observatory in Washington, D.C. (Vice President's House) is the official keeper of time. Check it out on the World Wide Web at http://tycho.usno.navy.mil/. To know the exact time, call 1-900-410-TIME or 1-202-653-1800.
1.6 Length
Length is the measure of distances in space. From here to there is a distance
which has a length, and for any two points in space the shortest distance connecting
them is along a unique path, according to Euclidean geometry.
This path
is called the displacement (
) and is a vector quantity because you CAN
point from here to there. The space of the
physical universe apparently occupies three dimensions and thus objects
have volume (with units of length^3) and surfaces have area (with units
of length^2). But the physical quantity 
is measured in units of
length, e.g. meters.
1.7 The Meter and the Speed of Light
The SI unit of length, the meter, is currently defined in terms of the
second based on Cs-133 and the speed of light, c. Light, as we'll
learn later, is a propagating change of electric and magnetic fields. This
velocity of propogation in a vacuum has a constant magnitude called the
speed of light.
The time
it takes light to travel a certain distance can be very precisely determined, more
precisely than the length of a specific piece of material or the wavelength of light
from a specific atomic transition.
Thus, since 1983, the SI unit of length has been defined in
terms of the time it takes light to travel that distance, i.e.
1 meter is 1/299,792,458 light seconds. The speed of light is
exactly 299,792,458 m/s.
1.8 Metrology
The science and art of metrology is continually refining the precision of the units for physical quantities. Using atomic quantities as the definitions of the second, the meter and so on is natural and important because in this way the standard can never be lost. A standard meter stick, for example, could be broken, melted etc. Currently, the standard kilogram remains a piece of material, but work is proceeding steadily to replace the standard kilogram with an atomic standard. The criteria is always that the most precise possible definition is the best (thus the definition of the meter as 1/299,792,458 light seconds).
Footnotes: