definite direction, then “velocity” is the proper term to use. For
practical purposes, speed and velocity mean the same thing.
Anything that travels must have “speed” or “velocity.”
When we say that a certain automobile is a speedy one, we are
not satisfied. By adding that it will make 80 miles an hour, our
curiosity is satisfied. One thing more; when we say that a non-
stop airplane is flying with a “speed” of 200 miles per hour
from New York to San Francisco, we have added direction. Then
we say that the “velocity is 200 miles per hour” and we have
completed our understanding of velocity.
Usually we like to express the velocity of any motion by
numbers. How is this done? By observing the distance covered
in a given time. Thus, the wave in the rope may travel 84 feet
in 2 seconds. Then we would say that the velocity of the wave
would be “ 84 feet per 2 seconds.” However, we would simplify
those numbers so that we would only speak of a single second
instead of two or more seconds. Therefore the velocity of the
wave in the rope would be also spoken of as “ 84/2 feet per second”
or “ 42 feet per second.” We readily realize that we divide the
one number into the other number. That is, we divide the number
of “distance units” (feet) by the number of “time units”
(seconds) in order to get the “velocity units” (feet/sec.).' In
the above case the velocity units are called “feet per second.”
This unit is usually written feet/sec. From this we can
see that the word per means “divided by.” The “dividing line”
in the unit means per. In a similar way we can now say that
“velocity” is “distance per time.” We can also let letters represent
the three quantities of “velocity,” “distance” and “time.”
Let V represent velocity, D the distance and T the time, then
V
=
D — T
The above equation is one which can be used to find the
velocity of any motion.
You will quite often see the expression per used in other
units, besides velocity units, so just keep its intention in mind
as meaning “divided by.”
The velocity of sound waves in air has been measured to be
1085 feet/sec. or nearly “ 1 mile per 5 seconds” (1/5 miles/sec.)
or 331 meters/sec.
