CHAPTER 2 - BASIC CONCEPTS RELATED TO FLOWING WATER AND MEASUREMENT

A dropped rock or other object will gain speed rapidly as it falls.
Measurements show that an object dropping 1 foot (ft) will reach a velocity
of 8.02 feet per second (ft/s). An object dropping 4 ft will reach
a velocity of 16.04 ft/s. After an 8ft drop, the velocity attained
is 22.70 ft/s. This gain in speed or acceleration is caused by the force
of gravity, which is equal to 32.2 feet per second per second (ft/s^{2}).
This acceleration caused by gravity is referred to as *g*.

If water is stored in a tank and a small opening is made in the tank
wall 1 ft below the water surface, the water will spout from the opening
with a velocity of 8.02 ft/s. This velocity has the same magnitude that
a freely falling rock attains after falling 1 ft. Similarly, at openings
4 ft and 8 ft below the water surface, the velocity of the spouting water
will be 16.04 and 22.68 ft/s, respectively. Thus, the velocity of water
leaving an opening under a given head, *h*, is the same as the velocity
that would be attained by a body falling that same distance. The equation
that shows how velocity changes with *h* and defines velocity head
is:

(2-12)

which may also be written in velocity head form as:

(2-13)