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Chapter 8 Power | Up
Basic Machines - Intro to machines and motion theories | Next
Figure 8-3.-A prony brake. |
Figure 8-2.-One horsepower.
HORSEPOWER
You measure force in pounds, distance in feet, and
work in foot-pounds. What is the common unit used for
measuring power? It is called horsepower (hp). If you
want to tell someone how powerful an engine is, you
could say that it is many times more powerful than a man
or an ox or a horse. But what man? and whose ox or
horse? James Watt, the man who invented the steam
engine, compared his early models with the horse. By
experiment, he found that an average horse, hitched to
a rig as shown in figure 8-2, could lift a 330-pound load
straight up a distance of 100 feet in 1 minute. Scientists
agree that 1 horsepower equals 33,000 foot-pounds of
work done in 1 minute.
Since 60 seconds equals a minute, 1 horsepower is
equal to 3S’W!W = 550 foot-pounds per second. Use
the following formula to figure horespower:
CALCULATING POWER
It isn’t difficult to figure how much power you need
to do a certain job in a given length of time. Nor is it
difficult to predict what size engine or motor you need
to do it. Suppose an anchor winch must raise a
6,600-pound anchor through 120 feet in 2 minutes. What
must be the theoretical horsepower rating of the motor
on the winch?
The first step is to find the rate at which the work
must be done using the formula:
Substitute the known values in the formula, and you
get:
So far, you know that the winch must work at a rate
of 396,000 ft-lb/min. To change this rate to horsepower,
you divide by the rate at which the average horse can
work—33,000 ft-lb/min.
8-2
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