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Technology: Engineering Our World
In metric, how much work would be done to move a 50 N resistance
through 4 meters?
Work = 50 × 4 = 200 joules
Machines make it easier to do work. However, no machine does as
much work as the energy put into it. If a machine did the same amount of
work as the energy supplied, it would be 100% efficient. Most machines
lose energy as heat or light. Approximate efficiencies of some common
machines are listed in Figure 8-57.
Another important term associated with work is power. Power is the r
rate at which work is done or the rate at which energy is converted from
one form to another. It can also be the rate at which energy is transferred
from one place to another. Power can be expressed in this formula:
Power =
Work
Time
P =
W
T
Mechanisms use or create motion. See Figure 8-58. The four basic kinds
of motion are:
• Linear—straight-line motion
• Rotary—motion in a circle
• Reciprocating—backward and forward motion in a straight line
• Oscillating—backward and forward arc motion, like a pendulum
Mechanisms are often used to change one kind of motion into another
kind. Some examples are shown in Figure 8-59.
Friction
Friction is a force that acts like a brake on moving objects. Your finger
will slide without much effort on a pane of glass. But if you do the same
thing on sandpaper, you can feel a resistance slowing your movement.
Watt’s Steam Engine 3%
Modern Steam Engine 10%
Gasoline Engine 30%
Nuclear Nu cl ea r Power Po we r Plant Pl an t 30% 30 %
Aircraf Ai Ai rc ra ft ft Gas G G as Turbine T T ur bi bi ne 36% 36 %
Diesel Di Di es es el el Engine E E ng ng in in e e 37% 37 37 % %
R Ro Ro Ro k ck ck ck t et et et Engi E E E ng ng g g in in in e e e 48% 48 48 48 % % %
Electri El El El El El El ec ec ec ec tr tr tr tr i ic ic ic ic ic Motor M M M M M M ot ot ot ot or or or or 80% 80 80 80 80 80 80 % % % % % %
gure 8-57. Scientists, engineers,
and manufacturers are always trying to
design more efficient engines, motors,
and machines.