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Chapter 3: Energy
Work 3.1
Work done on the object is defined as the products of the applied force and the parallel distance
through which the force acts:
work = force * distance
W = Fd
Mechanical work is the products of a force and the distance an object moves as a result of the force.
There are two important considerations to remember about this definition: (1) something
must move
whenever work is done, and (2) movement must be in the
same direction
as the direction of the force.
Ex: moving a book to a higher shelve in a book case. Applying vertically upward force equal to the
weight of the book. The work done on the book can be calculated by multiplying the weight of the
book by the distance it was moved.
Units of Work
In the metric system a force is measured in newtons (N) and distance is measured in meters (m) so the
unit of work is:
W = Fd
W = (newton)(meter)
W = (N)(m)
Newtonmeter is a unit of work. This unit name is called a joule (J)
1 joule = 1 newton meter
The units for a
newton are kg*m/s²
and the
unit for a meter is m
. Therefor the
units for a joule are
kg*m²/s²
. In the English system force is measured in pounds and distance in feet. Ft*lb does not have
its own name like the N*m does.
Power 3.2
Lifting of weight (force exerted) the vertical height of the stairs (distance through which the force is
exerted). A person who weighs 120 lb and climbs a stairway with a vertical distance of 10 ft will do
(120lb)(10ft) or 1,200 ft*lb of work. The amount of work will not change if the person were to run up
the stairs because the same amount of work is accomplished. The same amount of energy is used, but at
a greater
rate
when running. The rate that energy is transformed or the rate that the work is done is
called
power
. Power is measured as work per unit of time,
power = work
time
p = W
t
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View Full Document Considering just the work and time factors a 120lb person who ran up the 10ft stairs in 4 seconds
would have a power rating of
P = W
= (120 lb)(10 ft)
= 300 ft*lb
t
4 s
s
When the steam engine was created by James Watt a new measurement of horsepower was created.
Horsepower has a power rating of 550 ft*lb/s. To convert a power rating in English units to horsepower
simply divide the power rating by 550 ft*lb/s/hp. Example: the 120 lb person has a power rating of 400
ft*lb/s / 550 ft*lb/s/hp, or 0.7hp
The metric system measures power in joules per second. The unit J/s, has a name. It is called a
watt
(W). The watt is used with metric prefixes for large numbers: 1,000 W = 1 kilowatt (kW) and
1,000,000 W = 1 megawatt (MW). 746 W is equal to 1 horsepower. One kilowatt is equal to about 1
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This note was uploaded on 02/09/2011 for the course PSC 1121 taught by Professor Tulsian during the Spring '11 term at Daytona State College.
 Spring '11
 Tulsian

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