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WORK
Chapter Eight: Work
8.1 Work
8.2 Efficiency and Power
Chapter 8.1 Learning Goals
Tell what it means to “do work” in a
scientific sense.
Apply an equation to determine the
amount of work done by a force.
Infer that work requires energy.
Investigation 8A
Manipulating Forces
Key Question:
How do simple machines work?
8.1 Work
 In science, work is a
form of energy you
either use or get when
a force is applied over
a distance.
 You do 1 joule of work
if you push with a
force of 1 newton for
a distance of 1 meter.
8.1 Work
 When thinking about work, remember that
work is done by forces that cause
movement.
 If nothing moves (distance is zero), then
no work is done.
8.1 Work
Force (N)
Work (joules)
W=Fxd
Distance (m)
8.1 Work and energy
Doing work always means transferring
energy.
The energy may be transferred to the object
you apply the force to, or it may go
somewhere else.
8.1 Work and energy
You can do work to
increase an object’s
potential energy.
Then the potential
energy can be
converted to kinetic
energy.
8.1 Work
 A raised object’s potential
energy equals the amount of
work it can do as it moves
down.
 The amount of kinetic energy
an object has equals the
amount of work the object
can do by exerting force as it
stops.
8.1 Work
 If force is equivalent to the
weight of the object in
newtons, and
 height (h) is equivalent to
distance (d),
 Then multiplying the weight
by height gives you the
amount of work the object
can accomplish as it moves
down (as well as its potential
energy).
8.1 Work
 Force A does no work
because it does not
cause the block to move.
 Force B is applied at an
angle to the direction of
motion, so only part of
force B does work.
 The most effective force
to move the block is
force C.
Solving Problems
 How much work is done by a person
who pulls a cart with a force of 50
newtons if the cart moves 20 meters
in the direction of the force?
Solving Problems
1. Looking for:
 …work done by person
2. Given:
 …force = 50 N (forward);
 …distance = 20 m
3. Relationships:
 Work = force x distance
4. Solution
 50 N × 20 m = 1,000 joules.
Chapter Eight: Work
8.1 Work
8.2 Efficiency and Power
Chapter 8.2 Learning Goals
Describe the relationship between
work and power.
Apply a rule to determine the amount
of power required to do work.
Explain the meaning of efficiency in
terms of input and output work.
Investigation 8B
Work
Key Question:
How can a machine
multiply forces?
8.2 Efficiency and Power
 Every process that is done by machines can
be simplified in terms of work:
1. work input: the work or energy supplied to
the process (or machine).
2. work output: the work or energy that comes
out of the process (or machine).
8.2 Efficiency and Power
A rope and pulley
machine illustrates a
rule that is true for all
processes that
transform energy.
The total energy or
work output can never
be greater than the total
energy or work input.
8.2 Efficiency
65% of the energy in
gasoline is converted
to heat.
As far as moving the
car goes, this heat
energy is “lost”.
The energy doesn’t
vanish, it just does
not appear as useful
output work.
8.2 Efficiency
The efficiency of a machine is the
ratio of usable output work divided by
total input work.
Efficiency is usually expressed in
percent.
Output work (J)
efficiency = Wo
Wi
Input work (J)
x 100%
Solving Problems
 You see a newspaper advertisement for a
new, highly efficient machine. The
machine claims to produce 2,000 joules of
output work for every 2,100 joules of
input work.
 What is the efficiency of this machine?
 Is it as efficient as a bicycle?
 Do you believe the advertisement’s claim?
Why or why not?
Solving Problems
1. Looking for:
 …efficiency of machine
2. Given:
 …Wi = 2100 J, Wo = 2000 J
3. Relationships:
 % efficiency = Wo x 100
Wi
4. Solution
 2000 J ÷ 2100 J x 100 = 95% efficient
8.2 Power
The rate at which work is done is
called power.
It makes a difference how fast you
do work.
8.2 Power
Michael and Jim do
the same amount of
work.
Jim’s power is
greater because he
gets the work done in
less time.
8.2 Power
Power is calculated in watts.
One watt (W) is equal to 1 joule of work per
second.
James Watt, a Scottish engineer, invented
the steam engine.
Jame Watt explained power as the number
of horses his engine could replace.
One horsepower still equals 746 watts.
8.2 Power
Work (joules)
Power (watts)
P =W
t
Time (s)
Solving Problems
 Allen lifts his weight
(500 newtons) up a
staircase that is 5 meters
high in 30 seconds.
 How much power does
he use?
 How does his power
compare with a 100-watt
light bulb?
Solving Problems
1. Looking for:
 …power
2. Given:
 Fweight= 500 N; d = 5 m, t = 30 s
3. Relationships:
 W = F x d; P = W ÷ t
4. Solution
 W = 500 N x 5 m = 2500 Nm
 P = 2500 Nm ÷ 30 s = 83 watts
 Allen’s power is less than a 100-watt light bulb.
Investigation 8C
People Power
Key Question:
What’s your work and
power as you climb a
flight of stairs?
Human-powered Transportation
When we move our bodies along, whether by
walking, swimming, or skiing, we exert forces
over a distance and do work.