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Bellringer #51
 Read
“Careers Using Physics” on page 427
 Answer
 Leave
both questions in your journal
your books out and open
Bellringer #45

Read “What Machines are used on
Bicycles” on page 435

Answer both questions in your journal
What is work?

Which picture shows someone who is working?
What is work?

Work is being done when you
Push
 Lift
 Throw


You do work when you move something from
one place to another.

Your force makes the object move
What is work?
Work

If you push on the wall is it work?

Work is not done every time a force is applied.

A force must be exerted over a distance for it to
be work.

The force must be applied in the direction of the
object’s motion.
 You
pick up a bag of groceries.
 Your
arms are applying a force on
the bag to hold it up.
 You
carry the bag of groceries to
the car.
 Have
your arms done any work on
the bag?
What Is Work?
Work is calculated by multiplying the force by the
distance over which the force is applied.
work = force x distance, or W = Fd
Work is zero when an object is not moving.
Work is measured in joules (J):
1 N • m = 1 J = 1 kg • m2/s2
Math Skills
Imagine a father playing with his daughter by lifting her
repeatedly in the air. How much work does he do with
each lift if he lifts her 2.0 m and exerts an average force
of 190 N?
1. Underline and label knowns and circle unknowns.
2. Write the equation for work.
1) W = f x d
3. Insert the known values into
the equation.
2) W = 190 N x 2.0 m
4. Solve.
3) W = 380 J
Power

Does running up the stairs require more power
than walking up the stairs?
Power
-a quantity that measures the rate at which work is done
or energy is transformed
〉What
is the relationship between work
and power?
〉Power is the rate at which work is done,
or how much work is done in a given
amount of time.
work
W
power 
, or P 
time
t
Power is measured in watts (W): 1 W = 1 J/s
Watts the matter with you ?

Watt expresses electric power.

A 50-watt light bulb does the
work at a rate of 50 joules per
second.

Large quantities of power are
measured in kilowatts (kW)

1 kilowatt = 1000 watts

A bulldozer has more
power than a person
with a shovel.

If a bulldozer and a
person were working
for 1 hour. Which
would do more work?
Math Skills
Lifting an elevator 18 m takes 100 kJ. If doing so takes 20
s, what is the average power of the elevator during the
process?
1. Underline and label knowns and circle unknowns.
2. Write the equation for power.
P = work
time
3. Insert the known values into
the equation.
P = 1 x 105 J
20 s
4. Solve.
P  5  103 W  5 kW
Machines and Mechanical
Advantage
〉How
do machines make work easier?
〉Machines
help do work by changing the
size of an input force, the direction of the
force, or both.
mechanical advantage: a quantity that expresses
how much a machine multiplies force or distance
Math Skills
output force
input distance
mechanical advantage 

input force output distance
Mechanical Advantage
Calculate the mechanical advantage of a ramp that is 5.0 m
long and 1.5 m high.
input distance
mechanical advantage =
output distance
mechanical advantage =
5.0 m
1.5 m
Mechanical advantage = 3.3
Efficiency of Machines

Not all of the work done by a machine is useful
work.


because of friction, work output < work input
Efficiency is the ratio of useful work out to
work in.
Efficiency - a quantity, usually expressed as a
percentage, that measures the ratio of
useful work output to work input
useful work output
efficiency 
work input
Math Skills
Efficiency
A sailor uses a rope and an old, squeaky pulley to raise a sail
that weighs 140 N. He finds that he must do 180 J of work on
the rope to raise the sail by 1 m. (He does 140 J of work on the
sail.) What is the efficiency of the pulley? Express your
answer as a percentage.
useful work output
efficiency 
work input
efficiency 
140 J
 0.78
180 J
efficiency  0.78  100%  78%
Bellringer #46

Make a table in your journal that looks like
this
Incline Plane

Wedge
Screw
You will be watching a clip and writing down
all of the examples of the following simple
machines that are discussed in the clip.
What Are Simple Machines?

Simple machines are divided into two
families: the lever family and the inclined
plane family.
Lever family:
 simple lever
 pulley
 wheel and axle
Inclined plane family:
 simple inclined plane
 wedge
 screw

C:\Physical Science 2006-7\Physical
Science 2\Phys. Sci. Movies\ch. 13
(best)Work__Energy__and_the_Simple_M
achine__Lever__Wheel_and_Axle__Pulle
y_.asf
The Lever Family
〉Two
principal parts of all levers?
〉All
levers have a rigid arm that turns
around a point called the fulcrum.
•
Levers are divided into three classes.
The Lever Family, continued

Pulleys are modified levers.



The point in the middle of a pulley is like the fulcrum
of a lever.
The rest of the pulley behaves like the rigid arm of a
first-class lever.
A wheel and axle is a lever or pulley
connected to a shaft.

Screwdrivers and cranks are common wheel-and-axel
machines.

C:\Physical Science 2006-7\Physical
Science 2\Phys. Sci. Movies\ch. 13 (best)
Work__Energy__and_the_Simple_Machin
e__Inclined_Plane__Wedge__Screw_.asf
The Inclined Plane Family
〉How
does using an inclined plane
change the force required to do work?
〉Pushing
an object up an inclined
plane requires less input force than
lifting the same object does.
Wedge - is a modified inclined plane.
Screw - is an inclined plane wrapped around a
cylinder.
Pitch
M.A. = Pitch / Distance
Compound Machines
compound machine: a machine made of
more than one simple machine
Homework

Chapter 13 S.2 Concept Review and Quiz
Bellringer # 44
 How
do you think the Egyptians built
the pyramids? (hint: think about simple
machines)

Ideal MA = input distance/ output distance

Actual MA= output force/ input force

% Efficiency= actual MA/ ideal MA x 100
Output
force
And
Distance
Input force
and
distance
Homework

Section 2 Review Questions page 443

#1-8
Bellringer #48
 Read
“Energy Stored in Plants” on page 451
 Answer
both questions in your journal.
Bellringer #48

Make a table in your journal that looks like
this
Wheel and Axle

Pulley
Lever
You will be watching a clip and writing down
all of the examples of the following simple
machines that are discussed in the clip.
Is work being done?
Energy and Work
What is the relationship between energy and work?
Whenever work is done, energy is transformed or is
transferred from one system to another system.
energy: the capacity to do work
Energy is measured in joules (J).

If something does
not move, has work
happened?

If something does
not move, is energy
present?
Why Joules?

Energy can be
calculated by how
much work is done on
the object that energy
is transferred to .

Work and Energy are
expressed as Joules
Potential Energy
- the energy that an object has because of the position,
shape, or condition of the object

Any object that is stretched or compressed to increase or
decrease the distance between its parts has elastic potential
energy.

Examples: stretched bungee cords, compressed springs

Any system of two or more objects separated by a vertical
distance has gravitational potential energy.

Example: a roller coaster at the top of a hill
Gravitational potential energy depends on both mass and
height.
grav. PE = mass  free-fall acceleration  height, or
PE = mgh
The height can be relative.
Math Skills
A 65 kg rock climber ascends a cliff. What is the
climber’s gravitational potential energy at a point 35
m above the base of the cliff?
Math Skills
1) PE = mgh
2) PE = (65 kg)(9.8 m/s2)(35 m)
3) PE = 2.2  104 kg•m2/s2
4) PE = 2.2  104 J
Kinetic Energy
- the energy of an object due to the object’s motion
Kinetic energy depends on both the mass and the speed
of an object.
Equation:
KE = ½  mass  speed squared, or
KE= ½mv2
Math Skills
Kinetic Energy
What is the kinetic energy of a 44 kg cheetah running at 31
m/s?
1.
2.
Underline and label the given values and circle the
unknown values.
1
kinetic
energy

 mass  speed squared
Write the Equation
2
KE 
3.
1
mv 2
2
Insert the known values into the equation
KE = ½ (44 kg)(31 m/s)2 = 2.1 × 104 kg•m2/s2
4.
Solve
KE = 2.1 × 10 J
4
Other Forms of Energy
1. mechanical energy
- the amount of work an object can do because of the object’s
kinetic and potential energies.
2. nonmechanical energy
- Energy that lies at the level of the atom.
3. chemical energy
- Chemical reactions involve potential energy.
Other Forms of Energy
4. nuclear fusion
- nuclear reactions within the sun
5. electric energy (energy stored a field)
-Electrical energy results from the location of charged particles in an electric
field.
-When electrons move from an area of higher electric potential to an area of
lower electric potential, they gain energy.
6. electromagnetic waves
- Light energy traveling from the sun to Earth across empty space.
- Electromagnetic waves are made electric and magnetic fields, so light
energy is another example of energy stored in a field.
Homework

S.3 Concept Review and Quiz

Study your vocab for 6 minutes
Section 4: Conservation of Energy
Energy Transformations
Energy readily changes from one form to another.

Potential energy can become kinetic energy.


Kinetic energy can become potential energy.


Example: As a roller coaster car goes down a hill, PE changes
to KE.
Example: The KE of a roller coaster car at the bottom of a hill
can do work to carry it up another hill.
Mechanical energy can change to other forms of
energy.
Graphing Skills
Graphing Mechanical Energy
The bar graph shown here
presents data about a
roller coaster car.
What variables are plotted?
What does the legend tell
you about this graph?
1. Study the axes and
legend to determine
the variables.
Location is the variable
on the x-axis. Two
variables are plotted on
the y-axis: kinetic and
potential energy.
2. Consider the
relationship between
the variables.
The independent
variable is location,
because the car’s
kinetic energy and
potential energy
change with location.
3.
Examine the
legend and how it
relates to the graph.
The legend indicates
that the car’s
mechanical energy
consists of both kinetic
energy and potential
energy.
The Law of Conservation of Energy
Energy cannot be created or destroyed.
-In other words, the total amount of energy in the universe
never changes, although energy may change from one form
to another.
1. Energy does not appear or disappear.
2. Whenever the total energy in a system increases, it must be
due to energy that enters the system from an external
source.
3. Thermodynamics describes energy conservation.
4. For any system, the net change in energy equals the
energy
transferred as work and as heat. This form of the law of
energy conservation is called the first law of
thermodynamics.
The Law of Conservation of
Energy, continued
•
Systems may be open, closed, or isolated.
•
open system: energy and matter are exchanged with the
surroundings
•
closed system: energy but not matter is exchanged
•
isolated system: neither energy nor matter is exchanged
•
•
Most real-world systems are open.
Homework

S.4 Review Questions

Pg.461

#1-9
Bellringer #49

List the units (or a unit) for the following:
 Speed =
 Distance =
 Time =
 Acceleration =
 Force =
 Mass =
 Momentum=
 Weight =
 Work =
 Power =
 Mechanical Advantage =
 Energy =
 Efficiency =