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Chapter 6:
Energy, Work and
Simple Machines
6A
Work and Power
6A Objectives
Describe the relationship between work and
energy.
Display an ability to calculate work done by a
force.
Identify the force that does work.
Differentiate between work and power and
correctly calculate power used.
Concept Development Map
Applications
Chemical Energy
What is it?
The property of an object that
allows it to produce change in
itself or its environment.
Definitions
Latin energia =
en (in) + ergon (work)
= active
Thermal Energy
Energy
Internal or
Inherent Power
Nuclear Energy
Motion Energy
(momentum)
Potential Energy
Energetic, energize
Examples
I ran out of energy.
I need to re-energize.
I don’t feel very energetic.
Concept Development Map
Applications
Engines
What is it?
Definitions
The process of changing energy
of a system by means of forces.
Middle European
werk, wirk, work = to do
Springs; Pulleys
Exertion of Strength
Work
The matter on which someone
labors on.
Human Efforts
Examples
Torque
This is a work of art!
She’s a real
piece of work!
I need to get to work.
WORK
MOSHER’
S

Work Defined
Work (W): The product of the force on an object
and the distance through which the object is
moved.
Work  Force  distance
or in Symbols:
W  Fd
Let’s compare this new W with I (impulse)…
Work = Force X Distance
Where is the Force and where is
the distance in this picture?
Work Defined
Work Clarified
W  (F cos  )  d
Be careful!
Only the force in the same direction as the motion
counts towards the work.
Work Clarified
W  (F cos  )  d
Case 1. A man pushes against a car stuck in a snow
bank while his date sits nervously behind the
steering wheel trying not to make the tires spin.
However, the car does not move. How much work
did he do on the car?
Answer: The man did no work on the car since
d=0. He may have burned calories, converting
chemical energy into heat, but still, the car did
not move.
Work Clarified
W  (F cos  )  d
Case 2. Sally carries a 0.5 kg textbook under her
arm along a horizontal path. How much work was
done on the text book?
Answer: None, since both gravity and the force
Sally exerted against gravity are perpendicular to
the distance the book moved. (cosø = 0 so W = 0).
Work Clarified
W  (F cos  )  d
Case 3. An asteroid traveling at constant velocity
out of reach of gravitational fields [etc.]... How
much work is done on the satellite?

Answer: None, since F = 0, W = 0.
Power
Power - the rate of doing work. Work per
unit time. 1 Watt = 1 Joule/sec.
W Fd
P

 Fv
t
t
James Watts
Compare:

Pt  W
Units:
Nm
Ft  I
Units:
Ns
6A Conclusions
Work - a force applied over a certain distance.
Force times distance has units of 1 N m = 1
Joule.

Power - energy of an object due to its motion.
Units of 1 kg m2/s3 = 1 Watt=1 J/s.

6B Mechanical Energy: Potential and Kinetic
Mechanical Energy Defined
Mechanical Energy (ME): Energy due to the
position or the movement of something; potential
energy or kinetic energy or a combination of both.
ME  PE  KE
But just what is potential energy and kinetic

energy?
What do these have to do with Potential?
Potential Energy Defined
Potential Energy (PE): “Height” energy, position
energy. It is usually related to the relative
position of two things, such as a stone and the
earth (gravitational PE), or an electron and a
nucleus (Electric PE).
PE  mgh
h – relative to reference level;
hground = 0.
What
does
this
have to
do with
potential
Energy?
Work?
Kinetic Energy Defined
Kinetic Energy (KE): Motion energy. Equal to half
the mass multiplied by the speed (scalar!) squared.
KE  mv
1
2
2
Kinetic Energy Defined
When
does
potential
become
kinetic?
(when does
HEIGHT energy
become
MOTION energy)
When
does
potential
become
kinetic?
When
does
potential
become
kinetic?
Bill Nye: Energy (0:00 to 6:00)
6B Conclusions
Potential Energy – energy due to position = height
energy = mgh
Kinetic Energy - energy of an object due to its
motion. Units of 1 kg m2/s2 = 1 Joule.

Energy Transfer – Potential energy can be
turned into kinetic energy and visa versa.
Roller coasters and pendulums are examples
of this.
6C Energy Theorems and Conservation
6C Objectives
Solve problems using the work-energy
theorem.
Solve problems using the law of conservation
of energy.
Work versus Impulse
Starting with the Impulse-Momentum
Theorem:
I  Ft  mv  mv2  mv1
Multiplying both sides by d/t:

d
d
Ft 
 mv 
t
t
Work versus Impulse (cont’d)
This simplifies to:
d
Fd  mv 
t
Substitution of vavg = d/t = (v2+v1)/2:
1
1
Fd  mv  (v 2  v1)  m(v 2  v1)(v 2  v1 )
2
2

Work - Energy Theorem
This simplifies to:
1
Fd  m(v 2  v1)(v 2  v1 ) 
2
1
1
2
2
m(v 2  v1v 2  v1v 2  v1 )  m(v 22  v12 )
2
2
This is the Work-Energy Theorem:
1

Work  Fd  mv 2  K  Kinetic Energy Change
2
Work - Impulse Comparison
Let’s compare the two theorems:
Impulse-Momentum Theorem:
Ft  mv  mv2  mv1

Work-Energy Theorem:
1
1 2 1 2
2
Fd  mv  mv2  mv1
2
2
2
Kinetic Energy vs. Momentum
Another conclusion:
Kinetic Energy is the derivative
of Momentum
1
2
2
d(K.E.) d( mv )

 mv
dt
dt
Work - Impulse Comparison
Let’s compare the two graphs:
Impulse (Force versus Time)
Force
Time
Work (Force versus Distance)
Conservation of Mechanical Energy
Mechanical Energy is the sum of kinetic
energy and gravitational energy. It cannot
change in an ideal system.
ME  0  KE  PE  mv  mgh
1
2
The decrease in potential energy
is equal to the increase in kinetic energy.
2
The decrease in kinetic energy
is equal to the increase in potential energy.
Conservation in a Pendulum
Simple Harmonic Motion conserves
energy on each swing.
ME  0  KE  PE  12 mv 2  mgh

The decrease in potential
energy is equal to the
increase in kinetic energy.
The decrease in kinetic
energy is equal to the
increase in potential energy.
Momentum and Kinetic Energy
Conservation
Hew35: Bowling Ball
Conservation Of Energy
Hew36: Math Example
Conservation Of Energy
Bill Nye: Energy (6:00 to 12:56)
6D Machines
6D Objectives
Demonstrate Knowledge of why simple
machines are useful.
Communicate an understanding of
mechanical advantage in ideal and real
machines.
Analyze compound machines and describe
them in terms of simple machines.
Calculate efficiencies for simple and
compound machines.
6D Vocabulary
Machine - A device that changes the magnitude or
the direction of the force needed to do work,
making the task easier to accomplish.
Simple Machine - A lever, pulley, gear, wheel
and axle, inclined plane, wedge, or screw.
Compound Machine - A device that consists
of two or more simple machines linked so that
the resistance force of one machine becomes
the effort force of the second machine.
Japanese Rube Goldberg Machines
Rube Goldberg Machines #1
Rube Goldberg Machines #2
Mechanical Advantage - Pulleys
Mechanical Advantage - Pulleys
Mechanical Advantage
Mechanical Advantage - The ratio of the
resistance (r) force to the effort (e) force.
Fr
MA 
Fe
Ideal Mechanical Advantage - The ratio of
the resistance distance to the effort distance.

de
IMA 
dr
Torque Balance - The resistance torque equals the effort torque.
Fr dr  Fe de

Fr de
MA 
  IMA
Fe dr
Efficiency
Percent Efficiency - The ratio of the
output work to the input work times 100%.
W out
% Efficiency 
100%
W in

(Fd)out Fr dr MA


(Fd)in Fe de IMA
Energy Transfer in a Coupled Pendulum
Coupled Pendulum
Baseball-Basketball Bounce
Demo:
Place a baseball on top of a
basketball. Drop both at the
same time on the floor and
see what happens. What do
you think will happen? Why?
Baseball-Basketball Bounce
What does all this have to do with baseball or sports
in general?
When you bounce a baseball off a basketball, you are transferring
energy from the deformation of the basketball to the baseball. When
you bounce a baseball off a bat, you are transferring energy from the
bat to the baseball. How well a ball bounces off the basketball has to
do with timing. When the basketball hits the floor, it squashes the
bottom a bit. When it springs back to its original shape, it pushes off
the floor -- it bounces. The baseball indents into the basketball on the
top. When the basketball returns to its round shape all the energy is
transferred to the baseball. The effect is similar to a man on a
trampoline.
Conservation of Mechanical Energy
The Amazing Oscillating Spring Thing
ME  0  KE  PE  U e
 mv  mgh  U e
1
2

2
Efficiency
Ideal Machines:
(% Efficiency)IDEAL 100%
Real Machines:


(% Efficiency)REAL 100%
Bill Nye: Energy (12:56 to END)