Download Section 8.4

Monday, March 15th
Agenda
3Q Assessments
Section 8.4: Conservation of Energy
Efficiency
In-Class Assignments:
Practice pg. 270: #1
Pg. 280: #1, 2, 4, 7, 9
3Q Assessments
I feel like I did not give you enough time on
Thursday to properly complete the 3Q
assessments.
So, you will have another chance today to take
the 3Q assessment.
Whichever test is the better grade will be the
grade that you will get.
In addition, you will get ½ of your grade on the
assessment added as extra credit to your 3Q
grade.
Energy Transformations
Energy readily changes from one form to
another.
The tallest roller coaster in the
world is the Fujiyama, in Japan.
It spans 70 m from its highest to
lowest points.
Potential Energy Becomes Kinetic Energy
At the top of the tallest hill, almost all of the
energy of a car on the roller coaster is
potential energy.
The potential energy changes to
kinetic energy as the car moves
down the hill.
At the bottom of the hill, the
car has maximum kinetic energy
and minimum potential energy.
Kinetic Energy Becomes Potential Energy
When the car is at the lowest point on the
roller coaster, it has no more potential energy,
but it has a lot of kinetic energy.
This kinetic energy can do the work to carry
the car up another hill.
As the car climbs the hill,
the kinetic energy changes into
potential energy.
Energy Graphs
Notice that the total energy, either potential or
kinetic, is 354 kJ.
Energy is not created or lost, it just changes form.
Energy transformations explain the
flight of a tennis ball
As a tennis player tosses the ball up in the air
to serve, the kinetic energy will change to
potential energy until the ball reaches its
highest point.
As the ball falls back down,
the potential energy is again
transformed into kinetic
energy as the ball accelerates
downward.
Energy Transformations Explain a
Bouncing Tennis Ball
Before a serve, a tennis player usually bounces
the ball a few times.
As the ball is thrown downward, kinetic
energy is added to the potential energy that
the ball has at the height of her hand.
The kinetic energy will increase because the
potential energy is changing to kinetic energy.
When the ball hits the ground, the kinetic
energy changes to elastic potential energy and
then back to kinetic energy as the ball
bounces back up.
Mechanical energy can change to
other forms of energy.
If changes from kinetic energy to potential
energy and back again were always complete,
then balls would always bounce back to the
same height they were dropped from and cars
on roller coasters would keep gliding forever.
We know that this is not the way things really
happen.
Mechanical energy can change to
other forms of energy.
On a roller coaster, the total mechanical
energy constantly decreases due to friction
and air resistance.
When a ball bounces, some of the kinetic
energy compresses the air around the ball,
making a sound, and some of the energy
makes the ball, the air, and the ground hotter.
These forms of energy are considered nonmechanical energy.
The Law of Conservation of Energy
The law of conservation of energy states that
energy cannot be created or destroyed.
Energy doesn’t appear out of nowhere.
Whenever the total energy in a system
increases, it must be due to energy that
enters the system from an external source.
Energy doesn’t disappear,
but it can be changed to
another form.
The Law of Conservation of Energy
Scientist study energy systems.
Systems may be open or closed.
When the flow of energy into and out of a
system is small enough that it can be
ignored, the system is called a closed
system.
Most systems are open systems, which
exchange energy with the space that
surrounds them.
Efficiency
Because of friction and other factors, only
some of the work done by a machine is useful
work.
Efficiency: a quantity, usually expressed as a
percentage, that measures the ratio of useful
work output to work input
Efficiency Equation
Efficiency = useful work output
work input
To change to a percentage, multiply the
answer by 100 and add the percent sign, %.
Because every machine has some friction, no
machine has 100% efficiency.
Hint: Work output always has to be less than
work input.
Efficiency Example Pg. 279
A sailor uses a rope and an old, squeaky pulley
to raise a sail. He must do 180 J of work and
does only 140 J of work on the sail. The rest
of the work done was lost to friction. What is
the efficiency of the pulley?
Efficiency equation = useful work ouput
work input
Useful work output: 140 J
Work input: 180 J
Efficiency = 140 J/180 J = .78 X 100 = 78%
Additional Example
I takes 1,200 J of work to lift a car high enough
to change a tire. A person uses 4,800 J of
work to lift the car high enough. What is the
efficiency of the jack?
Efficiency equation = useful work output
work input
Useful work output = 1,200 J
Work input = 4,800 J
Solve: Efficiency = 1,200 J = .25 X 100 = 25%
4,800 J
Efficiency
Perpetual motion machines are impossible.
Energy is always lost to friction or air
resistance.
Machines need energy input.
Because energy always leaks out of a
system, every machine needs at least a
small amount of energy input to keep going.
In-Class Assignments
 Practice pg. 270: #1
 Pg. 280: #1, 2, 4, 7, 9
 If you don’t get these finished in-class, they WILL
BE homework.
Looking Ahead
Lab: Friday, March 19th
Chapter 8 test: Thursday, March 25th