Monday, March 15th

Agenda3Q AssessmentsSection 8.4: Conservation of Energy

EfficiencyIn-Class Assignments:

Practice pg. 270: #1Pg. 280: #1, 2, 4, 7, 9

3Q AssessmentsI 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 TransformationsEnergy readily changes from one form to

another.

The tallest roller coaster in theworld is the Fujiyama, in Japan.

It spans 70 m from its highest tolowest points.

Potential Energy Becomes Kinetic EnergyAt 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 movesdown the hill.

At the bottom of the hill, the car has maximum kinetic energyand minimum potential energy.

Kinetic Energy Becomes Potential EnergyWhen 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 non-mechanical energy.

The Law of Conservation of EnergyThe 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 EnergyScientist 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.

EfficiencyBecause 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. 279A 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 JWork input: 180 JEfficiency = 140 J/180 J = .78 X 100 = 78%

Additional ExampleI 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 JWork input = 4,800 JSolve: 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: #1Pg. 280: #1, 2, 4, 7, 9

If you don’t get these finished in-class, they WILL BE homework.

Looking AheadLab: Friday, March 19th

Chapter 8 test: Thursday, March 25th