1. 12 Conservation of Energy

2. Objectives Identify and describe transformation of energy
Explain the law of conservation of energy
Discuss where energy goes when it seems to disappear
Analyze the efficiency of machines

3. Energy transformations
Ex) roller coaster
All of the energy required for the entire ride comes from work done by the conveyor belt as it lifts the cars and passengers up the first hill
The energy from that initial work is stored as gravitational potential energy at the top of the 1st hill.
After that energy goes through many transformations; from potential to kinetic
Whatever from the energy takes during the ride is present from the beginning!

4. Potential energy can become kinetic energy and kinetic energy can become potential energy
Turn to page 401 in text book
In note book copy figure 19 A & B

5. Energy transformation explains the flight of a ball and the bouncing of a ball
The kinetic energy of a ball at the bottom of its path equals the potential energy at the top of the path

6. Mechanical energy can change to other forms of energy
Balls don’t bounce forever and roller coasters don’t glide forever!!!
Not all of the kinetic energy changes back to potential energy
Some kinetic energy goes to producing sound or heat.
These other forms of energy are not directly due to the motion or position of the ball
These forms of energy are considered to be nonmechanical

7. Ex) bouncing ball Ex) roller coaster
With each bounce of a tennis ball, some of the mechanical energy changes to nonmechanical energy
Ex) roller coaster
The total mechanical energy of a car on a roller coaster constantly decreases due to friction and air resistance. This energy doesn’t disappear; some increases the temperature of the track, the car’s wheels, and the air. Some of the energy compresses the air, making a roaring sound.

8. The law of conservation of energy
Energy cannot be created or destroyed
At any given time, the total energy should be the same in a system
Energy doesn’t appear out of no where
Energy doesn’t disappear

9. Systems
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 surround them (Ex. Earth)

10. Efficiency of Machines
Not all of the work done by a machine is useful work
Due to friction and the transformation to other energy forms
(heat, noise, vibration…etc)

11. Efficiency
Efficiency- a quantity, usually expressed as a percentage, that measures the ratio of useful work output to work input
Write down on index card!!!
Efficiency= useful work output
work input

12. Efficiency as a % Efficiency is usually expressed as a percentage.
To change an answer found using the efficiency equation into a percentage, just multiply the answer by 100 and add the percent sign, %.
Because every machine has friction, no machine has 100% efficiency

13. Efficiency Example
A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140N. He finds that he must do 180J of work on the rope in order to raise the sail by 1m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a %.
Given: work input= 180J
useful work output = 140J
unknown= efficiency %
Efficiency= useful work output
work input

14. Practice Problems Complete problems 1-3 on page 407
Extra credit opportunity!!! +12pts
Complete the chapter review pages
#s 1-24 (skip 14 &15)
Multiple choice, write questions and answers
Good way to study, bring up points for missed assignments or late current events!
Take advantage, due Thursday, no exceptions!