1. Section 9.4 Conservation of Energy
Work and Energy
Section 9.4 Conservation of Energy

2. Section 9.4 Objectives
Identify and describe transformations 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
Energy can change from one form to another.
Potential energy can become kinetic energy.
Kinetic energy can become potential energy.
At the top of the cliff the rock has potential energy. As the rock falls its potential energy is transformed into kinetic energy.

4. Energy Transformations explain a bouncing ball.
As the ball is thrown down, it adds kinetic energy to the potential energy it had at its initial height.
When the ball hits the ground energy transforms from kinetic to elastic potential energy stored in the compressed ball.
The elastic potential energy then quickly changes back to kinetic energy as the ball bounces upward.
Question: Why does the tennis ball’s height decrease after each bounce?
With each bounce of a tennis ball, some of the mechanical energy changes to non mechanical energy (sound, heat). PE PE KE KE
PE elastic

5. Law of Conservation of Energy
This means that the energy in a system, such as a roller coaster, at any time can be calculated by adding the kinetic and potential energy to get the total mechanical energy.
Law of Conservation of Energy states: Energy cannot be created or destroyed.
Total energy equals 50,000J at each point.

6. Open and Closed Systems
When the flow of energy into and out a system is small enough that it can be ignored.
Ex. Light in room
Open System
Exchange of energy with the outside
Ex. Earth – receives energy from sun & radiates energy into space

7. Efficiency of Machines
Not all work done by a machine is useful work.
Efficiency is the a quantity usually expressed as a percentage, that measures the ratio of useful work output to work input.
Only an ideal machine is 100% efficient which can never happen in real life.
Formula:
Efficiency = useful work output X 100 = %
work input
Work input = useful work output
efficiency
Useful work output = Efficiency x work input

8. Section 9.4 Summary
A machine cannot do more work than the work required to operate the machine. Because of friction, the work output of a machine is always somewhat less than the work input.
The efficiency of a machine is the ration of the useful work performed by the machine to the work required to operate the machine.
Energy readily changes from one form to another.
A mechanical system, potential energy can become kinetic energy, and kinetic energy can become potential energy.
Mechanical energy can change to non mechanical energy as a result of friction, air resistance, or other means.
Energy cannot be created or destroyed, although it may change form. This is called the law of conservation of energy.