12 Conservation of Energy

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

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!

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

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

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

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.

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

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)

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)

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

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

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

Practice Problems Complete problems 1-3 on page 407 Extra credit opportunity!!! +12pts Complete the chapter review pages 410-412 #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!