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.
• Scientist study energy systems.
• Boundaries define a system.
• 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 of Machines

• Not all of the work done by a machine is useful work.
• 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.
• Efficiency is the ratio of useful work out to work in.
• Efficiency is usually expressed as a percentage.
• The efficiency of a machine is a measure of how much useful work it can do.

Efficiency Equation

Efficiency = useful work output/ work input

• 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.

EXAMPLE - Efficiency

A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage.

List the given and unknown values.

Given: work input = 180 J

useful work output = 140 J

Unknown: efficiency = ? %

Write the equation for efficiency

Efficiency = useful work output/ work input

Insert the known values into the equation, and solve.

Efficiency = 140 J/180 J = .78

to express this as a percentage multiple by 100 and add the percent sign (%)

.78 * 100 = 78% efficiency