5.1 WorkWork – In science, work is defined as

force times distance.

Joule – The unit of work.

Efficiency and Loss of Input• When most or all of the work input into a

machine is transferred to output, it is said to be efficient. In real machines there is always less output than input. Friction and other forces use up some of the work energy.

• Efficiency is measured in a ratio of work output to work input. Divide output by input. Convert to a percent by multiplying by 100.

What is the efficiency of this machine?

%Efficiency = Work out (100)

Work in

= 3 J (100)

4 J

= 75%

PowerPower – The rate at which work is done

is called power.

Watt – The unit of power. It is equal to one joule of work per second.

5.2 Energy Conservation

• Energy is the ability to do work. It is the ability to do work so it is measured in joules as well.

Law of Conservation of energy• Energy can never be created or destroyed, just

transformed from one form into another. The sum of the potential and kinetic energy will always be the same, although some of the energy may be converted to frictional heat, but it is not destroyed.

Potential EnergyPotential energy comes from the

position of an object relative to the Earth. It is stored energy. Potential energy is not “used” until the object is moved to a lower position.

Kinetic EnergyThe energy of motion. An object in motion

stores energy in this motion. It is the energy of motion. It increases with speed. It also can increase with mass.

Conservation of EnergyThe total energy of a system can be calculated by

using the Potential energy equation for the object at it’s highest position.

m = 10 kg

30 m

EP = mgh

m =

g =

h =

10 kg

9.8 m/s2

30 m

= 10kg (9.8 m/s2) 30m

EP= 2940 J

What about if the ball fell one step? What would happen to the energy?

This is the total energy that the ball will ever have.

m = 10 kg

30 m

EP = mgh Can you find the potential energy of the ball on this step?

m =

g =

h =

10 kg

9.8 m/s2

20 m

20 m

EP= 10kg (9.8 m/s2) 20m

EP= 1960 J

What was the total energy of this system?

2940 JSo what happened to the rest of the energy? It was expressed as kinetic energy as

the ball fell.How much kinetic energy?

Conservation of energy states the total energy of the system should equal all the parts of the energy. ETotal= EP + EK

2940 J = 1960 J + EK

EK = 980 J