Energy Book Chapter 15. Energy Work is a transfer of energy. Mechanical energy is when objects have the ability to do work. There are two types of mechanical.

Energy

Book Chapter 15

Energy

Work is a transfer of energy.

Mechanical energy is when objects have the ability to do work.

There are two types of mechanical energy:1. Potential Energy (PE)2. Kinetic Energy (KE)

Kinetic Energy is energy in motion

Any object in motion has kinetic energy.The faster the object, or the more massive the

object is, the more kinetic energy it has.

KE = ½ (mass)(velocity)2 = ½ mv2

Kinetic energy can also be found in acoustical energy (sound), thermal energy (heat), etc.

Calculating Kinetic Energy

Mass = 1500 kg Velocity = 25 m/s

Kinetic Energy = ½ (1500)(25)2 = 468,750 J

Potential Energy is stored energy

Potential energy can be in many different forms.

Elastic potential energy: stored energy in an elongated rubber band or spring.

Chemical potential energy: the amount of energy stored in food (calories).

Gravitational potential energy: depends on how high an object is above the ground, or the amount of work it took to move that object to that height.

Gravitational Potential Energy

The higher the object is, the more work that can be done on another object. This height depends on what reference point is chosen.

PE = (mass)(gravity)(height) = mgh

Mass = 3 kg

Gravity = 9.8 m/s2

Height = 10 m

PE = 294 Joules (J)

Elastic Potential Energy

Elastic potential energy is the energy of an object that is compressed or stretched.

Example: A guitar string, a rubber band

Types of Energy

There are 6 main forms of energy:

1. Mechanical Energy

2. Heat/Thermal Energy

3. Chemical Energy

4. Electrical Energy

5. Electromagnetic Energy

6. Nuclear Energy

Law of Conservation of Energy

Energy can not be created or destroyed. It can be transformed from one form into another, but the total amount of energy in any system stays constant.

Total amount of energy is the sum of the potential and kinetic energies.

Conservation of Energy

Half way down:

PE = 5000 J

KE = 5000 JAt the bottom:

PE = 0 J

KE = 10,000 JAt the top:

PE = 10,000J

KE = 0 J

Total Energy of the system is always 10,000 J

Conservation of Energy

Quiz

Write the definition of Kinetic Energy.

Write the definition of Potential Energy

Calculate the Potential Energy of a 50 kg mass that is sitting on a 500 m cliff.

Does a bird sitting in a tree have kinetic energy?

The Pendulum

An object that is suspended so that it can swing back and forth about an axis is called a pendulum.

The swing of a pendulum is an example of simple harmonic motion, which means it repeats itself over and over.

Energy changes in a pendulum

The Pendulum

The time of a back-and-forth swing of a pendulum is called the period of a pendulum. It is measured in seconds.

The period of a pendulum only depends on the length of the pendulum and the acceleration of gravity. (see example)

Calculating the period of a pendulum

Use the equation:

Where

T= period of pendulum

l = length of pendulum

g = gravity

Calculating the period of a pendulum

An astronaut sets up a pendulum on the moon, where gravity is 1.6 m/s2. If the pendulum is 1 meter long, what will the period of the pendulum be?

sec97.4

)79.0(26.1

12

2

T

T

T

g

lT

Return to Home Page

Energy and Mass

Einstein’s equation, E=mc2, says that energy and mass are equivalent and can be converted into each other.

Energy is released as matter is destroyed, and matter can be created from energy.

Nonrenewable Energy Resources

Nonrenewable energy resources exist in limited quantities and, once used, cannot be replaced over the course of millions of years.

Nonrenewable energy resources include oil, natural gas, coal, and uranium. Oil, natural gas, and coal are known as fossil fuels.

Renewable Energy Resources

Renewable energy resources are resources that can be replaced in a relatively short period of time. Most renewable energy resources originate either directly or indirectly from the sun.

Renewable energy resourcesinclude hydroelectric, solar,geothermal, wind, biomass,and, possibly in the future,nuclear fusion.

Renewable Energy Resources

Energy obtained from flowing water is known as hydroelectric energy.

Solar energy is when sunlight is converted into usable energy.

Geothermal energy is the heat beneath the Earth’s surface.

Other Renewable Resources

Sunlight causes living things to grow, converting the sun’s electromagnetic energy into chemical energy. The chemical energy stored in living things is called biomass energy.

A hydrogen fuel cell by reacting hydrogen with oxygen.

Conserving Energy Resources

Energy resources can be conserved by reducing energy needs and by increasing the efficiency of energy use. Finding ways to use less energy or to use energy more efficiently is known as energy conservation.