Chapter 9 Energy

Section 1: Objectives• Explain the relationship between

energy and work.

• Compare kinetic energy and potential energy.

• Describe the different forms of energy.

Energy and Work• Energy is the ability to do work.

• Work is done when a force causes an object to move in the direction of the force. • Work is a transfer of energy.

• Energy and work are expressed in units of joules (J).

Energy and Work• Kinetic energy is the energy of

motion. • All moving objects have kinetic energy.

• Kinetic Energy Depends on Mass and Speed If you know an object’s mass (m) and its speed (v), you can calculate the object’s kinetic energy with the following equation:

Energy and WorkThe formula used to calculate

kinetic energy:KE = (mass)(volume)^2

2

Energy is also expressed in Joules.2

Calculating Kinetic Energy

Example # 1What is the KE of a car that has a

mass of 2,400 kg and is moving at 20 m/s?

Remember: KE = (mass)(volume)^2

2

Energy is expressed in Joules

Example # 2A) What is the KE of a 4,000 kg

elephant that is running at 2 m/s?

B) What about the same elephant running at 4 m/s?

Example # 3A)What is the KE of a 2,000 kg

bus that is moving at 30 m/s?

B) What about the same bus moving at 40 m/s?

Energy and Work• Potential energy is the energy an

object has because of its position.

• Gravitational Potential Energy The amount of GPE that an object has depends on its weight and its height.

Energy and Work• The equation to find gravitational

potential energy is:

GPE weight height

• Gravitational potential energy is equal to the amount of work done on an object to lift it a certain height.

Calculating GPE

Example # 1What is the GPE of a cat that

weighs 40 N and is standing on a table 0.8 meters off the ground?

Remember: GPE weight heightEnergy is expressed in Joules.

Example # 2What is the GPE of a diver who

weighs 500 N and is standing on a platform 10 meters off the ground?

Remember: GPE weight heightEnergy is expressed in Joules.

Example # 3What is the GPE of a diver who

weighs 600 N and is standing on a platform that is 8 meters off the ground?

Remember: GPE weight heightEnergy is expressed in Joules.

Energy and WorkWhen you find out an object’s

gravitational potential energy, the “ground” that you measure the object’s height from depends on where it is.

The height you use in calculating gravitational potential energy is a measure of how far an object has to fall.

Energy and Work• Mechanical energy is the total energy of

motion and position of an object. • Both kinetic energy and potential energy are

kinds of mechanical energy.

• The equation to find mechanical energy is:

mechanical energy potential energy kinetic energy

Energy and Work• The mechanical energy of an object

remains the same unless it transfers some energy to another object.

• But even if the mechanical energy of an object stays the same, the potential energy or kinetic energy can increase or decrease.

Energy and Work• Thermal Energy is all of the kinetic energy

due to random motion of the particles that make up an object.

• All matter is made up of particles that are always in random motion. • So, all matter has thermal energy.

• Thermal energy increases as temperature increases and increases as the number of particles increases.

Thermal Energy

Energy and Work• Chemical Energy is the energy of a

chemical compound that changes as its atoms are rearranged.

• Chemical energy is a form of potential energy because it depends on the position and arrangement of the atoms in a compound.

• The energy in food is chemical energy.

Energy and Work• Electrical Energy is the energy of moving

electrons. • Electrical energy can be thought of as potential

energy that is used when you plug in an electrical appliance and use it.

• Sound Energy is caused by an object’s vibrations. • The object’s vibrations transmit some kinetic

energy to the air particles, which also vibrate. These vibrations transmit sound energy.

Energy and Work• Light Energy is produced by the

vibrations of electrically charged particles.

• Nuclear Energy is energy that comes from changes in the nucleus of an atom. • Nuclear energy can be produced when

nuclei are joined in a fusion reaction or when a nucleus is split apart in a fission reaction.

Chapter 9 Sec. 1 Pop Quiz1) What 2 things must happen for

work to be done on an object?2) T/F All moving objects have

kinetic energy.3) What is the formula used to

calculate KE?4) What 2 things does GPE

depend on?5) What is the formula used to

calculate GPE?

Chapter 9 Sec. 1 Pop Quiz6) List 2 kinds of mechanical

energy.7) What is the formula used to

calculate mechanical energy?8) Describe the particles that

make up matter.9) Why is chemical energy a form

of potential energy?10) List 2 ways nuclear energy

can be produced.

Section 2: Objectives• Describe an energy conversion.

• Give examples of energy conversions for the different forms of energy.

• Explain how energy conversions make energy useful.

• Explain the role of machines in energy conversions.

Energy Conversions• An energy conversion is a change from

one form of energy to another.

• Any form of energy can change into any other form of energy.

• As the skateboarder on the next slide travels up and down the half-pipe, his energy changes back and forth between kinetic energy and potential energy.

Energy Conversions

Energy Conversions• Elastic Potential Energy Stretching

a rubber band stores elastic potential energy in the rubber band.

• When you let the rubber band go, it goes back to its original shape.

• It releases its stored-up potential energy as it does so.

Energy Conversions• Chemical energy is stored in the food you eat.

• Your body uses this chemical energy to function.

• Energy Conversion in Plants The chemical energy in the food you eat comes from the sun’s energy.

• Plants use photosynthesis to convert light energy into chemical energy.

Energy Conversions

Energy Conversions• Plants change light energy into chemical

energy.

• The chemical energy in the food you eat is changed into another kind of chemical energy that your body can use.

• Your body then uses that energy to give you kinetic energy that you use in everything you do.

Energy Conversions• Energy conversions are needed for

everything we do.

• Heating our homes, getting energy from a meal, and many other things use energy conversions.

• Machines help harness energy and make that energy work for you.

Energy ConversionsConversions Involving Electrical Energy Some

common energy conversions that involve electrical energy are shown in the table below.

Alarm clockelectrical energy light and sound energyBatterychemical energy electrical energyLight bulbelectrical energy light and thermal energyBlenderelectrical energy kinetic and sound energy

Energy Conversions• A machine can make work easier by changing

the size or direction (or both) of the force needed to do the work.

• Some machines allow you to use less force over a greater distance to do the same amount of work.

• Machines as Energy Converters Some machines help you use energy by converting it into the form of energy that you need.

Energy Conversions