Chapter 4Chapter 4EnergyEnergy

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Sec 1.Sec 1. The Nature of Energy The Nature of Energy EnergyEnergy—the ability to cause change.—the ability to cause change. Energy has several different forms such as Energy has several different forms such as

Electrical—making toast or turning on a lightElectrical—making toast or turning on a light Chemical—energy stored in food or gasolineChemical—energy stored in food or gasoline Thermal—from the sun, warming the planetThermal—from the sun, warming the planet

It can be transferred from one form to It can be transferred from one form to another.another.

Kinetic EnergyKinetic Energy Kinetic EnergyKinetic Energy—energy in the form of motion.—energy in the form of motion.

Examples—a spinning bicycle wheel, kicking a ball.Examples—a spinning bicycle wheel, kicking a ball. The amount of kinetic energy (KE) depends on 2 The amount of kinetic energy (KE) depends on 2

quantities:quantities: The mass of the moving object and its velocity.The mass of the moving object and its velocity. The more mass, the more kinetic energy.The more mass, the more kinetic energy.

Example: a truck and a motorcycle are both Example: a truck and a motorcycle are both moving at 100 km/hr. Which one has more KE?moving at 100 km/hr. Which one has more KE? Answer: the truck, because it has more mass.Answer: the truck, because it has more mass.

Calculating Kinetic EnergyCalculating Kinetic Energy

You can calculate kinetic energy using You can calculate kinetic energy using this equation.this equation. Kinetic Energy = ½ mass x velocity squaredKinetic Energy = ½ mass x velocity squared KE = ½ m x vKE = ½ m x v22

The SI unit of energy is the joule (J)The SI unit of energy is the joule (J)

Potential EnergyPotential Energy Energy does not always involve motion.Energy does not always involve motion. Potential EnergyPotential Energy—stored energy due to —stored energy due to

position.position. The amount of potential energy (PE) depends on the The amount of potential energy (PE) depends on the

objects position.objects position. There are different types of potential energy.There are different types of potential energy. Elastic potential energyElastic potential energy—energy stored by —energy stored by

something that can compress or stretch something that can compress or stretch (ex. rubber band)(ex. rubber band)

Chemical potential energyChemical potential energy—energy stored in —energy stored in chemical bonds chemical bonds (ex. gasoline is stored until you start your car)(ex. gasoline is stored until you start your car)

Gravitational Potential Gravitational Potential EnergyEnergy

Gravitational Potential EnergyGravitational Potential Energy (GPE) (GPE)—the —the energy stored by objects that are above Earth’s energy stored by objects that are above Earth’s surface.surface.

The amount of GPE depends on 3 thingsThe amount of GPE depends on 3 things Mass, acceleration due to gravity, & height above Mass, acceleration due to gravity, & height above

the ground.the ground. The amount of GPE can be calculatedThe amount of GPE can be calculated

GPE = mass x 9.8m/sGPE = mass x 9.8m/s22 x height x height GPE = m x 9.8 m/sGPE = m x 9.8 m/s22 x h x h or GPE = mghor GPE = mgh

Changing GPEChanging GPE

Sec 2. Conservation of EnergySec 2. Conservation of Energy Changing forms of energyChanging forms of energy

When you turn on a light bulb it When you turn on a light bulb it converts electrical energy into light converts electrical energy into light energy and thermal energy.energy and thermal energy.

Fuel stores energy in chemical Fuel stores energy in chemical bonds. A car engine transforms bonds. A car engine transforms chemical PE into KE.chemical PE into KE.

Mechanical EnergyMechanical Energy—the total —the total amount of potential and kinetic amount of potential and kinetic energy in a system.energy in a system. Mechanical energy = PE + KEMechanical energy = PE + KE

Falling ObjectsFalling Objects As an object falls, its potential As an object falls, its potential

energy decreases and its kinetic energy decreases and its kinetic energy increases.energy increases.

The amount of mechanical energy of The amount of mechanical energy of a falling object always remains the a falling object always remains the same.same.

Energy transformation also occurs Energy transformation also occurs when a baseball is hit.when a baseball is hit.

Close to the ground it has mostly Close to the ground it has mostly KE. At its highest point it has KE. At its highest point it has mostly PE, but the amount of mostly PE, but the amount of mechanical energy is constant.mechanical energy is constant.

SwingingSwinging On a swing or a pendulum the mechanical On a swing or a pendulum the mechanical

energy is also constant.energy is also constant. To get moving, you need KE. As you rise To get moving, you need KE. As you rise

higher, your GPE increases. As you move higher, your GPE increases. As you move down, your KE increases. Mechanical Energy down, your KE increases. Mechanical Energy is constant.is constant.

Law of Conservation of EnergyLaw of Conservation of Energy

Law of Conservation of energyLaw of Conservation of energy——states that energy cannot be created states that energy cannot be created or destroyed.or destroyed.

Sometimes it is hard to see energy Sometimes it is hard to see energy conserved.conserved.

Ex. Friction can cause some Ex. Friction can cause some mechanical energy to change into mechanical energy to change into thermal energy (heat).thermal energy (heat).

Ex. The sun can convert a small Ex. The sun can convert a small amount of mass into a large amount amount of mass into a large amount of energy using nuclear fusion.of energy using nuclear fusion.

Law of Conservation of EnergyLaw of Conservation of Energy Continued Continued

Your body follows the law Your body follows the law of conservation of energy.of conservation of energy. Ex. Chemical potential Ex. Chemical potential

energy is stored in the food energy is stored in the food you eat.you eat.

Ex. Your body stores energy Ex. Your body stores energy in fat and other chemicals.in fat and other chemicals.

Ex. Your body converts the Ex. Your body converts the energy to make you move.energy to make you move.

Google “Energy Changes”Google “Energy Changes” Click on secret lives of energy (2Click on secret lives of energy (2ndnd)) Find the Energy Change GameFind the Energy Change Game Download and PlayDownload and Play Need the score of …____… for your points!Need the score of …____… for your points!