Chapter 4

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# Chapter 4 - PowerPoint PPT Presentation

Chapter 4. Energy. http://www.nsf.gov/news/special_reports/olympics/snowboarding.jsp. Sec 1. The Nature of Energy. Energy —the ability to cause change. Energy has several different forms such as Electrical—making toast or turning on a light Chemical—energy stored in food or gasoline

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### Chapter 4

Energy

http://www.nsf.gov/news/special_reports/olympics/snowboarding.jsp

Sec 1. The Nature of Energy
• Energy—the ability to cause change.
• Energy has several different forms such as
• Electrical—making toast or turning on a light
• Chemical—energy stored in food or gasoline
• Thermal—from the sun, warming the planet
• It can be transferred from one form to another.
Kinetic Energy
• Kinetic Energy—energy in the form of motion.
• Examples—a spinning bicycle wheel, kicking a ball.
• The amount of kinetic energy (KE) depends on 2 quantities:
• The mass of the moving object and its velocity.
• The more mass, the more kinetic energy.
• Example: a truck and a motorcycle are both moving at 100 km/hr. Which one has more KE?
• Answer: the truck, because it has more mass.
Calculating Kinetic Energy
• You can calculate kinetic energy using this equation.
• Kinetic Energy = ½ mass x velocity squared
• KE = ½ m x v2
• The SI unit of energy is the joule (J)
Potential Energy
• Energy does not always involve motion.
• Potential Energy—stored energy due to position.
• The amount of potential energy (PE) depends on the objects position.
• There are different types of potential energy.
• Elastic potential energy—energy stored by something that can compress or stretch (ex. rubber band)
• Chemical potential energy—energy stored in chemical bonds (ex. gasoline is stored until you start your car)
Gravitational Potential Energy
• Gravitational Potential Energy (GPE)—the energy stored by objects that are above Earth’s surface.
• The amount of GPE depends on 3 things
• Mass, acceleration due to gravity, & height above the ground.
• The amount of GPE can be calculated
• GPE = mass x 9.8m/s2 x height
• GPE = m x 9.8 m/s2 x h

or GPE = mgh

Sec 2. Conservation of Energy
• Changing forms of energy
• When you turn on a light bulb it converts electrical energy into light energy and thermal energy.
• Fuel stores energy in chemical bonds. A car engine transforms chemical PE into KE.
• Mechanical Energy—the total amount of potential and kinetic energy in a system.
• Mechanical energy = PE + KE
Falling Objects
• As an object falls, its potential energy decreases and its kinetic energy increases.
• The amount of mechanical energy of a falling object always remains the same.
• Energy transformation also occurs when a baseball is hit.
• Close to the ground it has mostly KE. At its highest point it has mostly PE, but the amount of mechanical energy is constant.
Swinging
• On a swing or a pendulum the mechanical energy is also constant.
• To get moving, you need KE. As you rise higher, your GPE increases. As you move down, your KE increases. Mechanical Energy is constant.
Law of Conservation of Energy
• Law of Conservation of energy—states that energy cannot be created or destroyed.
• Sometimes it is hard to see energy conserved.
• Ex. Friction can cause some mechanical energy to change into thermal energy (heat).
• Ex. The sun can convert a small amount of mass into a large amount of energy using nuclear fusion.

Law of Conservation of Energy Continued

• Your body follows the law of conservation of energy.
• Ex. Chemical potential energy is stored in the food you eat.
• Ex. Your body stores energy in fat and other chemicals.
• Ex. Your body converts the energy to make you move.