Chapter 4 Energy

Sec 1. The Nature of Energy Energy—the ability to cause change. Energy—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 light Electrical—making toast or turning on a light Chemical—energy stored in food or gasoline Chemical—energy stored in food or gasoline Thermal—from the sun, warming the planet Thermal—from the sun, warming the planet It can be transferred from one form to another. It can be transferred from one form to another.

Kinetic Energy Kinetic Energy—energy in the form of motion. Kinetic Energy—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 quantities: The amount of kinetic energy (KE) depends on 2 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 moving at 100 km/hr. Which one has more KE? 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. Answer: the truck, because it has more mass.

Calculating Kinetic Energy You can calculate kinetic energy using this equation. You can calculate kinetic energy using this equation. Kinetic Energy = ½ mass x velocity squared Kinetic Energy = ½ mass x velocity squared KE = ½ m x v 2 KE = ½ m x v 2 The SI unit of energy is the joule (J) The SI unit of energy is the joule (J)

Potential Energy Energy does not always involve motion. Energy does not always involve motion. Potential Energy—stored energy due to position. Potential Energy—stored energy due to position. The amount of potential energy (PE) depends on the objects position. The amount of potential energy (PE) depends on the objects position. There are different types of potential energy. There are different types of potential energy. Elastic potential energy—energy stored by something that can compress or stretch (ex. rubber band) 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) 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. Gravitational Potential Energy (GPE)—the energy stored by objects that are above Earth’s surface. The amount of GPE depends on 3 things The amount of GPE depends on 3 things Mass, acceleration due to gravity, & height above the ground. Mass, acceleration due to gravity, & height above the ground. The amount of GPE can be calculated The amount of GPE can be calculated GPE = mass x 9.8m/s 2 x height GPE = mass x 9.8m/s 2 x height GPE = m x 9.8 m/s 2 x h GPE = m x 9.8 m/s 2 x h or GPE = mgh or GPE = mgh

Changing GPE

Sec 2. Conservation of Energy Changing forms of energy Changing forms of energy When you turn on a light bulb it converts electrical energy into light energy and thermal 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. 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—the total amount of potential and kinetic energy in a system. Mechanical energy = PE + KE Mechanical energy = PE + KE

Falling Objects As an object falls, its potential energy decreases and its kinetic energy increases. 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. The amount of mechanical energy of a falling object always remains the same. Energy transformation also occurs when a baseball is hit. 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. 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. 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. 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. Law of Conservation of energy— states that energy cannot be created or destroyed. Sometimes it is hard to see energy conserved. Sometimes it is hard to see energy conserved. Ex. Friction can cause some mechanical energy to change into thermal energy (heat). 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. 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. Your body follows the law of conservation of energy. Ex. Chemical potential energy is stored in the food you eat. Ex. Chemical potential energy is stored in the food you eat. Ex. Your body stores energy in fat and other chemicals. Ex. Your body stores energy in fat and other chemicals. Ex. Your body converts the energy to make you move. Ex. Your body converts the energy to make you move.