Conservation of Energy Chapter 4 Section 3 Law of Conservation of Energy Law of Conservation of Energy: states that energy cannot be created or destroyed Energy can only be converted from one for to another or transferred from one place to another Energy Transformations Mechanical energy: the sum of the kinetic energy and potential energy of objects in a system This includes kinetic energy of objects, elastic potential energy, and gravitational potential energy (GPE) Mechanical energy of a system often remains constant or nearly constant Mechanical energy and total energy are NOT the same (there are types of energy not mechanical) Falling Objects Think about the falling apple example As an object falls and gets closer to the earth the GPE of the apple-Earth system decreases The potential energy is transformed into kinetic energy as the apples speed increases When potential energy converts to kinetic energy the mechanical energy of the system does not change The form of mechanical energy changes but the total amount remains constant Projectile Motion Energy transformations also occur during projectile motion when an object moves in a curved path Swings Other Energy Transformations Think about the swing example Eventually the swing slows down and eventually comes to a stop The mechanical energy of the swing-Earth system decreases It may appear that energy is being destroyed However, recall that there are other forms of energy aside from mechanical energy The mechanical energy from the swing is transformed into the other forms of energy when the swing stops The Effect of Friction If the mechanical energy of the earth- swing system decreases then some other forms of energy must increase by an equal amount to keep the total energy the same Think about friction and air resistance These two things convert mechanical energy into thermal energy With every pass of the swing, the temperature of the hooks and the air increases slightly Thus mechanical energy is not destroyed only converted Transforming Electrical Energy Electric stoves and toasters transform electrical energy into thermal energy Electronics such as TVs transform electrical energy into radiant energy and sound energy The electric motor in a washing machine transforms electrical energy into mechanical energy Transforming Chemical Potential Energy Fuel (such as gasoline) stores energy in the form of chemical potential energy A car engine transforms this chemical potential energy into thermal energy and into mechanical energy for the cars motion If you eat an ear of corn, the chemical potential energy from the corn is transferred to your body Your body then extracts this energy for functions such as breathing, pumping blood, etc. Power- How Fast Energy Changes Power: the rate at which energy is converted Watt = Joule/sec Power Example You transform 950 J of chemical energy into mechanical energy to push a sofa. If it took you 5.0s to move the sofa, what was your power? P = ? E = 950 J t = 5.0s Energy Conversions in Your Body The chemical potential energy of food supplies the cells in your body with energy that they need to function Energy from food is often measured in Calories (C) 1 Calorie = 4,000 J Every gram of fat in a food supplies a person with about 10C (40,000J) Calories you need for different activities depend on your weight, body type, and degree of physical activity In class/Homework For the rest of the period we will work on Assignment 4. 3 If you are having trouble or have any questions come ask me If you do not finish this in class it is homework Wrap Up What is the law of conservation of energy? What does it state? What is mechanical energy? Is it the same as total energy? Are are potential and kinetic energy related? What is the unit for Power? How many Joules are in a Calorie?