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Describing Energy When something is able to change its surroundings or itself, it has energy. Energy is the ability to cause change. Without energy nothing would ever change. When work is done energy is transferred. So, energy can also be described as the ability to do work. Because of this, we measure energy in the same units as work, which are...Joules. When we calculate energy usage, or the amount of energy transferred, we measure within a system. A system is anything around which you can imagine a boundary. a system can be a single object, like a tennis ball, or a group of objects, like the solar system. different forms of energy There are many different forms of energy. The four main types are: Mechanical energy Electrical energy Chemical energy Radiant energy Regardless of the type of energy, it is all the same and is only transferred between types. Kinetic and Potential Energy Objects in motion have energy. kinetic energy is energy due to motion. Even motionless objects have energy. potential energy is energy that is stored due to the interactions between objects. there are 3 types of potential energy: Elastic potential energy is energy that is stored by compressing or stretching an object. Chemical potential energy is energy due to chemical bonds. Gravitational potential energy is energy due to gravitational forces between objects. An apple hanging from a tree has gravitational potential energy. A stretched rubber band has elastic potential energy. Food that will be digested has chemical potential energy. A spring that is compressed has elastic potential energy. A vase on a shelf has gravitational potential energy. HW: p 119 # 20-23 Conservation of Energy The Law of Conservation of Energy As your height above the ground changes your gravitational potential energy changes. As your speed changes the kenetic energy changes Think of a rollercoaster. When the rollercoaster is high above the ground the GPE is large and the KE is small. When the cars are low, GPE is small and KE is large. Energy changes back and forth between GPE and KE The total energy remains constant The law of conservation of energy states that energy cannot be created or destroyed. Energy can only be converted from one form to another or transferred from one place to another. Conserving resources You may have heard about or been told you to conserve energy. These ideas are related to using energy resources (coal and oil) wisely. Energy transformations Falling vase, a race car roaring by or the sun warming your skin are all energy transformations. Mechanical energy transformations Bicycles, roller coasters, and swings can be described in terms of mechanical energy. Mechanical energy is the sum of the kinetic energy and the potential energy of the objects in the system. It includes kinetic energy, elastic potential energy and gravitational potential energy. It does not include nuclear energy, thermal energy or chemical potential energy. Mechanical energy is not the same as total energy because of the types of energy that are not mechanical- these are still counted in total energy. Because of this mechanical energy is not necessarily conserved, but it does usually remain constant or nearly constant. When this is the case energy is only transformed between different types of mechanical energy. Falling objects Think of an apple hanging from a treeThe apple earth system has gravitational potential energy. It does not have kinetic energy while the apple is hanging from the tree because the apple is not moving. When the apple falls the gravitational potential energy decreases, so it is transformed into kinetic energy as the apples speed increases. The mechanical energy does not change as the apple falls, the potential energy that is lost is gained back as kinetic energy. The form changes, but the amount of mechanical energy stays the same Projectile Motion During projectile motion an object moves in a curved path (like a base ball through the air- it arcs) When a baseball is hit the ball is moving fast as the ball leaves the bat, so the system’s kinetic energy is relatively large. The ball’s speed decreases as it rises, so the KE decreases. But as the ball goes higher the GPE increases. At the top of the curve the GPE is ________ and the KE is _________. Then when the baseball falls the GPE decreases and the KE increases The mechanical energy of the system stays the same. Swings At the top of the swing you have high _______ and low ________ At the bottom of the swing curve you have high ________ and low _________ Other energy transformations The effect of frictionFriction and air resistance convert some of the mechanical energy into thermal energy Thermal energy is the energy of heat and hot objects When this happens mechanical energy is not destroyed just transformed into thermal energy, which is soon transferred into the air Transforming electrical energy Many of the devices we use every day are transforming electrical energy. They transform it into light, heat (radiant energy), mechanical energy and sound energy Transforming chemical potential energy Most of the transformation of CPE is in the form of fuel Gasoline, or coal Also, plants turn radiant energy from the sun into chemical potential energy. Power-how fast energy changes Power is the rate at which energy is converted P=e/t Power is measured in watts (1 watt =1joule per second) You transform 950 J of chemical energy into mechanical energy to push a sofa. If you took 5 seconds to move the sofa what was your power? If a runner’s power is 400 W as she runs, how much chemical energy does she convert into other forms in 10.0 minutes? One horsepower is a unit of pwer equal to746 W. How much energy can a 150 horsepower engine transform in 10.0 seconds? P 127 # 28-33 by end of class