Energy, Work, and Power PDF

# Energy, Work and Power ## Work Work is the transfer of energy that occurs when a force makes an object move. ### To Do Work: 1. A force must be applied 2. The motion must be in the same direction as the applied force ### Work Done in a System - The SI Unit for work is Joules or J. - The work...

# Energy, Work and Power ## Work Work is the transfer of energy that occurs when a force makes an object move. ### To Do Work: 1. A force must be applied 2. The motion must be in the same direction as the applied force ### Work Done in a System - The SI Unit for work is Joules or J. - The work done on a system only depends on forces and distances. ### Calculating Work Work occurs when a force moves an object some distance (in the same direction). $W = Fd$ - W = Work (Joules) - F = Force (Newtons) - d = distance (Meters) #### Example: Jessica picks up a book on the floor. She applies a force of 2 N and moves the book 1.5 m higher. How much work did she do on the book? Jessica moved the book sideways with a force of 0.5 N to a position 0.5 m away. How much work did she do on the book? ## Power Power is the rate work is done. Amount of work done in a unit of time. - # Joules of energy used in 1 sec (J/s) - Power is measured in Watts (W) or kilowatts (kW). ### Calculating Power - Power is the rate of work done. - Power is the rate of Joules used per second. $P = W/t$ - P = power (Watts) - w = work (Joules) - t = time (seconds) #### Example: How long will it take a runner to do 1,000 J of work if his power is 125 W? Jessica did 3J of work when she lifted the book. It takes her 2 seconds to move the book. Calculate her power. ## Energy What do you think energy is? ### What Is Energy? 1. Energy is the ability to cause change. 2. Energy can change the object itself or its surroundings. - Energy is measured in Joules. ### Forms of Energy - Some of the many forms of energy are electromagnetic, chemical, nuclear, mechanical, and heat. - Energy can change between these forms. #### Electromagnetic Energy Power lines carry electromagnetic energy into your home in the form of electricity. A difference in potential energy causes electrons to flow. #### Chemical Energy Chemical energy is required to bond atoms together. And, when bonds are broken, energy is released. - Fuel and food are forms of stored chemical energy. #### Nuclear Energy The nucleus of an atom is the source of nuclear energy. - When the nucleus splits (fission), nuclear energy is released in the form of heat energy and light energy. - Nuclear energy is also released when nuclei collide at high speeds and join (fusion). - Nuclear energy is the most concentrated form of energy. - Most of us live within 10 miles of the Shearon Harris Nuclear Power Plant which converts nuclear energy into electromagnetic energy. - The sun's energy is produced from a nuclear fusion reaction (thermonuclear) in which hydrogen nuclei fuse to form helium nuclei. #### Mechanical Energy When work is done to an object, it acquires energy. This energy can be potential or kinetic. The combined energy it acquires is known as mechanical energy. - When you throw a bowling ball, you transfer energy to it. When that bowling ball then hits the pins, some of the energy is transferred to the pins (transfer of momentum). - When you kick a football, you transfer mechanical energy to the football to make it move. #### Heat Energy The internal motion of the atoms is called heat energy, because moving particles produce heat. - Heat energy can be produced by friction. - Heat energy causes changes in temperature and phase of any form of matter. ### Kinetic Energy Kinetic energy is the energy of motion. $KE = 1/2mv²$ - m = mass (kg) - v = velocity (m/s) #### Example: Find the kinetic energy of a 10 kg bowling ball that is rolling at 3 m/s. #### Practice Problems - A ball rolls down a hill and ends up moving 4.5 m/s. If the mass of the ball is 3 kg, what is the ball's kinetic energy? - If a ball is moving 5 m/s and is recorded as having 55J of energy, what is the boxes mass? ### Potential Energy - Potential energy is stored in an object. - Potential energy comes from an object's position. - “Can or could that object change or do something?” #### Types of Potential Energy - Elastic PE - Electrical PE - Gravitational PE ### Gravitational Potential Energy Gravitational Potential Energy (GPE) is the stored energy in objects that can fall. $GPE = mgh$ - m = mass (kg) - h = height (meters) Remember: g = 9.8 m/s² #### Example: My slinky sits on my desk, 1 m above the floor. The mass of the slinky is 0.5 kg. Find its GPE. ## Law of Conservation of Energy The total amount of energy in a closed system never changes. - Energy is never created or destroyed, only transferred or transformed. ### Transfer of Energy - Energy can only transfer forms, not be created or destroyed. ### Transforming Energy - Energy can change types - it can be transformed. - Energy in = Energy out #### Example: A hairdryer transforms electrical energy into thermal energy, kinetic energy, and sound energy. ### Mechanical Energy Mechanical Energy is the total amount of kinetic energy and potential energy in a system. A system is a group of objects that work together. #### Calculating Mechanical Energy - Add kinetic energy and potential energy together. - $ME = KE + PE$ - $ME = 1/2 mv² + mgh$ ### Mechanical Energy for a Falling Object - GPE decreases because height decreases. - KE increases because gravity increases the velocity. #### Example Problem - What is the mechanical energy of a box that has a potential energy of 15J and a kinetic energy of 32J? - What is the mechanical energy of a 75 kg man on a diving board 15 m above the ground? - The man jumps. After he has fallen 5 m he is traveling about 9.9 m/s. Find the mechanical energy of the man. ## Conservation of Energy Why do we lose mechanical energy? - Energy can be taken out of a system if work is done, for example, by friction. - Friction produces heat that takes energy. #### Example: A person on a sled going down a snowy hill. They lose mechanical energy due to friction between the sled and the unpacked snow. This friction causes the sled to slow down, converting mechanical energy into heat energy.