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Energy Chapter 15 Chapter 15 Pretest 1. How much work is done when a weightlifter holds a barbell motionless over his head? – No work is done! 2. Calculate the work done on a 2-N mass when it is lifted to a height of 2 m. –4J 3. Calculate the average speed of a bicycle that travels 100 m in 20 s. – 5 m/s Chapter 15 Pretest 4. Is weight a force? – yes 5. What is the formula for calculating weight? – W = mg 6. How does the temperature of an object change when it is acted on by friction? – The temperature increases. Chapter 15 Pretest 7. True or false: In a closed system, the loss of momentum of one object equals the gain in momentum of another object. – True 8. How is power related to work? – Power is the rate at which work is done 9. True or false: The amount of work done on a machine (work in) always equals the amount of work done by the machine (work out). – False How is Energy Related to Work? • Energy is defined as the ability to do work. Recall that work is the product of force and distance. If a force acts through a greater distance, it has done more work. You can use work to measure changes in energy. • Place two identical books on the table so there is a gap of about 8 cm between books. Place a sheet of notebook paper on the books so it covers the gap shown. Now drop a penny from a height of 10 cm onto the paper above the gap. Note what happens. Next, drop the penny from a height of 30 cm and observe the results. How is Energy Related to Work? 1. How did the height of the penny affect the distance the paper moved? – The paper moved farther when the penny was dropped from a greater height. 2. How did lifting the penny affect the work it did on the paper? – Lifting the penny allowed it to do more work on the paper. 3. How did lifting the penny affect its energy? – Lifting the penny increased the penny’s energy. 15.1- Energy and Its Forms • Energy is the ability to do work. – Energy is transferred by a force moving an object through a distance. • Work and Energy are closely related. – When work is done on an object, energy is transferred to that object. – Work is a transfer of energy. • Energy is typically measured in joules (J) just like work. 15.1 – Kinetic Energy • The energy of motion is called kinetic energy. • The kinetic energy of any moving object depends upon its mass and speed. KE = ½ mv2 • KE = joules (J), mass (m) = kg, speed (v) = meters/sec (m/s) – Double the mass, double the KE – Double the speed, quadruple the KE Kinetic Energy Math Practice 1. A 70.0 kilogram man is walking at a speed of 2.0 m/s. What is his kinetic energy? – KE = 1/2mv2 = 0.5(70kg)(2 m/s)2= 140 J 2. A 1400 kilogram car is moving at a speed of 25 m/s. How much kinetic energy does the car have? – KE = 1/2mv2 = 0.5(1400kg)(25 m/s)2= 437,500 J 3. A 50.0 kilogram cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running? – v = √2(KE)/m = √2(18,000 J)/50.0 kg = 27 m/s 15.1 – Potential Energy • Potential energy is the energy that is stored as a result of position or shape. – PE has the potential to do work – Examples: plucking a guitar string, lifting a book • Two forms of potential energy are: – Gravitational Potential Energy – Elastic Potential Energy 15.1 – Gravitational Potential Energy • Potential energy that depends on an object’s height is called gravitational potential energy. – Gravitational Potential Energy increases when an object is raised to a higher level. • An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity. PE = mgh • PE = joules (J), mass (m) = kilograms (kg), acceleration due to gravity (g) = 9.8 m/s2 on Earth, height (h) = meters • Note: height is measured from the ground or floor, and GPE is measured relative to that same reference level. 15.1 – Elastic Potential Energy • The potential energy of an object that is stretched or compressed is known as elastic potential energy. • Something is said to be elastic if it springs back to its original shape after it is stretched or compressed. • Examples: rubberband, spring, basketball, shock absorber, wind-up toy 15.1 – Forms of Energy • All energy can be considered to be kinetic energy, potential energy, or the energy in fields such as those produced by electromagnetic waves. • The major forms of energy are: – – – – – – mechanical energy thermal energy chemical energy electrical energy electromechanical energy nuclear energy. 15.1 – Mechanical Energy • The energy associated with the motion and position of everyday objects is mechanical energy. • Mechanical energy is the sum of an object’s potential energy and kinetic energy. • Examples: speeding trains, bouncing balls, sprinting athletes • Mechanical energy is NOT limited to machines. • Mechanical energy does not include thermal energy, chemical energy, or other forms of energy associated with the motion or the arrangement of atoms or molecules. 15.1 – Thermal Energy • The total potential and kinetic energy of all the microscopic particles in an object make up its thermal energy. – Almost all of the matter around you contains atoms. – These particles are always in random motion and thus have kinetic energy. • When an object’s atoms move faster, its thermal energy increases and the object becomes warmer. • Examples: molten metal, toasting marshmallows 15.1 – Chemical Energy • Chemical energy is the energy stored in chemical bonds. – When bonds are broken, the released energy can do work. • All chemical compounds store energy. • Examples: fuels such as coal or gasoline, wood 15.1 – Electrical Energy • Electrical energy is the energy associated with electric charges. – Electric charges can exert forces that do work. • Examples: batteries, lightning bolts 15.1 – Electromagnetic Energy • Electromagnetic energy is a form of energy that travels through space in the form of waves. • Examples: visible light, x-rays, sun 15.1 – Nuclear Energy • The energy store in atomic nuclei is known as nuclear energy. • Examples: heat and light of the sun, nuclear power plant operations – nuclear fission – process that releases energy by splitting nuclei apart – nuclear fusion – releases energy when less massive nuclei combine to form a more massive nuclei 15.2 – Energy Conversion • Energy can be converted from one form to another. • The process of changing energy from one form to another is energy conversion. • Examples: wind-up toys, light bulbs, striking a match 15.2 – Conservation of Energy • When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur. • The law of conservation of energy states that energy cannot be created or destroyed. – Energy can be converted from one form to another. – In a closed system, the amount of energy present at the beginning of a process is the same as the amount of energy at the end. 15.2 – Energy Conversions • One of the most common energy conversions is between potential energy and kinetic energy. • The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls. • Examples: avalanche, compressed spring, gull/oyster shell 15.2 – Energy Conversion • A pendulum consists of a weight swinging back and forth from a rope or a string. • Pendulum Examples: rope swing, clock 15.2 – Energy Conversion Calculations • Mechanical Energy = KE + PE • *Conservation of Mechanical Energy (KE + PE)beginning = (KE + PE)end * Friction is neglected Conservation of Mechanical Energy • A 10 kg rock is dropped and hits the ground below at a speed of 60 m/s. Calculate the gravitational potential energy of the rock before it was dropped. You can ignore the effects of friction. PEbeginning = KEend = 1/2mv2 =(0.5)(10kg)(60m/s)2 = 18,000 J Conservation of Mechanical Energy • A diver with a mass of 70.0 kg stands motionless at the top of a 3.0 m high diving platform. Calculate his potential energy relative to the water surface while standing on the platform, and his speed when he enters the pool. (Hint: Assume the diver’s initial vertical speed after diving is zero). PEbeginning = mgh At the beginning, KE = 0 and at the end, PE = 0, so PEbeginning = KEend = 1/2mv2 2100 J = (0.5)(70.0kg)v2 v = 7.7m/s Conservation of Mechanical Energy • A pendulum with a 1.0 kg weight is set in motion from a position of 0.04 m above the lowest point on the path of the weight. What is the kinetic energy of the pendulum at the lowest point? (Hint: Assume there is no friction.) PEbeginning=mgh =(1.0 kg)(9.8 m/s2)(0.04m) = 0.4 J At the beginning, KE = 0; at the lowest point, PE = 0; PEbeginning = KEend = 0.4 J 15.2 – Energy and Mass • Special theory of relativity developed by Albert Einstein in 1905. • E = mc2 c = 3.0 x 108 m/s • E = energy, m = mass, c = speed of light • Einstein’s equation says that energy and mass are equivalent and can be converted into each other. – Energy is released as matter is destroyed – Matter can be created from energy 15.3 – Energy Resources • Energy resources can be classified as renewable or nonrenewable. • Nonrenewable – exist in limited supply and cannot be replaced quickly – Examples: oil, natural gas, coal, uranium – Fossil Fuels: oil, natural gas, coal • Usually readily available and inexpensive, but their use creates pollution 15.3 – Renewable Energy Resources • Renewable – energy resources that can be replaced in a relatively short period of time. – Most originate directly or indirectly from the sun – Examples: hydroelectric, solar, geothermal, wind, biomass, nuclear fusion 15.3 - Definitions • Hydroelectric energy – energy obtained from flowing water • Solar energy – sunlight that is converted into usable energy • Geothermal energy – thermal energy beneath Earth’s surface • Biomass energy – chemical energy stored in living things • Hydrogen fuel cell – generates electricity by reacting hydrogen with oxygen. 15.3 – Conserving Energy Resources • Energy resources can be conserved by reducing energy needs and by increasing the efficiency of energy use. • Things you can do: – Turning off lights, etc. when not in use – Walk or bike on short trips – Carpool or mass transportation – Fuel-efficient automobiles – Energy-efficient purchases