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Chapter 15

Chapter 15. Energy. Objectives. Describe the relationship between work & energy Analyze how potential energy is related to an object’s position and give examples of gravitational and elastic potential energy Relate kinetic energy to mass and speed

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Chapter 15

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  1. Chapter 15 Energy

  2. Objectives • Describe the relationship between work & energy • Analyze how potential energy is related to an object’s position and give examples of gravitational and elastic potential energy • Relate kinetic energy to mass and speed • Calculate and solve equations for both kinetic and potential energy

  3. Energy • perhaps the concept most central to all of science • matter is substance, and energy is the mover of substance • cannot see, smell, or feel most forms of energy • difficult to define, because it is not a thing but a process • persons, places, and things all have energy but we usually observe energy only when it changes forms ex. radiant energy to thermal energy

  4. Energy Cont. • Definition - ability to do work - when work is done on an object, energy is transferred) ex. carrying your backpack up a flight of stairs • Units - Joule (J), same as work • 2 types of energy - mechanical: the sum of the kinetic and potential energy of large scale objects in a system - non-mechanical: energy that lies at the level of atoms and that does not affect motion

  5. Potential Energy • Definition - stored energy resulting from the relative position of objects in a system • 2 types of potential energy 1. elastic potential energy - energy stored in any type of stretched or compressed elastic material ex. rubber band, spring 2. gravitational potential energy - energy resulting from the gravitation attraction between the object and the earth ex. apple falling from tree

  6. Potential Energy • Formula - PE = mgh - PE = mass x free fall acceleration x height -notice the mg is the weight of the object due to gravity, hence it is really just a calculation of force times distance which is work

  7. Potential Energy - height is usually measured from the ground ex. an apple at the top of the tree has a greater gpe then that on a lower branch ex. two apples of differing weights are on the same branch: the apple weighing more has higher gpe

  8. Potential Energy Problems Q: Calculate the gravitational potential energy in the following systems a. a car with a mass of 1200 kg at the top of a 42 m high hill (1200 kg)( 9.8m/s/s)(42 m) = 4.9 x 105 b. a 65 kg climber on top of Mt. Everest (8800 m high) (65 kg) (9.8m/s/s) (8800 m) = 5.6 x 106 J c. a 0.52 kg bird flying at an altitude of 550 m (.52 kg) (9.8m/s/s)(550) = 2.8 x 103 J

  9. Kinetic Energy • Definition - energy of a moving object due to its motion • Formula - KE = ½ mv2 • Units - Joules (J)

  10. Kinetic Energy Cont. • depends on the object’s mass and speed squared • small changes in speed produce large changes in KE ex. car moving along a road * if you double the speed, you increase its KE by 4 (22) • graph

  11. Kinetic Energy Problems Q: Calculate the KE in joules of a 1500 kg car moving at the following speeds: a. 29 m/s ½(1500 kg)(29 m/s)2 = 6.3 x 105 J b. 18 m/s ½(1500 kg)(18 m/s)2 = 2.4 x 105 J c. 42 km/h (convert) 42 km/h(1000 m/km)(1 hr/3600 s) = 12 m/s ½(1500 kg)(12 m/s)2 1.1 x 105 J

  12. Objectives • Explain how the major forms of energy are produced • Give examples of the major forms of energy

  13. Forms of Energy • Mechanical Energy - sum of an object’s potential energy and kinetic energy ex. bouncing balls • Thermal Energy - the total potential and kinetic energy of all the microscopic particles ex. molten metal • Chemical Energy - the energy stored in the chemicalbonds in compounds ex. gasoline

  14. Forms of Energy Cont. • Electrical Energy - energy associated with electriccharges ex. bolts of lightening • Electromagnetic Energy - form of energy that travels through space in the form of waves ex. X-rays • Nuclear Energy - energy stored in the atomicnuclei ex. fusion

  15. Conservation of Energy • Law of conservation of energy - energy cannot be created or destroyed closed system: - all energy remains in the system - nothing can enter or leave open system: - energy present at the beginning of the system may not be at present at the end

  16. Conversion of Energy • Definition - the process of changing energy from one form to another ex. wind up toys: elastic potential energy toy moves: potential changes into kinetic - conversion can happen in multiple steps ex. striking a match - chemical energy to kinetic to thermal * Most common is changing GPE to KE

  17. Conversion of Energy Cont. • Energy conversions • Energy conversions in pendulums

  18. Energy Conversions Calculations • Formula (KE + PE) beginning = (KE + PE) end • Problem A 1000 kg car is coasting at 10 m/s toward a hill that is 10 m high. Will the car make it to the top of the hill if the driver does not step on the gas pedal? No, KE = 50,000J. At a height of 10 m, PE would be about 100,000 J. Ignoring friction the car would only reach 5 m.

  19. Energy Calculation Cont. • At a construction site a 1.50 kg brick is dropped from rest and hit the ground at a speed of 26.0 m/s. Assuming air resistance can be ignored, calculate the gravitational potential energy of the brick before it was dropped? • PE = ½ (1.50 kg) (26.0m/s)2 = 507 J

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