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Chapter 5

Chapter 5. Work and Energy. Work and Energy. Physics—study of matter, energy, and the interactions between them Energy is a fundamental concept to all fields of physics Both work and energy are quantifiable i.e. they’re not fuzzy concepts, but real, definable things.

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Chapter 5

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  1. Chapter 5 Work and Energy

  2. Work and Energy • Physics—study of matter, energy, and the interactions between them • Energy is a fundamental concept to all fields of physics • Both work and energy are quantifiable • i.e. they’re not fuzzy concepts, but real, definable things

  3. What are we talking about? • Work—the act of doing something useful • We’ll just talk about mechanical work for now • By “doing something,” we mean exerting a force on an object • By “useful,” we mean actually changing the motion of an object Work is rarely done before the due date

  4. What are we talking about? • Energy—the ability to do something useful • Mechanical energy—associated with kinematics • Thermal energy—associated with heat • Electrical energy—associated with electrical charge • Work—a change of energy from one form to another • Energy can be used to do work • All energy has units of Joules (J) Reading your electric meter

  5. Power • Power—rate at which energy is used • Rate at which work is done • Units of Watts (W) • 1 horsepower (hp) = 746 Watts • 1 W = 1 J/s

  6. Mechanical Work • Mechanical work—exerting a force on an object over a given distance • For a constant force, W = Fd • Units of Joules (J) • Kilowatt-hour (kWh) is also a common unit • 1 kWh = 3.6 x 106 J • Only consider component of force in the direction of displacement Ex: work done on a crate, p. 147

  7. Mechanical Energy • Kinetic Energy (KE) • Energy based on movement • m = mass of object • v = velocity of object • Work-Energy theorem • If Mechanical work is done on an object, its mechanical KE must change • Mechanical work done is defined as the change in KE of an object

  8. Mechanical Energy • Potential Energy (PE) • Energy based on position • Gravitational PE: the higher you go, the more PE you have • m = mass of object • g = 9.8 m/s2 • h = height above reference point • Spring PE: the more a spring is compressed or stretched, the more PE it has

  9. Conservation of Energy • Total energy remains constant • Energy can be changed, but not destroyed • Mechanical Energy is sum of kinetic and potential

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