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Advanced Physics

Advanced Physics. Chapter 6 Work and Energy. Work and Energy. 6-1 Work done by a Constant Force 6-2 Work done by a Varying Force 6-3 Kinetic Energy, and the Work-Energy Principle 6-4 Potential Energy 6-5 Conservative and Nonconservative Forces

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Advanced Physics

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  1. Advanced Physics Chapter 6 Work and Energy

  2. Work and Energy • 6-1 Work done by a Constant Force • 6-2 Work done by a Varying Force • 6-3 Kinetic Energy, and the Work-Energy Principle • 6-4 Potential Energy • 6-5 Conservative and Nonconservative Forces • 6-6 Mechanical Energy and Its Conservation • 6-7 Problems Solving Using Conservation of Mechanical Energy • 6-8 Other Forms of Energy • 6-9 Energy Conservation with Dissipative Forces: Solving Problems • 6-10 Power

  3. 6-1 Work done by a Constant Force Work • Describes what is accomplished by the action of a force when it acts on an object as the object moves through a distance • The transfer of energy by mechanical means • The product of displacement times the component of the force parallel to the displacement • Both work and energy are scalar quantities

  4. 6-1 Work done by a Constant Force Work • W = Fd • Or • W = Fd cos • where  is the angle between the direction of the applied force and the direction of displacement

  5. 6-1 Work done by a Constant Force Work • W = Fd cos  Force  Displacement

  6. 6-1 Work done by a Constant Force Work • Units: joule (N•m) • 1 joule = 0.7376 ft•lb

  7. 6-1 Work done by a Constant Force Work • Negative work? • What about friction? • Work done on Moon by Earth? • Work done by gravity depends only on height of hill not incline angle.

  8. 6-2 Work done by a Varying Force • Work done by a variable force in moving an object between 2 points is equal to the area under the curve of a Force (parallel) vs. displacement graph between the two points. • Why? • Or we will have to do some calculus on it!

  9. 6-2 Work done by a Varying Force

  10. 6-3 Kinetic Energy, and the Work-Energy Principle Energy • The ability to do work (and work is?) Kinetic Energy • Energy of motion; a moving object has the ability to do work Translational Kinetic Energy (TKE) • Energy of an object moving with translational motion (?)

  11. 6-3 Kinetic Energy, and the Work-Energy Principle Translational Kinetic Energy (KE) • KE = ½ mv2

  12. 6-3 Kinetic Energy, and the Work-Energy Principle Work-Energy Principle • The net work done on an object is equal to the change in its kinetic energy • Wnet = Kef – Kei = KE • TKE  m and v2 • But…what about potential energy????

  13. 6-4 Potential Energy Potential Energy • Energy associated with forces that depend on the position or configuration of a body (or bodies) and the surroundings Gravitational Potential Energy • Potential energy due to the position of an object relative to another object (gravity)

  14. 6-4 Potential Energy Gravitational Potential Energy • Potential energy due to the position of an object relative to another object (gravity) • PEgrav = mgy

  15. 6-4 Potential Energy Potential Energy • In general the change in potential energy associated with a particular force is equal to the negative of the work done by the force if the object is moved from one point to another. • W = -PE

  16. 6-4 Potential Energy Elastic Potential Energy • Potential energy stored in an object that is released as kinetic energy when the object undergoes a change in form or shape • For a spring: • Elastic PE = ½ kx2 • Where k is the spring constant

  17. 6-4 Potential Energy Elastic Potential Energy • For a spring: • The force that the spring exerts when it is pushed or pulled is called the restoring force (Fs) and is related to the stiffness of the spring (spring constant-k) and the distance it is compressed or expanded • Fs = -kx

  18. 6-4 Potential Energy Elastic Potential Energy • For a spring: • Fs = -kx • This equation is called the spring equation or Hooke’s Law

  19. 6-5 Conservative and Nonconservative Forces Conservative Forces • Forces for which the work done by the force does not depend on the path taken, only upon the initial and final positions. Examples: • Gravitational • Elastic • Electric

  20. 6-5 Conservative and Nonconservative Forces Nonconservative Forces • Forces for which the work done depends on the path taken Examples: • Friction • Air resistance • Tension in a cord • Motor or rocket propulsion • Push or pull by a person

  21. 6-5 Conservative and Nonconservative Forces Work-Energy Principle (final) • The work done by the nonconservative forces acting on a object is equal to the total change in kinetic and potential energy. • Wnc = KE + PE

  22. 6-6 Mechanical Energy and Its Conservation Total Mechanical Energy (E) • E = KE + PE

  23. 6-6 Mechanical Energy and Its Conservation Principle of Conservation of Mechanical Energy • If only conservative forces are acting, the total mechanical energy of a system neither increase nor decreases in any process. It stays constant—it is conserved • KE1 + PE1 = KE2 + PE2 • KE = -PE

  24. 6-7 Problems Solving Using Conservation of Mechanical Energy • E = KE + PE = 1/2mv2 + mgy • KE = -PE • 1/2mv21 + mgy1 = 1/2mv22 + mgy2 • Sample problems: P.160 – 165

  25. 6-8 Other Forms of Energy Other Forms of Energy: • According to atomic theory, all types of energy is a form of kinetic or potential energy. Electric energy • Energy stored in particles due to their charge • KE or PE? Nuclear energy • Energy that holds the nucleus of an atom together • KE or PE?

  26. 6-8 Other Forms of Energy Other Forms of Energy: Thermal energy • Energy of moving (atomic/molecular) particles • KE or PE? Chemical energy • Energy stored in the bonds between atoms in a compound (ionic or covalent) • KE or PE?

  27. 6-8 Law of Conservation of Energy Law of Conservation of Energy • The total energy is neither increased nor decreased in any process. • Energy can be transformed from one form to another, and transferred from one body to another, but the total remains constant • This is one of the most important principles in physics! 

  28. 6-9 Energy Conservation with Dissipative Forces: Solving Problems Dissipative Forces • Forces that reduce the total mechanical energy Examples: • Friction • Thermal energy

  29. 6-9 Energy Conservation with Dissipative Forces: Solving Problems Problem Solving (Conservation of Energy) • Draw a diagram • Label knows (before/after) and knowns (before/after) • If no friction (nonconservative forces) then… KE1 + PE1 = KE2 + PE2 • If there’s friction (nonconservative forces) then add into equation • Solve for the unknown

  30. 6-10 Power Power • The rate at which work is done • The rate at which energy is transferred • Units (what?) • 1W = 1J/s • 746 W = 1 hp

  31. 6-10 Power Power • P = W/t = Fd/t • P = F v (since v =d/t)

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