1 / 31

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

claire
Télécharger la présentation

Advanced Physics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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)

More Related