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PHYS16 – Lecture 14

PHYS16 – Lecture 14. Ch. 7 Work and Energy. Announcements. Interested in Biophysics Research? Come hear me give a talk on Thursday 4:45 pm in Merrill 2 “Using Lasers to Observe the Mechanics of Life”. Ch. 7 Work and Energy. Mechanical Energy Potential Energy Kinetic Energy Work

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PHYS16 – Lecture 14

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  1. PHYS16 – Lecture 14 Ch. 7 Work and Energy

  2. Announcements • Interested in Biophysics Research? • Come hear me give a talk on Thursday 4:45 pm in Merrill 2 • “Using Lasers to Observe the Mechanics of Life”

  3. Ch. 7 Work and Energy • Mechanical Energy • Potential Energy • Kinetic Energy • Work • Constant Force → Dot Product • Variable Force → Integral • Work-Mechanical Energy theorem

  4. Energy pre-question • A projectile is shot at an angle of θ with an initial speed of v0. What is the kinetic energy of the projectile at the top of the arc? A) B) C) D) E) None of the above

  5. Energy pre-question • Some forces in nature depend on the inverse-square of the distance between objects (1/r2). If you were to calculate the potential for this force it would depend on: • 1/r • 1/r3 • r • r2 • None of the above

  6. Cheat Sheet • Mechanical Energy • Potential Energy, Gravitational • Potential Energy, Spring • Kinetic Energy • Work • Constant Force • Variable Force

  7. Review of Work • Back to Jorge and his crate… Jorge pushes on a 10-kg crate 5 m with a 50 N force at 21 degrees. The coefficient of kinetic friction is 0.2. What is the total work? http://www.wsu.edu/~jtd/Physics205/Chap4/Image189.gif

  8. Discussion: Does the normal force do work? • How much work is done by friction in the Jorge/crate example? How much work is done by the normal force? • I push on a wall. Does the normal force do work? Why am I tired? • I push on a wall while on a skateboard, does the normal force do work? • Does the normal force ever do work?

  9. Discussion: Mechanical Advantage and Work • If I increase my mechanical advantage do I decrease the amount of work I do? • What is the mechanical advantage for the incline? What is the work?

  10. Discussion: Mechanical Advantage and Work • What about for a pulley? θ = 60 degrees F mass

  11. Work-Mechanical Energy Theorem

  12. Energy can be transformed • Wyle E. Coyote • http://www.youtube.com/watch?v=Jnj8mc04r9E&feature=related

  13. Work – Mechanical Energy Theorem • Work = the transfer of Energy • Energy = the ability to do work Work done by system is negative

  14. Work and grav. potential energy • If I lift an object, how much work did I do on the object? • Use work-energy theorem to derive gravitational potential energy Notice Sign

  15. Discussion: Denver vs. Boston • So if I drop a 1-kg ball from 2 m then the change in the potential energy is 20 J. What if this classroom where in Denver instead of in Boston, a mile higher, would the potential energy change? http://upload.wikimedia.org/wikipedia/commons/c/c8/ReliefUSA_map.gif

  16. Zero of Potential Energy • Wherever you set your zero point for potential energy will determine your coordinate system • U=0 J at y=0 point for gravitational pot. energy • U=0 J at equilibrium point for spring pot. Energy • All calculations of ΔU will be relative so it won’t matter where you put 0 J just make sure not to switch back and forth!!!

  17. Conclusion • Mechanical Energy • Potential Energy, Gravitational • Potential Energy, Spring • Kinetic Energy • Work • Constant Force • Variable Force • Work-Mechanical Energy Thereom

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