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”

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

Energy pre-question A projectile is shot at an angle of θ with an initial speed of v 0. What is the kinetic energy of the projectile at the top of the arc? A) B) C) D) E) None of the above

Energy pre-question Some forces in nature depend on the inverse- square of the distance between objects (1/r 2 ). If you were to calculate the potential for this force it would depend on: A)1/r B)1/r 3 C)r D)r 2 E)None of the above

Cheat Sheet Mechanical Energy – Potential Energy, Gravitational – Potential Energy, Spring – Kinetic Energy Work – Constant Force – Variable Force

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?

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?

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?

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

Work-Mechanical Energy Theorem

Energy can be transformed Wyle E. Coyote

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

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

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?

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!!!

Conclusion Mechanical Energy – Potential Energy, Gravitational – Potential Energy, Spring – Kinetic Energy Work – Constant Force – Variable Force Work-Mechanical Energy Thereom