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PHYS16 – Lecture 16

PHYS16 – Lecture 16. Work, Energy, and Power October 15, 2010. This Week…. What is Energy? What is Work? Work-Energy Theorem – how energy and work are related What is Power?. Definition of Work. Mechanical Work (W) – energy transferred to an object due to the action of a force

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PHYS16 – Lecture 16

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  1. PHYS16 – Lecture 16 Work, Energy, and Power October 15, 2010

  2. This Week… • What is Energy? What is Work? • Work-Energy Theorem – how energy and work are related • What is Power?

  3. Definition of Work • Mechanical Work (W) – energy transferred to an object due to the action of a force (+) transfer to object (-) transfer from object

  4. Work with a Constant Force • Force = Constant, then can take force outside integral

  5. Practice Question • I pull a 4.0 kg sled a distance of 5.0 m. I pull the sled using a rope at a 30.0 degree angle with a force of 5.0 N. What is the work done by me? A) 0 J B) 20 J C) 25 J D) 22 J

  6. Work with a Variable Force • Force = Constant, then can take force outside integral Fx x

  7. Practice Question • A force is given by Fx = 3x2+2. What is the work done by the force for moving an object from x=0.0 m to x=4.0 m? A) 72 J B) 50 J C) 0 J D) 200 J

  8. Work – Energy Theorem • Work = the transfer of Energy • Energy = the ability to do work Work done by External Force Change in Energy to the system

  9. 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 Force and displacement are both downward

  10. Work and spring potential energy • If mass on a spring moves, how much work is done by spring? • Use work-energy theorem to derive spring potential energy Work done by system is negative Force and displacement are in opposite directions

  11. Work and Kinetic energy • If an object speeds up, how much work is done on object? • Use work-energy theorem to derive kinetic energy Assume K=mv2/2 and prove left side = right side Just multiply and divide by dt since dt/dt=1 Now take derivative and remember to use chain rule

  12. Let’s define Power • Power – the rate at which work is performed • Units = W = J/s • Scalar

  13. Lingenfelter Corvette… • What is the power of the 1420 kg Lingenfelter Corvette that can go 0 to 26.8 m/s in 1.97 s? Approach #1 Approach #2 Answer = 259 kW Spec says 1100 kW, why?

  14. Horsepower • 1 hp = 33,000 ft lbs/min • Comes from how much weight a horse could pull up a mine shaft a particular amount of feet in a given minute Horse Force

  15. Estimating horsepower • Car = 250-1475 hp • 1 hp = 746 W • Lingenfelter corvette = 1475 hp • Small compact= 250 hp • Lawnmower = 5-25 hp • Person = ~1 hp • Estimate using time it takes to run up stairs

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