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Work and Energy ENERGY The ability to do work WORK A change in energy Work Energy Circular Definition! Radiant Thermal Electrical Nuclear Chemical Sound Mechanical Magnetic At home – check out http://www.youtube.com/watch?v=3ff_ AXVlo9U for the Singing Tesla Coil! Potential energy + Kinetic energy = Mechanical energy Potential energy + Kinetic energy = Mechanical energy Example of energy changes in a swing or pendulum. 2 Kinds of Mechanical Energy • Kinetic energy (KE) • Potential Energy (PE) –Gravitational PE (PEg) –Elastic PE (PEelastic) Kinetic Energy Energy due to motion KE = ½ 2 mv Potential Energy Energy due to position Bowling Ball – Conservation of Energy Conserving Energy Gravitational PE PEg = mgh A 25 kg boy runs with a speed of 6.5 m/s. How much kinetic energy does the boy have? 530 Joules How much potential energy does a 0.50 kg apple have if it hangs from a tree 13m above the ground? 64 Joules A 220 gram fly has a kinetic energy of 55 Joules. How fast is the fly flying? 22 m/s ENERGY IS CONSERVED!! WORK W = FDs (Work = Force x displacement) Units: Joule (J) = N· m Work is done only when components of a force are parallel to a displacement. Force helps motion Force opposes motion Force 90° to motion Object not in motion Positive Work Negative Work No work No work A student drags his book bag 15 meters across the classroom by pulling on it with a force of 5.0 Newtons. How much work does the student do on the bag? 75 Joules Larry carries his 35N physics book across the room 25m away. How much work does he do on the book? 0 Joules Work-Kinetic Energy Theorem Wnet = DKE POWER The rate at which work is done W P= Dt Or… DKE P= Dt Or… DPE P= Dt Or… P = Fv = Force * velocity The SI unit of power is the Watt, named in honor of James Watt. One Watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s. Unit of Power Watt = 1 J/s Horsepower (hp) = 746 W How much power is generated by a motor that can lift a 5300 kg crate of bricks with a velocity of 15 m/s? 780,000 W How much power is generated by a car engine that does 5500 Joules of work in 35 seconds? 160 W How long will it take a 36 Watt motor to pull a box across the floor 25 m with a force of 16 N? 11 s Homework: Chapter 8, Hewitt: Read 8.1-8.6 Do RQ 1-13; P&C 1-3,5-7; T&E 3-8; T&S 1-4