Ch 06 Work and Energy

# Ch 06 Work and Energy - 1 LECTURE NOTES 6 Goals Overview...

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1 s F LECTURE NOTES 6 Goals Overview energy. Study work as defined in physics. Relate work to kinetic energy. Consider work done by a variable force. Study potential energy. Understand energy conservation. Include time and the relationship of work to power. Introduction: In previous chapters we studied motion Sometimes force and motion are not enough to solve a problem. We introduce energy as the next step. Definition The work done on an object by a constant force F is: W = F. s = ( F cos ) s where F is the magnitude of the force, s is the magnitude of the displacement, and is the angle between the force and the displacement. Scalar quantity . SI units of Work: newton(N).meter(m) = joule (J) CGS units: dyne(dyn).centimeter(cm) = erg Work done by a constant force

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2 Trigonometry Reminder! When the force points in the same direction as the displacement, then = 0 o , and W = F s Work can be positive, negative, or zero 1. If the force has a component in the same direction as the displacement of the object, the work done by the force is positive. 2. On the other hand, if a force component points in the direction opposite to the displacement , the work is negative. 3. If the force is perpendicular to the displacement , the force has no component in the direction of the displacement, and the work is zero ( = 90 o ) F N (zero work) f s (positive work) f s (negative work) W (zero work) Pulling a Suitcase-on-Wheels Displacement s cos0 1 cos90 0 cos180 1 
3   cos0 W F s Fs    cos180 W F s Fs  

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4 F s f F F F s f s f + y W f k F N m g sin m g cos m g + x A falling object A mountain climber Lift f s Examples to think about F s A satellite moving in a circular or elliptic orbit about the earth http://home.cvc.org/science/kepler.htm Ice skater Thrust Drag Weight A skier coasting down a slope
5 The work-energy theorem and kinetic energy Work done by net external force = 2 2 2 2 2 1 2 1 2 . . ). ( o f o f mv mv v v m s ma s F = final KE initial KE Definition of kinetic energy The kinetic energy K of an object with mass m and speed v ( scalar quantity ) is given by: K = ½ m v 2 SI units of kinetic energy: joule (J) The work-energy theorem When a net force does work W on an object, the kinetic energy of the object changes from its initial value of K o to a final value of K f , the difference between the two values being equal to the work: 22 11 f o f o W K K mv

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## This note was uploaded on 12/03/2010 for the course PHY phy135 taught by Professor Weighgabriel during the Fall '10 term at University of Toronto- Toronto.

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Ch 06 Work and Energy - 1 LECTURE NOTES 6 Goals Overview...

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