chapter5_sildes

chapter5_sildes - 9/28/2009 Chapter 5. Energy, Work, Power...

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9/28/2009 1 Chapter 5. Energy, Work, Power x F W ) cos( x f f i i PE KE PE KE F One of the most important concepts in science Various form of energy - kinetic energy - potential energy - chemical energy - thermal energy - electromagnetic energy - nuclear energy, etc Energy can be transformed from one kind to another - Light bulb: Electrical energy to light/heat - Motor: Electric energy to mechanical energy - Nuclear power plant: nuclear energy to electrical energy - Solar cell: light energy to electrical energy, … Total amount of energy in the universe is a constant
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9/28/2009 2 Work: The amount of energy transferred by a force acting through a displacement . “Work” in physics is very different from its everyday meaning If the crate does NOT move, the girl does NO work to the crate! - no matter how hard she pulls the crate. A truck driver does no “mechanical work” even if he drives for several hours: F x = 0 x F W SI unit: Joule (J) 1 J = 1 Newton•meter What happens if the force is not in the same direction with the displacement? This poor man does no work to the books if he stands still . x F x F W x ) cos( 0 x 0 W You carry a bucket of water, walking with a constant velocity horizontally 0 0 ) cos( W F F x Can be positive, negative , or zero
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9/28/2009 3 Work done by a force on a object can be NEGATIEVE How about the work done by the dog to the man? - Positive! x F x F W x ) cos( ) 90 270 if 0 ) cos( ) cos( o o x x F x F W x F This man does negative work to the dog. 1 ) 180 cos( o A force F of 100 N is applied to pull a block. The block moves along the x-direction through displacement of 5 m. What is the work done by the force? What is the work done by the force if it is along the x-direction? x F x F W x ) cos( o 30 ) 30 cos( o x F F F J 433 m 5 N 6 . 86 x F W x ) cos( F F x x N 6 . 86 866 . 0 100 F x J 500 m 5 N 100 x F W
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4 If there are multiple forces acting on an object, the total work done is the algebraic sum of the amount of work done by each force x F x F W ) cos( ) cos( 2 2 1 1 1 F 2 F x 1 2 Suppose the coefficient of kinetic friction m k is 0.2 The object moves 5 m along the x-direction m = 5 kg F = 100 N What is the work done by the force of kinetic friction on the block? The magnitude of the friction force: F x k F mg F F k n k k m n F g m F g x F W k k ) cos( o 180 N 8 . 9 8 . 9 5 2 .
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chapter5_sildes - 9/28/2009 Chapter 5. Energy, Work, Power...

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