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Work and Power Physics Mrs. Coyle
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What is the direction of the component of the force that causes the sled to move?
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What component of F applied causes this object to move?
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Work The work done on an object by an external constant force: W= F d cos is the angle between the displacement d and the force F.
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Work Is a scalar quantity. Unit: Joule= Newton ∙ meter J= N∙m James Prescott Joule English, 19thCentury
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When the force is in the direction of the displacement, =0 o : cos 0= 1 => W= F d and work is positive Which force acting on the sled does positive work?
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When the force acts opposite to the direction of the displacement, =180 o : cos 180 = -1 => W= F d and work is negative Which force acting on the sled does negative work?
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When the force acts perpendicular to the displacement, then the work done by that force is zero. What force(s) acting on the sled does no work?
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Question Does the centripetal force do work?
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Meaning of Sign of Work When the work done by an external force onto the system is: + : the system gains energy - : the system loses energy
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Example 1 How much work was done by the 30N applied tension force to move the 6kg box along the floor by 20m? (A: 300J) How much work did the weight do in the same case? If µ=0.2, how much work did the friction do? (Hint: Use F xnet to calculate N) (A: -136J)
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Work done by Resultant W= W 1 + W 2 + W n Since work is a scalar, the net work done by a resultant force is equal to the sum of the individual works done by each individual force acting on the object.
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Example 2 How much total work is done on a sled that moves at constant velocity?
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Work is the area under an F vs d graph.
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Power, P= W = Fdcos t t Rate Scalar Unit : J/s = W ( Watt) James Watt (18 th Century) developed the steam engine.
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Power P= W = Fdcos Fvcos t t
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kiloWatt∙ hour Does a kWatthour (kWh) measure: Power or Work
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Pile Driver
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Example 3 A pile driver drives a pile in 10 m with a force of 2000N in 0.05s. 1.Calculate the work done by the driver. (A: 2 x10 4 J) 2.Calculate the power of the driver. (A: 4x10 5 W)
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Example 4 An engine moves a boat through the water at a speed of 12m/s. The force exerted by the engine is 5500N. What is the power of the engine? Answer: 6.6 x 10 4 W
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