Work and Power Physics Mrs. Coyle. What is the direction of the component of the force that causes...
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![Page 1: Work and Power Physics Mrs. Coyle. What is the direction of the component of the force that causes the sled to move?](https://reader036.fdocuments.in/reader036/viewer/2022082800/5a4d1b707f8b9ab0599b5052/html5/thumbnails/1.jpg)
Work and Power
PhysicsMrs. 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 JouleEnglish, 19thCentury
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• When the force is in the direction of thedisplacement, =0o: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 ofthe displacement, =180o:cos 180 = -1 => W= F d and work is negative
Which force acting on the sleddoes negative work?
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When the force acts perpendicular to thedisplacement,
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 Fxnet to calculate N)(A: -136J)
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Work done by Resultant
W= W1+ W2+ Wn
Since work is a scalar, the net work done by a resultant force is equal to the sum of theindividual works done by each individualforce 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 (18th Century) developed the steam engine.
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Power
P= W = FdcosFvcos 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 x104J)
2. Calculate the power of the driver. (A: 4x105W)
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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 104 W