chapter 14 work, power and simple machines do work, son!
TRANSCRIPT
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Chapter 14Work, Power and Simple
Machines• Do work, son!
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14.1: Work and Power
• What is work?– the product of force and distance
• Question: How does a stationary object begin moving? (Recall...From Chapter 12)– When an unbalanced force acts on it.
• Work is done when a force acts on an object in the direction the object moves.
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Is work being done?
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Work Requires Motion
Question: Does a weight lifter do work on the barbell to lift it over his head?
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Answer: yes, force is up and barbell moves up
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Stationary Objects
• Question: Is the weight lifter doing work while he holds the barbell stationary over his head?
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ANSWER
• Answer: NO, the barbell is stationary
• For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is NO movement, NO work is done!!!
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Work Depends on Direction
• The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement.
• A force does not have to act entirely in the direction of movement to do work.
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Is work being done?
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Is work being done????
• The force acts upward and to the right.
• The suitcase only moves to the right.
• Any part of a force that does not act in the direction of motion does NO work on an object
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Calculating Work
• Work = Force x Distance
• Units of Force and Distance– SI unit for force is newtons– SI unit for distance is meters
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Unit of Work
• The SI unit for work is – newton-meter or the JOULE (J)
• When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.
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Practice Problem• Imagine the weight lifter. The weight lifter lifts a
1600 newton barbell over his head. Assume the barbell is lifted to a height of 2.0 meters. What is the work done?
• Work = Force x Distance• Work = 1600 N x 2.0 m• Work = 3200 N m = 3200 J
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What is Power?
• Power: – the RATE of doing work
• Doing work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time
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Q: Does a person shoveling snow do work? Explain.
• Answer: YES, because the shovel is moving in the same direction as the force being applied
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Q: Does a snow blower do work? Explain
Answer: YES, but because the snow blower does the work in less time it has more POWER!!!
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Calculating Power
• Power = Work / Time – Work is in joules (J)– Time is in seconds (s)
• The SI unit for POWER is the watt (W) = one joule per second– Thus, a 40-watt light bulb requires 40 joules
each second that it is lit.
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Practice Problem
• You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?
• Power = work / time • Where is work in the problem?• Can we calculate work?
– W = F x d
– W = 72 N x 1.0 m = 72J
• Now plug work into the formula for power• Power = 72J/ 2.0s• P = 36 Watts
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James Watt and Horsepower
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Horsepower
• Horsepower (hp): common unit for power. One horsepower is equal to about 746 watts.
• FYI...Interesting side note: Horsepower is literally
based on the power output of a very strong horse!!!