Work and Power Chapter 5 Section 1. (c) 2007 brainybetty.com ALL RIGHTS RESERVED. 2 What is Work?...

Work and Power

Chapter 5 Section 1

(c) 2007 brainybetty.com ALL RIGHTS RESERVED.

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What is Work?

• No, we are not talking about the J-O-B word, but something that you do and use everyday.


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• If you were to press down on your desk as hard as you can, and although your muscles may start to ache later, you have not done any work.


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• People often make the misconception that work is a push or pull. We know better because a push or pull is the definition for force.

• Work is the transfer of energy that occurs when a force makes an object MOVE !!!!!


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• So, no matter how much force you put on an object, if it fails to move, then no work is done on the object.

• Ex. If I pick up a load of laundry and hold it in the air, are you doing work?


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• Ex. If you are just holding a basket in the air, are you doing work?


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• Two conditions have to be satisfied for work to be done on an object. One is that the object has to move, and the other is that the motion of the object must be in the same direction as the applied force.

• Again, if you are picking something up, you are doing work, but if you are just holding something in the air, you are not doing work.


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• What if you are holding the basket and start walking, are you doing work?


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Work and Energy

• How are work and energy related? When work is done, a transfer of energy always occurs. This is easy to understand when you think about how you feel after carrying a heavy box up a flight of stairs.


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• You increased its potential energy by increasing its height.

• When you do work on an object, you increase its energy. If you do work, your energy decreases. Energy is always transferred from the object that is doing the work to the object on which the work is done.


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Formula:

• Work = Force x distance

• Or

• W = F x D


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Problem:

• You move a 75 – kg refrigerator 35 meters. This requires a force of 90 N.

• How much work, in joules, was done while moving the refrigerators?


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• Work =?

• Force = 90 N

• Distance = 35 m


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• Work = 90 x 35

• Work = 3,150 Joules.

• Work is done on an object only when a force is being applied to the object.


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Power

• Power is the amount of work done in a certain amount of time.

• It is a rate –the rate at which work is done.

• The SI unit for power is watt.

http://images.google.com/imgres?imgurl=http://www.electricscotland.com/history/men/pics/James%2520Watt.gif&imgrefurl=http://www.electricscotland.com/history/men/james_watt.htm&usg=__mIsoCEmPOX5N0SamqmTtza1U_lA=&h=346&w=300&sz=105&hl=en&start=9&um=1&tbnid=E6iRfZgHE4MIUM:&tbnh=120&tbnw=104&prev=/images%3Fq%3Dwatt%26hl%3Den%26safe%3Dactive%26rls%3Dcom.microsoft:*%26sa%3DN%26um%3D1


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Formula:

• Power = work/time

• Or

• P= W/t


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Problem

• : It took five minutes to move a refrigerator. You did 3,150 joules of work in the process. How much power was required to move the refrigerator?


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Answer

• Power =?

• Work = 3,150

• Time = 5 min. = 5 x 60 = 300s

• P = 3,150 / 300

• P = 10.5 W or watt


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Power and Energy

• Doing work is a way of transferring energy from one object to another. For example, a light bulb uses electrical energy to produce heat and light, but no work is done. Power is produced or used any them energy is transferred form on e object to another. Power is the energy transferred divided by the time needed for the transfer to occur.


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Formula:

• Power = energy / time

• Or

• P = E/t


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• Which of the following has work being done?

• A person shovels snow off a sidewalk.

• A worker lifts bricks, one at a time, from the ground to the back of the truck.

• A roofer’s assistant carries a bundle of shingles across a construction site.


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Practice Problem:

• A passenger weighing 500 N is inside an elevator weighing 24,500 N that rises 30 m in 1 min. How much power is needed for the elevator’s trip?

• Write down what you know:


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• Power = work / time

• Power =?

• Work =?

• Time = 1 min. = 60s


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Q: How do we solve this problem?

• We need work, but it doesn’t tell us directly in the question. What do you need to do?

• You need to figure out work.

• Work = Force x Distance


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• W =?

• Force = 500 N

• Distance = 30 m

• W= 500 x 30

• W= 15000 J


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• Now….

• P = W/t

• P = 15000 / 60


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What am i?


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Work and Power Chapter 5 Section 1. (c) 2007 brainybetty.com ALL RIGHTS RESERVED. 2 What is Work?...

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