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# Physics Work and Energy

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• When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. For this question, students may discuss the top speed of the cars or the time required for the cars to accelerate from 0 to 60 miles per hour. See if you can get them to talk about energy consumption. How quickly would each consume a gallon of gas? They likely will mention horsepower, but it is unlikely that any will consider watts as a unit of power.
• Remind students that W = Fd, and ask them to substitute this for W in the power formula. Then ask what d / t represents. At this point, move on to the next slide, which shows the alternative form of the power equation ( P = Fv ).
• Be sure students understand that P = Fv is not a new definition. It is simply a different but equivalent formula that makes calculations easier in some cases. Do not just show the units. Ask students to figure them out. It should be easy for them to get J/s but the basic units of kg•m 2 /s 3 will be more difficult. This provides a good opportunity to review the units for joules and newtons. The horsepower was based on the work a good horse could do lifting coal out of a mine. A good horse could lift 275 pounds of coal at 2.0 ft/s, so it could do 550 ft•lb/s. This is equivalent to 746 J/s or 746 W.
• Remind students that a joule is the energy required to lift an apple a distance of a meter. Light bulbs are also rated for the amount of light they produce. Fluorescent bulbs using 22 watts can produce as much light as incandescent bulbs using 100 watts.
• These two problems can be done in either order. Most students will use P = Fv, and then use P = W / t to get the work done. However, it is worth showing them that they could calculate the work done by using W = Fd , where d = 2.0 m/s  600.0 s = 1.2 x 10 3 m. After getting the work, then they could use the fundamental definition of power ( P = W / t ) to get the power.
• Having more power will allow the car to consume more energy per second. This will produce greater accelerations and higher energy costs. Automobiles generally measure the power in hp. However, small motors are sometimes rated in watts and sometimes rated in hp.
• ### Physics Work and Energy

1. 1. Work and Energy Section 4 What do you think? • Two cars are identical with one exception. One of the cars has a more powerful engine. How does having more power make the car behave differently? – What does power mean? – What units are used to measure power?Mr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
2. 2. Work and Energy Section 4 Power • The rate of energy transfer – Energy used or work done per secondMr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
3. 3. Work and Energy Section 4 Power • SI units for power are J/s. – Called watts (W) – Equivalent to kg•m2/s3 • Horsepower (hp) is a unit used in the Avoirdupois system. – 1.00 hp = 746 WMr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
4. 4. Work and Energy Section 4 Watts • These bulbs all consume different amounts of power. • A 100 watt bulb consumes 100 joules of energy every second.Mr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
5. 5. Work and Energy Section 4 Classroom Practice Problems • Two horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min. – Calculate the power delivered by the horses. – How much work is done by the two horses? • Answers: 1.0 x 103 W and 6.0 x 105 JMr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
6. 6. Work and Energy Section 4 Now what do you think? • Two cars are identical with one exception. One of the cars has a more powerful engine. How does having more power make the car behave differently? – What does power mean? – What units are used to measure power?Mr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
7. 7. Work and Energy Section 4 The End Review At www.ScienceByThompson.comMr. ThompsonsPublishing Company © Houghton Mifflin Harcourt Physics Class
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