10 Work, Energy, and Machines BIGIDEA

Transcription

1 10 Work, Energy, and Machines BIGIDEA Write the Big Idea for this chapter. Use the What I Know column to list the things you know about the Big Idea. Then list the questions you have about the Big Idea in the What I Want to Find Out column. As you read the chapter, fill in the What I Learned column. K What I Know W What I Want to Find Out L What I Learned 149

2 10 Work, Energy, and Machines 1 Work and Energy MAINIDEA Write the Main Idea for this section. REVIEW VOCABULARY law of conservation of momentum Recall and write the definition of the Review Vocabulary term. law of conservation of momentum NEW VOCABULARY work joule energy work-energy theorem kinetic energy translational kinetic energy power watt Use your book to define each term. work joule energy work-energy theorem kinetic energy translational kinetic energy power watt 150

3 Student Edition, pp Complete the sentence. When a force applied at an angle causes an object to move horizontally, only the of the force does work. GET IT? Determine the work you do when you exert a force of 3 N at an angle of 45 from the direction of motion for 1 m. Draw a force diagram showing the force you exert (F me ) to the right on a box and the force your friend exerts (F friend ) to the left on the box. As a result of these two forces, the box moves to the right. Also show the gravitational force and the normal force. Explain why some forces do no work on the box. GET IT? Explain why you do positive work on the box and your friend does negative work on the box. GET IT? Describe another scenario in which you do work on a system, and explain how much work is done on the system. 151

4 Student Edition, pp Summarize how you can use a force-displacement graph to determine the work done in each case. If the force is constant If the force increases linearly Use with Example Problem 2. Use this column for scratch work and sketches. TRY IT! Problem An ice skater slides toward a sled sitting on the ice and hits against it. The skater exerts a 12.6-N force on the sled at an angle of 15.3 below the horizontal. The sled then moves 15.4 m forward. How much work did the skater do on the sled? Assume friction is negligible. 1. ANALYZE AND SKETCH THE PROBLEM KNOWNS UNKNOWNS F = d = =? θ = What is the system? What is the force that is doing work on the system? 2. SOLVE FOR THE UNKNOWN Write the equation for work. Then insert the known quantities. 3. EVALUATE THE ANSWER Explain why your units for work are correct. Explain why the sign of the answer is correct. 152

5 Student Edition, pp Give an example of a force that does work on a system. Then use the work-energy theorem to your example. Write a sentence using the word energy with its science usage and a sentence using the word energy with its common usage. Science usage Common usage Describe the relationship between each of the following pairs of quantities. work and energy power and energy power and work 153

6 Student Edition, pp Write work, energy, or power beside each unit to identify the quantity that it measures. Some quantities are measured by more than one unit. Some units are used for more than one quantity. Units Quantities kg m 2 /s 2 W J/s J Use with Example Problem 3. Use this column for scratch work and sketches. TRY IT! Problem Moving ocean water exerts a force of 375 N on a boat, causing the boat to move a distance of 34.7 m in 8.34 s. What power does the boat produce in kw? 1. ANALYZE AND SKETCH THE PROBLEM KNOWNS UNKNOWNS d = =? t = F = 2. SOLVE FOR THE UNKNOWN Use the definition of power. 3. EVALUATE THE ANSWER Explain why your units for power are correct. Explain why the sign of the answer is correct. 154

7 Student Edition, p. 273 Identify the equation you would use to calculate power if you know the following quantities. work and time force, distance, and time time and change in energy force and velocity SUMMARIZE How does the MAINIDEA for this section relate to the chapter s BIGIDEA? REVIEW IT! 15. MAINIDEA If the work done on an object doubles its kinetic energy, does it double its speed? If not, by what ratio does it change the speed? 16. Work Murimi pushes a 20-kg mass 10 m across a floor with a horizontal force of 80 N. Calculate the amount of work done by Murimi on the mass. 17. Work Suppose you are pushing a stalled car. As the car gets going, you need less and less force to keep it going. For the first 15 m, your force decreases at a constant rate from N to 40.0 N. How much work did you do on the car? Draw a force-displacement graph to represent the work done during this period. 155

8 18. Work A mover loads a 185-kg refrigerator into a moving van by pushing it at a constant speed up a 10.0-m, friction-free ramp at an angle of inclination of 11. How much work is done by the mover on the refrigerator? 19. Work A kg ball falls 2.5-m. How much work does the force of gravity do on the ball? 20. Work and Power Does the work required to lift a book to a high shelf depend on how fast you raise it? Does the power required to lift the book depend on how fast you raise it? Explain. 21. Power An elevator lifts a total mass of kg a distance of 40.0 m in 12.5 s. How much power does the elevator deliver? 22. Mass A forklift raises a box 1.2 m and does 7.0 kj of work on it. What is the mass of the box? 23. Work You and a friend each carry identical boxes from the first floor of a building to a room located on the second floor, farther down the hall. You choose to carry the box first up the stairs, and then down the hall to the room. Your friend carries it down the hall on the first floor, then up a different stairwell to the second floor. How do the amounts of work done by the two of you on your boxes compare? 24. Critical Thinking Explain how to find the change in energy of a system if three agents exert forces on the system at once. 156