Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Important forms of energy How energy can be transformed and transferred Definition of work Concepts of kinetic, potential, and thermal energy The law of conservation of energy Elastic collisions Chapter 10 Energy Topics: Sample question: When flexible poles became available for pole vaulting, athletes were able to clear much higher bars. How can we explain this using energy concepts? Slide 10-1

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 10-7 Examples of Energy Bar Charts & solving Energy Problems

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Energy Bar Charts for a swinging pendulum Slide 10-7 Demonstration: Smash the Professor - Part 2

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Energy Bar Charts and Energy Transformations For the following questions, start by drawing energy bar graphs and identifying energy transformations. Then answer the question. 1.A block slides down a frictionless ramp of height h. It reaches velocity v at the bottom. To reach a velocity of 2v, the block would need to slide down a ramp of height A h B. 2 h C. 3 h D. 4 h E. 6 h Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Energy Bar Charts and Energy Transformations For the following questions, start by drawing energy bar graphs and identifying energy transformations. Then answer the question. 2.A block is shot up a frictionless 40 degree slope with initial veloctiy v. It reaches height h before sliding back down. The same block is shot with the same velocity up a frictionless 20 degree slope. On this slope, the block reaches height A. 2 h B. h C. 1/2 h D. Greater than h, but I connot predict an exact value. E. Less than h, but I can't predict an exact value Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Energy Bar Charts and Energy Transformations For the following questions, start by drawing energy bar graphs and identifying energy transformations. Then answer the question. 3.Two balls, one twice as heavy as the other, are dropped from the roof of a building. Just before hitting the ground, the heavier ball has _________ the kinetic energy of the lighter ball. A. one-half B. the same amount as C. twice D. four times Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Conceptual Example Problem A car sits at rest at the top of a hill. A small push sends it rolling down a hill. After its height has dropped by 5.0 m, it is moving at a good clip. Write down the equation for conservation of energy, noting the choice of system, the initial and final states, and what energy transformation has taken place. Slide 10-25

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Example: Roller Coaster Demonstration: which ball reaches the end of the track first A. The one in front B. The one in back C. Neither, they both reach the end of the track at the same time Using Conservation of Energy to find the speed of a roller coaster Slide 10-23

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Spring Force The magnitude of the spring force is proportional to the stretch of the spring: Slide 8-14 F sp = k ∆L = k (L - L 0 )

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The spring force is directed oppositely to the stretch of the spring. In this case, we can then write Hooke’s law as Hooke’s Law Slide 8-15 (F sp ) x = –k ∆x

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Beyond the Elastic Limit Slide 8-25

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding Which spring has the largest spring constant? Slide 8-16

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Which spring has the largest spring constant? Slide 8-17 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding The same spring is stretched or compressed as shown below. In which case does the force exerted by the spring have the largest magnitude? Slide 8-18

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The same spring is stretched or compressed as shown below. In which case does the force exerted by the spring have the largest magnitude? Slide 8-19 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Spring Problem 1 Slide A 20-cm long spring is attached to the wall. When pulled horizontally with a force of 100 N, the spring stretches to a length of 22 cm. What is the value of the spring constant?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Spring Problem 2 Slide The same spring is used in a tug-of-war. Two people pull on the ends, each with a force of 100 N. How long is the spring while it is being pulled?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Spring Problem 3 Slide The same spring is now placed vertically on the ground and a 10.2 kg block is balanced on it. How high is the compressed spring?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Spring Problem 4 Slide The same spring is placed vertically on the ground and a 10.2 kg block is held 15 cm above the spring. The block is dropped, hits the spring, and compresses it. What is the height of the spring at the point of maximum compression?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Spring Problem 5 A spring with spring constant k = 125 N/m is used to pull a 25 N wooden block horizontally across a tabletop. The coefficient of friction between the block and the table is µ k = By how much does this spring stretch from its equilibrium length? Slide 8-22