Important forms of energy How energy can be transformed and transferred Definition of work

1. Chapter 7 Energy • 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 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

2. A. B. C. D. E. Clicker Question 4 A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? Slide 10-14

3. Answer A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? D. Slide 10-15

4. Conservation of Mechanical Energy • KE1 + PEg1 = KE2 + PEg2

5. Choosing the System Slide 10-16

6. Solving Problems Slide 10-22

7. How can we check to see if the Sum of KE + PE is conserved? • Energy Bar Charts • Equation • Example - pendulum • Ball thrown up in the air Slide 10-13

8. Conceptual Example 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 10-17

9. Example A 200 g block on a frictionless surface is pushed against a spring with spring constant 500 N/m, compressing the spring by 2.0 cm. When the block is released, at what speed does it shoot away from the spring? Slide 10-23

10. Transferring Energy into of out of a system • Heat => Q • Work => W = F||x Energy that changes form within the system is said to be transformed from one form to another Energy that enters or leaves the system is transferred from the system to the environment or vice versa. Need to distinguish what is the system and what is the environment. Forces from the environment can act on the system or objects in the system (external forces) -- Can also add heat from the environment Slide 10-4

11. Work and Work-Energy Theorem • Work = Force x displacement (parallel)We either need the component of force parallel to the displacement or the component of displacement parallel to the force • Work is the energy equivalent of ImpulseImpulse = momentum * Delta tWork = Force x displacement (parallel) • Work by net force = Delta KE • Impulse of net force = Delta P • Machines => Work in = Work outPulley Example Slide 10-23

12. Clicker Question 5 Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Slide 10-18

13. Answer Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Slide 10-19

14. Question • Does an automobile consume more fuel when it’s air conditioner is turned on? • When it’s lights are turned on? • When it’s radio is turned on while it is sitting in the parking lot? Note that fuel economy improves when tires are inflated to maximum pressure. Why?

15. Power and Efficiency • Power = work done (or energy change) / time interval • Two cars accelerate from rest to highway speed • One does it in five seconds (Porsche) • One does it in ten seconds (4 cylinder SUV) • Which uses more power? • Efficiency = useful energy output / Total Energy Input • At best 30-35% of the Chemical Energy in gas => Moving a carCar engines are at best 30-35% EfficientElectric motors can have efficiencies approaching 99% Slide 10-23

16. Energy And Real Life • Tour de FranceA Tour de France bicycle racer needs to eat 8000 calories per day to maintain their weight. About 65-70% of this energy is used to maintain body temperature. • Bow and ArrowsAbout 60-75% of the PE in a drawn bow goes into the KE of an Arrow. The rest of the energy heats the bow. • Guns and BulletsOnly about 30% of the energy in a firearms discharge is transferred to the projectiles they fire Slide 10-23

17. Einstein’s Big Idea Video Einstein's Big Idea Video - Part 1 Einstein’s Big Idea Video - Part 2 Take a look at this and let me know if you would like me to schedule time for us to view the whole video

18. On to Heat and Temperature In this demonstration, we will use thermometers (temperature probes) connected to the computer to measure and display the temperature of objects. We will define temperature as the reading on the thermometer.

19. Mixing Equal Amounts of Hot & Cold Water Predicting the final temperature: Describe in your own words what you think will happen to two cups that contain equal amounts of water at different temperatures when they are mixed together. Suppose one cup of water is at a temperature T1 and the other at a higher temperature T2. How will the temperature of each change? The program plots the temperature measured by each probe as a function of time. If you measured the temperatures in the experiment described above, sketch what you think the temperature vs. time grapht would look like. Describe the direction of temperature change that will occur. In your experience, is it ever possible that when a hot and cold object are put together that the hot object becomes hotter and the cold object becomes colder?

20. Mixing Equal Amounts of Hot & Cold Water Observing the temperature change: Describe the result. How does the temperature change recorded by one probe compare with the temperature change recorded by the second? Explain in your own words why you think this occurs. 3. At an instant a few seconds after you mixed the two cups of water together, the thermometers showed different temperatures. With the water all mixed together, how did each thermometer recognize its "own" water? Why weren't the two temperature readings the same as soon as the water was mixed?

21. Mixing Different Amounts of Hot & Cold Water • Predicting the final temperature: • Describe in your own words what you think will happen to one cup of hot water and two cups of cold water when mixed together. How will the temperature of each change? • On a set of axes like those shown below, sketch what you think the plot of the temperatures will look like.

22. Mixing Different Amounts of Hot & Cold Water Observing the temperature change: 1. Describe the results as shown on the screen. Account for any discrepancies with your prediction. 2. Measure the change in temperature of the hot water. Measure the change in temperature of the cold water. Consider half of the original cold water. Compare its temperature change with the temperature change of the entire original cup of cold water. Explain. Consider two cups of water at different temperatures. The mass of the cold water is m1 and the mass of the hot water is m2. Suppose the two cups are mixed. Call the temperature change of the hot and cold water T1 and T2, respectively. Use your earlier results to help you find the relationship between m1, m2, T1, and T2.