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12/6 do now Is conservation of mechanical energy likely to hold in this situation? An ice skater glides across freshly made ice. Explain your reasoning.

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Presentation on theme: "12/6 do now Is conservation of mechanical energy likely to hold in this situation? An ice skater glides across freshly made ice. Explain your reasoning."— Presentation transcript:

1 12/6 do now Is conservation of mechanical energy likely to hold in this situation? An ice skater glides across freshly made ice. Explain your reasoning.

2 5.3 Conservation of energy – due Monday 12/2 1.What is the meaning of the word “conserved”? Give example(s) of some quantity that are conserved. 2.What is Mechanical Energy? Is it a new form of energy? What are some forms of non-mechanical energy? 3.When is mechanical energy conserved? 4.When an object encounters friction, its mechanical energy is lost. What does mechanical energy become? 5.Is total energy conserved even when mechanical energy is not conserved?

3 objectives 1.Relate the concepts of energy, time, and power. 2.Calculate power in two different ways. 3.Explain the effect of machines on work and power. Lab 10 – energy of a tossed ball Homework: 5.4 essay – due Mon 12/9 Castle Learning – extra credit on test if everyone does the castle learning assignment

4 questions Joe and Joey weights the same and both run up the stairs. Joe runs in 30 second and Joey runs in 20 second. Who does more work? What is different?

5 Power Power is the rate at which work is done or the rate of energy transfer by any method. It is the work/time ratio. Mathematically, it is computed using the following equation. The standard metric unit of power is the Watt. Example: A 60 watt light bulb transfers 60 J every second of electrical energy into light energy.

6 All machines are typically described by a power rating. The power rating indicates the rate at which (how fast) that machine can do work upon other objects. Machines with different power rating do the same work in different time intervals. A 12 watt vacuum can do the job 2 times faster than a 6 watt vacuum. The power rating of a car relates to how rapidly the car can be accelerated. The power of a light bulb indicate how fast electrical energy can be transferred to light energy. A 60 watt light bulb transfers 60 J of energy per second. A 100 watt light bulb transfers 100 J of energy per second. Some people are more power-full than others. That is, some people are capable of doing the same amount of work in less time or more work in the same amount of time.

7 Question Many mountain roads are built so that they zigzag up the mountain rather than go straight up toward the peak. What is the advantages of such a design from the view point of energy conservation and power? Assuming mechanical energy is conserved, the same amount of energy is needed to reach the top in both cases. Because the same amount of work must be done, the path with a longer distance takes more time and hence requires less power.

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9 Example 1 Ben Pumpiniron elevates his 80-kg body up the 2.0- meter stairwell in 1.8 seconds. What is his power? It can be assumed that Ben must apply an (80 kg x 9.81 m/s 2 ) - Newton downward force upon the stairs to elevate his body.

10 Example 2 Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most work? ______________ Which student delivers the most power? ______________ Explain your answers.

11 Example 3 When doing a chin-up, Julia lifts her 42.0-kg body a distance of 0.25 meters in 2.0 seconds. What is the power delivered by Julia’s biceps?

12 Example 4 A 193 kg curtain needs to be raised 7.5 m, at constant speed, in as closed to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, ad 5.5 kW. Which motor is best for the job?

13 Another equation for power

14 kilowatt-hour is unit for energy Your household's monthly electric bill is often expressed in kilowatt-hours. One kilowatt-hour is the amount of energy delivered by the flow of l kilowatt of electricity for one hour. Use conversion factors to show how many joules of energy you get when you buy 1 kilowatt-hour of electricity.

15 12/11 do now A 1.00 × 10 3 kg sports car accelerates from rest to 25.0 m/s in 7.50 s. What is the average power output of the automobile engine?

16 12/12 do now Water flows over a section of Niagara Falls at a rate of 1.20 × 10 6 kg/s and falls down a height of 50.0 m. What is the power of the waterfall?

17 Objectives Work, energy and power review Homework – castle learning assignments – 4 new assignments – due Fri. Chapter test is on Friday

18 Class work Page 189 Practice 5F # kW x 10 4 W x 10 8 s x 10 3 s 5.a x 10 4 J; b x 10 4 W Page 189-Section Review #1-3; Page 195 # s; 2450 J W; 2450 J s x 10 8 W Page 193 #1-6 pp #11-18 Page 194 #26-32

19 Lab 9 – forces on a spring Finish – fix you lab in your lab group

20 Lab 10 - Energy of a Tossed Ball OBJECTIVES 1.Measure the change in the kinetic and potential energy as a ball moves up and down in free fall. 2.Graph potential energy, kinetic energy, and total energy. 3.Analyze the graph to determine how much kinetic energy is lost. 4.Reach conclusions regarding the amount of energy possessed by the ball as it moves in air. MATERIALS Ball, Computer with Vernier Program, Interface, motion detector, basket

21 PRELIMINARY QUESTIONS For each question, consider the free-fall portion of the motion of a ball tossed straight upward, starting just as the ball is released to just before it is caught. Assume that there is very little air resistance. 1.What form or forms of energy does the ball have while momentarily at rest at the top of the path? 2.What form or forms of energy does the ball have while in motion near the bottom of the path? 3.Sketch a graph of position vs. time for the ball. 4.Sketch a graph of velocity vs. time for the ball. 5.Sketch a graph of kinetic energy vs. time for the ball. 6.Sketch a graph of potential energy vs. time for the ball. 7.Sketch a graph of total energy vs. time for the ball. 8.If there are no frictional forces acting on the ball, how is the change in the ball’s potential energy related to the change in kinetic energy?

22 To Nick


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