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.

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?

objectives1. 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

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?

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.

• 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.

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.

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/s2) -Newton downward force upon the stairs to elevate his body.

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.

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?

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?

kWs

msmkgP

tmgd

tFd

tWP

8.20.5

)1)(5.7)(/81.9)(193(

0coscos

2

Another equation for power

coscos

vF

tdF

tWP

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.

12/11 do now• A 1.00 × 103 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?

avgvFP tWP

KEW tvmmaF

12/12 do now• Water flows over a section of Niagara Falls at a

rate of 1.20 × 106 kg/s and falls down a height of 50.0 m. What is the power of the waterfall?

Objectives

• Work, energy and power review

• Homework – castle learning assignments – 4 new assignments – due Fri.

• Chapter test is on Friday

Class workPage 189 Practice 5F #1-5 1. 65 kW2. 2.38 x 104 W3. 2.61 x 108 s4. 3.6 x 103 s5. a. 7.50 x 104 J; b. 2.50 x 104 WPage 189-Section Review #1-3; Page 195 #35-362. 12.3 s; 2450 J3. 613 W; 2450 J35. 17.2 s36. 5.9 x 108 W

Page 193 #1-6 pp. 193-194 #11-18

Page 194 #26-32

Lab 9 – forces on a spring

• Finish – fix you lab in your lab group

Lab 10 - Energy of a Tossed BallOBJECTIVES1. 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

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?

To Nick