Phys. Chapter 08 - Beau Chene High School Home Page

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Efficiency is a measure of how well a machine worksThe simple machines we have considered so far are ideal, frictionless machines.Real machines, however, are not frictionless. They dissipate energy. When theparts of a machine move and contact other objects, some of the input energy isdissipated as sound or heat. The efficiency of a machine is a measure of howmuch input energy is lost compared with how much energy is used to performwork on an object. It is defined by the following equation:NSTATOPIC: Simple machinesGO TO: www.scilinks.orgsciLINKS CODE: HF2083eff = ⎯ W out⎯WinIf a machine is frictionless, then mechanical energy is conserved. This meansthat the work done on the machine (input work) is equal to the work done bythe machine (output work) because work is a measure of energy transfer. Thus,the mechanical efficiency of an ideal machine is 1, or 100 percent. This is thebest efficiency a machine can have. Because all real machines have at least a littlefriction, the efficiency of real machines is always less than 1.Section Review1. Figure 8-16 shows an example of a RubeGoldberg machine. Identify two types ofsimple machines that are included in thiscompound machine.2. The efficiency of a squeaky pulley system is73 percent. The pulleys are used to raise amass to a certain height. What force is exertedon the machine if a rope is pulled 18.0 m inorder to raise a 58 kg mass a height of 3.0 m?Figure 8-163. A person lifts a 950 N box by pushing it up an incline. If the personexerts a force of 350 N along the incline, what is the mechanical advantageof the incline?4. You are attempting to move a large rock using a long lever. Will the workyou do on the lever be greater than, the same as, or less than the workdone by the lever on the rock? Explain.5. Physics in Action A bicycle can be described as a combination ofsimple machines. Identify three types of simple machines that are used topropel a typical bicycle.Copyright © by Holt, Rinehart and Winston. All rights reserved.Rotational Equilibrium and Dynamics301
Earlier in this chapter, we discussed angular momentum and its effects in themacroscopic world of your everyday experience. In the early twentieth century,scientists realized that they must modify their ideas about angular momentumwhen working with the microscopic world of atoms and subatomic particles.Electron orbital angular momentumIn 1911, Ernest Rutherford proposed a model of the atom in which negativelycharged particles called electrons orbit a positively charged nucleus containingparticles called protons, much as the Earth orbits the sun, as shown inFigure 8-17(a). Because the electron orbits the nucleus, it has an orbital angularmomentum.Further investigations into the microscopic realm revealed that the electroncannot be precisely located in space and therefore cannot be visualized asorbiting the proton. Instead, in modern theory the electron’s location isdepicted by an electron cloud, as shown in Figure 8-17(b), whose densityvaries throughout the cloud in proportion to the probability of finding theelectron at a particular location in the cloud. Even though the electron doesnot orbit the nucleus in this model, the electron still has an orbital angularmomentum that is very different from the angular momentum discussed earlierin this chapter.For most of the history of science, it was assumed that angular momentumcould have any possible value. But investigations at the atomic level haveshown that this is not the case. The orbital angular momentum of the electroncan have only certain possible values. Such a quantity is said to be discrete,and the angular momentum is said to be quantized. The branch of moderntheory that deals with quanta is called quantum mechanics.(a)(b)Figure 8-17(a) In Rutherford’s model of the atom, electronsorbit the nucleus. (b) In quantum mechanics, anelectron cloud is used to show the probability thatthe electron will be at different points. The densestregions of the cloud represent the most probablelocations for the electron.302Chapter 8Copyright © by Holt, Rinehart and Winston. All rights reserved.
Page 1 and 2: Copyright © by Holt, Rinehart and
Page 3 and 4: 8-1Torque8-1 SECTION OBJECTIVES•
Page 5 and 6: •INTERACTIVEFigure 8-4In each exa
Page 7 and 8: PRACTICE 8ATorque1. Find the magnit
Page 9 and 10: MOMENT OF INERTIAmoment of inertiat
Page 11 and 12: inertia of a thin rod is larger if
Page 13 and 14: This value for the force in the wir
Page 15 and 16: •INTERACTIVE8-3Rotational dynamic
Page 17 and 18: MOMENTUMHave you ever swung a sledg
Page 19 and 20: Equate the initial and final angula
Page 21 and 22: SAMPLE PROBLEM 8EConservation of me
Page 23 and 24: 8-4Simple machines8-4 SECTION OBJEC
Page 25: Human ExtendersA cyborg, as any sci
Page 29 and 30: CHAPTER 8SummaryKEY TERMSangular mo
Page 31 and 32: CENTER OF MASS ANDROTATIONAL EQUILI
Page 33 and 34: 36. A 65 kg woman stands at the rim
Page 35 and 36: 60. A cylindrical 5.00 kg pulley wi
Page 37 and 38: 72. The efficiency of a pulley syst
Page 39 and 40: PROCEDUREInclined plane3. Set up th

Phys. Chapter 08 - Beau Chene High School Home Page

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