1Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 1Hour Exam #1• Hour Exam I, Wed. Feb. 14, in-class (50 minutes)• Material Covered: Chap 1, 3-6• One page of notes (8.5” x 11”) allowed• 20 multiple choice questions• Scantron sheets will be used -bring #2 HB pencils and calculator• In-class review, Monday Feb. 12On-line review questions, previous exams, at course web siteuw.physics.wisc.edu/~rzchowski/phy107Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 2From last time…• Energy and work• Work = Force x Distance– Units of newton-meters (N-m) = Joules (J)– Work is done on an object only when the object moves• Energy = amount of work a system can do– Has units of Joules (J), same as workWed, Feb. 7 Phy107, Spring 2007 Lecture 8 3QuestionI hold up a bowling ball in a fixed position.The work I do on the ball A. Depends on the weight of the ball. B. Depends on how long I holds it. C. Is zero.Although I exert a force against gravity to hold the bowling ball up,I have not moved the ball over any distance.So the work I do on the ball is zero.Work = Force x DistanceWed, Feb. 7 Phy107, Spring 2007 Lecture 8 4Lifting an objectI lift a body weighing 1 N upward at a constant verticalvelocity of 0.1 m/s. The net force on the body isA. 1 N upwardB. 1 N downC. 0 ND. 0.1 N upE. 0.1 N downSince the acceleration is zero, the net forcemust be zero. I apply a force upward thatexactly cancels the gravitational forcedownwardWed, Feb. 7 Phy107, Spring 2007 Lecture 8 5Doing work on an objectI lift the 1 N body a total distance of 1 m.What work have I have done on the body?A. 0 JB. 0.1 JC. 1 JD. 10 JWork = Force x Distance = 1 N x 1 m = 1 N-m = 1 JouleWed, Feb. 7 Phy107, Spring 2007 Lecture 8 6How much work can I do?2Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 7EnergyA object’s energy is the amount of work it can do.Energy comes in many formsIt can be converted into other forms without loss(i.e it is conserved)Energy of motionGravitational Energy Electrical EnergyThermal EnergySolar EnergyChemical EnergyNuclear EnergyWed, Feb. 7 Phy107, Spring 2007 Lecture 8 8Electrical Energy• Electricity is the flow of charged particles.• These have electromagnetic force between themsimilar to the gravitational force.• Electromagnetic force can do work.• Doing work against this force can store energy in thesystem.• The energy can be removed at any time to do work.Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 9Thermal Energy• Otherwise known as heat.• The temperature of an object is related to theamount of energy stored in the object.• The energy is stored by the microscopic vibratorymotion of atoms in the material.• This energy can be transferred from one object toanother by contact.• It can also be turned into work by contact.Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 10Chemical Energy• Chemical energy transformed into– Heat energy– Light energy– Sound energy– Energy of motionWed, Feb. 7 Phy107, Spring 2007 Lecture 8 11Energy of motionIn outer space: Applya force of 1 N to a 1 kg rock (initially at rest) for a distance of 1 m.How much work have I done?Work = Force x Distance = (1 N)x(1 m)= 1 Joule of work.My energy decreased by 1 Joule,the rock has one more Joule of energy.Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 12The rock• Rock now has energy of motion (kinetic energy)• As a result of my work,rock moves at constant speed.• Observation:– rock gains velocity as work is done on it.– energy from me has been transferred to the rock• velocity & energy of motion are related3Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 13kinetic energyWork done= Force x Distance= (mass x acceleration) x Distance! vfinaltime! vaverage" time! vfinal2" timexmassx! =12m vfinal( )2Kinetic energy =! 12mv2Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 14Kinetic energy• The kinetic energy of an object is! 12mv2• Positive work done on isolated object– object speeds up, kinetic energy increases• Negative work done on isolated object– object slows down, kinetic energy decreasesWed, Feb. 7 Phy107, Spring 2007 Lecture 8 15ExampleA motor does 2000 J of work accelerating a 1000 kg carfrom rest. How fast is the car moving?A. 0.5 m/sB. 1 m/sC. 2 m/sD. 4 m/sE. 10 m/sWork done = change in kinetic energy2000J = (1/2)mv2 - 0 =(1/2)(1000 kg)(v2) 4 m2 / s2 = v2Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 16Work-Energy relation• Suppose object initially moving at vinitialIt has kinetic energy• As the result of a net work Wnet, the velocityincreases to vfinal,and the Kinetic Energy increases to! 12mvinitial2! 12mvfinal2! Wnet=12mvfinal2"12mvinitial2The change in kinetic energy of an isolatedobject is equal to the net work done on it.Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 17Lifting againI do work when I lift a bowling ball.But bowling ball does not change velocitykinetic energy has not changed.Why not?A. Something else did negative workB. I didn’t do any net workC. Air resistanceD. Velocity change too small to measure.E. Quantum black holes sucked up the energyGravitational force -mgLifting force mg1 NGravity does negative work on the ball.Net work is zeroWed, Feb. 7 Phy107, Spring 2007 Lecture 8 18Where’s the energy?• When I did work, I transferred energy to the ball.• But zero net work done on the ball.• Ball’s kinetic energy has not changed.• Energy is ‘stored’ as potential energy.• Can think of this as energy stored in thegravitational field.4Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 19Where does the energy go?Water tower and pumphouseWater is pumped into tower whenelectricity cost is lowElectrical energy transformed intogravitational energy.Work is extracted when needed totransport the water to homes.Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 20Ball falls down in gravity• An object in a gravitational field can do work whenit falls.• Ball initially held at rest.– vinitial==0 --> Kinetic energy = 0• Ball released.• Gravitational force = mg, falls with acceleration g• Work done by gravitational force in falling distanceh is Force x Distance = mgh.• Final kinetic energy = mgh =! 12mvfinal2Wed, Feb. 7 Phy107, Spring 2007 Lecture 8 21QuestionTwo marbles, one twice as heavy as the other, aredropped to the ground from the roof of a building. Justbefore hitting the ground, the heavier marble hasA. same kinetic energy as the lighter oneB. 2 times as much kinetic energy as the lighter oneC. 1/2 as
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