2 What is Work?transfer of energy to a body by application of a force that causes body to move in direction of force.W = F d
3 What is Work? Chapter 12 SI units: joules (J). 1 J = 1 N•m = 1 kg•m2/s2
4 F W d WORK W = ? W = Fd F = 190 N W = (190 N) (2.0 m) d = 2.0 m Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?GIVEN:W = ?F = 190 Nd = 2.0 mWORK:W = FdW = (190 N) (2.0 m)W = 380 JFWd
5 F W d WORK W = ? W = Fd F = 5200 N W = (5200 N) (25 m) d = 25 m A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder?GIVEN:W = ?F = 5200 Nd = 25 mWORK:W = FdW = (5200 N) (25 m)W = 130,000 J or1. 3 x 105 JFWd
6 F W d WORK W = ? W = Fd F = 1 N W = (1 N) (1 m) d = 1m W = 1 J An apple weighing 1 N falls through a distance of 1 m. How much work is done on the apple by the force of gravity?GIVEN:W = ?F = 1 Nd = 1mWORK:W = FdW = (1 N) (1 m)W = 1 JFWd
7 PowerChapter 12rate at which work is done or energy is transformed.SI Unit:watts.watt (W) = 1 J/sPower = worktimep= W/t
8 p W t POWER p = ? p = W/t W = 1 x 105 J p = 1 x 105 J / 20 s t = 20 s It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process?GIVEN:p = ?W = 1 x 105 Jt = 20 sWORK:p = W/tp = 1 x 105 J / 20 sp = 5 x 103 W or 5 kWpWt
9 p W t POWER p = ? p = W/t W = 3960 J p = 3960 J / 60 s t = 60 s While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts?GIVEN:p = ?W = 3960 Jt = 60 sWORK:p = W/tp = 3960 J / 60 sp = 66.0 WpWt
10 p W t POWER p = ? p = W/t W = 5350 J p = 5350 J / 50 s t = 50 s Using a jack, a mechanic does 5350 J of work to lift a car m in 50.0 s. What is the mechanic’s power output?GIVEN:p = ?W = 5350 Jt = 50 sWORK:p = W/tp = 5350 J / 50 sp = 107 WpWt
11 Machines Machines multiply and redirect forces. help people by redistributing work put into them.change either size or direction of input force.allows same amount of work to be done byeither decreasing distance while increasing force orby decreasing force while increasing distance.
13 Mechanical Advantage id od of if ma ma tells how much a machine multiplies force or increases distance.mechanical advantage = output force = input distanceinput force output distancemaidodmaofif
14 MECHANICAL ADVANTAGE id od ma = ? ma = id/od id = 5.0 m Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.GIVEN:ma = ?id = 5.0 mod = 1.5 mWORK:ma = id/odma = 5.0 m / 1.5 mma = 3.3maidod
15 MECHANICAL ADVANTAGE id od ma = ? ma = id/od id = 6.0 m Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.GIVEN:ma = ?id = 6.0 mod = 1.5 mWORK:ma = id/odma = 6.0 m / 1.5 mma = 4maidod
16 MECHANICAL ADVANTAGE of if ma = ? ma = of/if of = 9900 N Determine the mechanical advantage of an automobile jack that lifts a 9900 N car with an input force of 150 N.GIVEN:ma = ?of = 9900 Nif = 150 NWORK:ma = of/ifma = 9900 N / 150 Nma = 66maofif
18 The Lever Family simple machines One of six basic types of machines which are basis for all other forms of machines.have a rigid arm and a fulcrum.six types divided into two families.lever family:inclined plane family:simple lever(3 types)simple inclined planepulleywedgewheel and axlescrew
19 First Class Leversfulcrum located between points of application of input and output forces.
20 fulcrum is at one end of arm and input force is applied to other end. Second Class Leversfulcrum is at one end of arm and input force is applied to other end.
21 Third Class Levers multiply distance rather than force. have a mechanical advantage of less than 1.
22 Pulleys are modified levers. point in middle of a pulley is like fulcrum of a lever.single, fixed pulley has a m. a. of 1.block and tackle: Multiple pulleys working together
23 Wheel & Axle a lever or pulley connected to a shaft. steering wheel of a car, screwdrivers, and cranks
24 The Inclined Plane Family multiply and redirect force.turns small input force into large output force by spreading work out over a large distance.
25 Simple Inclined PlaneChanges both magnitude & direction of force
26 Wedge Functions as two inclined planes back to back. Turns single downward force into two forces directed out to sides.
27 Screwan inclined plane wrapped around a cylinder.
28 Compound Machines Chapter 12 machine made of more than one simple machineExamples :scissorstwo first class levers joined at a common fulcrumcar jackcombination of lever with a large screw
30 Energy Chapter 12 Energy: ability to do work. When you do work on an object, you transfer energy to that object.Whenever work is done, energy is transformed or transferred to another system.SI Units: joules (J)
31 Potential Energy Elastic potential energy: energy that an object has because of position, shape, or conditionstored energy.Elastic potential energy:energy stored in any type of stretched or compressed elastic material, (spring or a rubber band).Gravitational potential energyenergy stored in gravitational field which exists between any two or more objects.
32 Gravitational Potential Energy depends on both mass and height.PE = mghThe height can be relative.height used in above equation is usually measured from ground.However, it can be a relative height between two points, such as between two branches in a tree.
33 GRAVITATIONAL POTENTIAL ENERGY A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?GIVEN:m = 65 kgh = 35 mg= 9.8 m/s2PE = ?WORK:PE = mghPE = (65 kg) (35 m) (9.8 m/s2)PE = 2.2 x 104 kg•m2/s22.2 x 104 J
34 Kinetic Energy energy of a moving object due to object’s motion depends on mass and speed.depends on speed more than mass.
35 KE = ? m = 44 kg v= 31 m/s KE = ½ mv2 KE = ½ (44 kg) (31 m/s)2 KINETIC ENERGYWhat is the kinetic energy of a 44 kg cheetah running at 31 m/s?GIVEN:KE = ?m = 44 kgv= 31 m/sWORK:KE = ½ mv2KE = ½ (44 kg) (31 m/s)2KE = 2.1 x 104 kg x m2/s2 or 2.1 x104 J
36 KE = ? m = 1500 kg v= 29 m/s KE = ½ mv2 KE = ½ (1500 kg) (29 m/s)2 KINETIC ENERGYCalculate the kinetic energy in joules of a 1500 kg car moving at 29 m/s.GIVEN:KE = ?m = 1500 kgv= 29 m/sWORK:KE = ½ mv2KE = ½ (1500 kg) (29 m/s)2KE = 6.3 x105 J
37 KE = ? m = 1500 kg v= 18 m/s KE = ½ mv2 KE = ½ (1500 kg) (18 m/s)2 KINETIC ENERGYCalculate the kinetic energy in joules of a 1500 kg car moving at 18 m/s.GIVEN:KE = ?m = 1500 kgv= 18 m/sWORK:KE = ½ mv2KE = ½ (1500 kg) (18 m/s)2KE = 2.4 x105 J
38 Other Forms of Energy mechanical energy: amount of work an object can do because of object’s kinetic & potential energiesyou can SEE itLarge scale basisnonmechanical energy.you CANNOT SEE itX rays, microwavesSmall scale basis (atomic)
39 Other Forms of Energy Chapter 12 Photosynthesis Atoms and moleculeskinetic energy of particles related to heat and temperature.Chemical reactionsBreaking bonds exothermic/endothermicPhotosynthesisturn energy in sunlight into chemical energy.
40 Other Forms of Energy nuclear fusion reactions Combining of atomic nucleiElectricity.derived from flow of charged particlesbolt of lightning or in a wire.electromagnetic waves.Light energy from sun
42 Energy Transformations readily changes from one form to another.Potential energy changes into kinetic energy.car goes down a hill on a roller coaster, potential energy changes to kinetic energy.Kinetic energy changes into potential energy.kinetic energy a car has at bottom of a hill can do work to carry car up another hill.
43 Energy Transformations Mechanical energy can change tononmechanical energy as a result offriction,air resistance,or other means.
44 The Law of Conservation of Energy energy cannot be created or destroyed.doesn’t disappear, it changes to another form.if total energy in a system increases, it must be due to energy that enters the system from an external source.
45 SYSTEMS open systems (most) closed systemwhen flow of energy into and out of a system is small enough that it can be ignoredopen systems (most)exchange energy with the space that surrounds them.
46 Efficiency of Machines Not all of the work done by a machine is useful work.cannot do more work than work required to operate machine.Because of friction, work output of a machine is always somewhat less than work input.Efficiency:ratio of useful work out to work in.measure of how much useful work it can do.expressed as a percentage.
47 Efficiency of Machines Efficiency EquationMachines need energy input.Because energy always leaks out of a system, every machine needs at least a small amount of energy input to keep going.
48 eff = ? uwo = 140 J wi= 180 J wi eff = uwo/wi eff = 140 J / 180 J EFFICIENCYA sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage.GIVEN:eff = ?uwo = 140 Jwi= 180 JWORK:eff = uwo/wieff = 140 J / 180 Jeff = 0.78 or 78 %effuwowi
49 eff = ? uwo = 1800 J wi= 2400 J wi eff = uwo/wi eff = 1800 J / 2400 J EFFICIENCYAlice & Jim calculate that they must do 1800 J of work to push a piano up a ramp. However, because they must also overcome friction, they must actually do 2400 J of work. What is the efficiency of the ramp?GIVEN:eff = ?uwo = 1800 Jwi= 2400 JWORK:eff = uwo/wieff = 1800 J / 2400 Jeff = 0.75 or 75 %effuwowi
50 eff = 25% uwo = 1200 J wi= ? wi wi = uwo/eff wi = 1200 J / .25 EFFICIENCYIt takes 1200 J of work to lift the car high enough to change a tire. How much work must be done by the person operating the jack if the jack is 25% efficientGIVEN:eff = 25%uwo = 1200 Jwi= ?WORK:wi = uwo/effwi = 1200 J / .25wi = 4800 Jeffuwowi