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EnergyEnergy
Chapter 9Chapter 9
ObjectivesObjectives Define and describe work.Define and describe work. Define and describe power.Define and describe power. State the 2 forms of mechanical energy.State the 2 forms of mechanical energy. State 3 forms of potential energy.State 3 forms of potential energy. Describe how work and kinetic energy are Describe how work and kinetic energy are
related.related. State the work-energy theorem.State the work-energy theorem. State the law of conservation of energy.State the law of conservation of energy. Describe how a machine uses energy.Describe how a machine uses energy. Explain why no machine can have an Explain why no machine can have an
efficiency of 100%.efficiency of 100%. Describe the role of energy in living Describe the role of energy in living
organisms.organisms.
9.1 Work9.1 Work Recall that a change in an object’s Recall that a change in an object’s
momentum is related to an impulse –momentum is related to an impulse –a force and how long the force acts.a force and how long the force acts.
““How long” does not always mean How long” does not always mean time, however. It can also mean time, however. It can also mean distancedistance..
Force x distance = workForce x distance = workWork is the product of the force on an Work is the product of the force on an
object and the distance through which object and the distance through which the object is moved.the object is moved.
9.1 Work9.1 Work
For work to be done For work to be done in Physicsin Physics, , a force acts on an object and the a force acts on an object and the object moves object moves in the direction of in the direction of the forcethe force..Work is done on a load when you Work is done on a load when you
lift it against Earth’s gravity. lift it against Earth’s gravity. No work is done on the load to hold No work is done on the load to hold
it once it has been lifted or to carry it once it has been lifted or to carry the load around the room.the load around the room.
9.1 Work9.1 Work Work is done only in Work is done only in
lifting the barbell.lifting the barbell. The physics of a The physics of a
weightlifter holding a weightlifter holding a stationary barbell is no stationary barbell is no different than the physics different than the physics of a table supporting a of a table supporting a barbell's weight. NO NET barbell's weight. NO NET FORCE acts on the barbell FORCE acts on the barbell (forces are balanced!!), (forces are balanced!!), no work is done, and no no work is done, and no change in its energy change in its energy occurs.occurs.
Is This Work???Is This Work???
A student applies a force to a A student applies a force to a wall and becomes exhausted.wall and becomes exhausted.
A book falls off a table and lands A book falls off a table and lands on the ground.on the ground.
You do some push-ups.You do some push-ups.A waiter carries a tray full of A waiter carries a tray full of
meals across the room.meals across the room.
9.1 Work9.1 Work
The waiter does NO The waiter does NO work because the work because the object is NOT object is NOT moving in the moving in the direction of the direction of the force. (The force is force. (The force is against gravity – its against gravity – its in an up & down in an up & down direction.) direction.)
9.1 Work9.1 Work
Mathematically, work = force x Mathematically, work = force x distance.distance.W = FdW = Fd
The unit for work combines the unit The unit for work combines the unit for force, N, with the unit for for force, N, with the unit for distance, m.distance, m.
A N-m is also called a joule (J).A N-m is also called a joule (J).1 kJ equals 1000 J.1 kJ equals 1000 J.
W = F x dW = F x d Recall that when lifting an object against Recall that when lifting an object against
gravity, the force required equals the object’s gravity, the force required equals the object’s weight.weight.
FFgravgrav = Weight = mg = Weight = mg Therefore, W = mgdTherefore, W = mgd So, if the mass is doubled, the required force So, if the mass is doubled, the required force
and work are doubled.and work are doubled. If the distance is doubled, the work is doubled.If the distance is doubled, the work is doubled. Force (F) and distance (d) are both Force (F) and distance (d) are both directly directly
proportionalproportional to work. to work.
9.1 Work9.1 Work There are 2 kinds of work.There are 2 kinds of work.
1.1. Work done against another forceWork done against another force• To pick up an object you do work against To pick up an object you do work against
gravitygravity..• To push an object, you do work against To push an object, you do work against
frictionfriction..2.2. Work is done to change the speed of an Work is done to change the speed of an
object.object.• Work is done to speed up or slow down a Work is done to speed up or slow down a
car.car. In either case, when work is done, a
transfer of energy occurs between an object and its surroundings.
PracticePractice A weight lifter lifts a barbell. How much A weight lifter lifts a barbell. How much
work is done if he lifts a barbell that is work is done if he lifts a barbell that is twice as heavy the same distance? If he twice as heavy the same distance? If he lifts a barbell that is twice as heavy and 2x lifts a barbell that is twice as heavy and 2x as far?as far?
If you apply a 50 N force to a package, If you apply a 50 N force to a package, pushing it across the floor 3 m, how much pushing it across the floor 3 m, how much work do you do?work do you do?
A tugboat pulls a ship with a constant net A tugboat pulls a ship with a constant net horizontal force of 5000 N. How much horizontal force of 5000 N. How much work does the tugboat do on the ship if work does the tugboat do on the ship if each moves a distance of 3.00 km?each moves a distance of 3.00 km?
9.2 Power9.2 Power When you climb a When you climb a
staircase, you do staircase, you do the same amount the same amount of work whether of work whether you get to the top you get to the top in 30 sec. or 2 in 30 sec. or 2 minutes.minutes.
What is different in What is different in each case is each case is how how fastfast the work is the work is done or the done or the power.power.
9.2 Power9.2 Power
Power is the rate at which work is done.Power is the rate at which work is done. Power equals the amount of work done Power equals the amount of work done
divided by the time interval during which the divided by the time interval during which the work is done: P = work is done: P = FdFd = = mgdmgd
t tt t
9.2 Power9.2 Power
Power = Work/Time. This means Power = Work/Time. This means power is power is directly proportionaldirectly proportional to work to work and and indirectly proportionalindirectly proportional to time. to time.
Having twice the power, then, can Having twice the power, then, can mean several things.mean several things.Twice the work can be done in the same Twice the work can be done in the same
amount of time.amount of time.The same work can be done in ½ the The same work can be done in ½ the
time.time.
9.2 Power9.2 Power The unit of power is the The unit of power is the
joule per second or watt joule per second or watt (W).(W).
For historical reasons, the For historical reasons, the horsepower is horsepower is occasionally used to occasionally used to describe the power describe the power delivered by a machine.delivered by a machine.
One horsepower is One horsepower is equivalent to equivalent to approximately 750 Watts.approximately 750 Watts.
To lift a ¼ lb. cheeseburger a distance of 1 m in 1 s
requires 1 watt of power!
PracticePractice
Two physics students, Will and Ben, are in the Two physics students, Will and Ben, are in the weightlifting room. Will lifts the 100-pound weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Ben barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most in 10 seconds. Which student does the most work? Which student delivers the most power?work? Which student delivers the most power?
If little Nellie Newton lifts her 40-kg body a If little Nellie Newton lifts her 40-kg body a distance of 0.25 meters in 2 seconds, then what distance of 0.25 meters in 2 seconds, then what is the power delivered by little Nellie's biceps? is the power delivered by little Nellie's biceps?
Joe elevates his 80-kg body up the 2.0 meter Joe elevates his 80-kg body up the 2.0 meter stairwell in 1.8 seconds. What is his power? stairwell in 1.8 seconds. What is his power?
9.3 Mechanical Energy9.3 Mechanical Energy
When work is done on an object, the When work is done on an object, the object acquires the ability to do work object acquires the ability to do work on something else.on something else.For example, when you do work to lift a For example, when you do work to lift a
hammer, the hammer now has the hammer, the hammer now has the ability to do work on a nail.ability to do work on a nail.
The property of an object that The property of an object that enables it to do work is enables it to do work is energy.energy.Energy is measured in joules.Energy is measured in joules.
9.3 Mechanical Energy9.3 Mechanical Energy
Mechanical energyMechanical energy is the energy due is the energy due to the to the positionposition or or movementmovement of an of an object.object.
The 2 forms of mechanical energy The 2 forms of mechanical energy are are kinetic energykinetic energy and and potential potential energyenergy..
9.4 Potential Energy9.4 Potential Energy
Energy that is stored Energy that is stored and held in readiness and held in readiness is called is called potential potential energy (PE).energy (PE).
Stored energy has the Stored energy has the potential to do WORK.potential to do WORK.
3 kinds of PE are: 3 kinds of PE are: elastic PE, chemical elastic PE, chemical PE, and gravitational PE, and gravitational PE.PE.
Elastic Potential EnergyElastic Potential Energy
A compressed or A compressed or stretched spring stretched spring has the potential to has the potential to do work.do work.
Other examples Other examples include a stretched include a stretched rubber band or a rubber band or a stretched string on stretched string on a bow and arrow.a bow and arrow.
Chemical EnergyChemical Energy Fuels (like oil or food) Fuels (like oil or food)
have chemical potential have chemical potential energy.energy.
This energy is available This energy is available to do work when a to do work when a chemical reaction chemical reaction breaks the bonds breaks the bonds between the atoms in a between the atoms in a fuel and thereby fuel and thereby releases the energy releases the energy stored in them.stored in them.
Gravitational Potential EnergyGravitational Potential Energy
Work is required to elevate objects against Work is required to elevate objects against gravity. The lifted objects then have PEgravity. The lifted objects then have PEgravgrav..
The work required (above) will be the same The work required (above) will be the same in each case because the work is equal to the in each case because the work is equal to the forceforce (weight of the object) times the (weight of the object) times the distancedistance it is moved against gravity (3 m in it is moved against gravity (3 m in each case).each case).
Gravitational Potential EnergyGravitational Potential Energy
The amount of gravitational PE The amount of gravitational PE possessed by an elevated object is possessed by an elevated object is equal to the work done in lifting it.equal to the work done in lifting it.
PEPEgravgrav = Fd = Fd
Since F = weight & weight = mg, Since F = weight & weight = mg,
PEPEgravgrav = mgh = mgh
(where h = height or distance lifted)(where h = height or distance lifted)
Gravitational Potential EnergyGravitational Potential Energy The height is the The height is the
distance above a distance above a chosen reference level.chosen reference level.
Often this reference Often this reference level is the ground or level is the ground or floor.floor.
A book on a table will A book on a table will have no height relative have no height relative to the table and no PE. to the table and no PE. It does, however, have It does, however, have a positive PE relative to a positive PE relative to the floor.the floor.
Uses of PEUses of PEgravgrav
Hydroelectric power Hydroelectric power makes use of the makes use of the gravitational potential gravitational potential energy of water.energy of water.
Falling water drives a Falling water drives a turbine, which is turbine, which is connected to a connected to a generator. The generator. The generator converts the generator converts the energy from the falling energy from the falling water into electrical water into electrical energy.energy.
PracticePractice
Knowing that the potential energy at the top of the tall pillar is 30 J, what is the potential energy at the other positions shown on the hill and the stairs.
PracticePractice
If a force of 15 N is used to lift a load If a force of 15 N is used to lift a load to height 3 m from the ground, what to height 3 m from the ground, what is the PE of the load?is the PE of the load?
A 10 kg mass is suspended 5 m from A 10 kg mass is suspended 5 m from the ground.the ground.How much work was done to the mass?How much work was done to the mass?If it is lifted in 2 sec., how much power is If it is lifted in 2 sec., how much power is
expended?expended?What is the PEWhat is the PEgravgrav of the mass? of the mass?
9.5 Kinetic Energy9.5 Kinetic Energy If an object is moving, it is capable of doing work. If an object is moving, it is capable of doing work. It has energy of motion or It has energy of motion or kinetic energy (KE).kinetic energy (KE). The amount of kinetic energy which an object has The amount of kinetic energy which an object has
depends upon two variables: depends upon two variables: the mass (m) of the object the mass (m) of the object the speed (v) of the object. the speed (v) of the object.
KE = ½mvKE = ½mv22
9.5 Kinetic Energy9.5 Kinetic Energy
The kinetic energy of a moving The kinetic energy of a moving object is equal to the work required object is equal to the work required to bring it to its speed from rest or to to bring it to its speed from rest or to the work it can do while it is being the work it can do while it is being brought to rest.brought to rest.
KE = WKE = W
1/2mv1/2mv22 = Fd = Fd
1/2mv1/2mv22 = Fd = FdWhat this means:What this means:
– If the speed of a vehicle If the speed of a vehicle is doubled, its KE is is doubled, its KE is quadrupled.quadrupled.
– So, 4x as much work So, 4x as much work must be done to stop the must be done to stop the vehicle.vehicle.
– If the braking force used If the braking force used to stop the vehicle is the to stop the vehicle is the same, the distance same, the distance required for stopping will required for stopping will be 4x as great.be 4x as great.
What if the speed is What if the speed is tripled?tripled?
Types of Kinetic EnergyTypes of Kinetic Energy Thermal energy Thermal energy SoundSound LightLight ElectricityElectricity
Kinetic energy often appears “hidden” in one of its different forms, such as heat, sound, light, and electricity.
•Random molecular motion is sensed as heat.
•Sound consists of molecules vibrating in rhythmic patterns.
•Light energy originates in the motion of electrons within atoms.
•Electrons in motion make electric currents.
Kinetic Energy
PracticePractice Determine the kinetic energy of a 1000-kg Determine the kinetic energy of a 1000-kg
roller coaster car that is moving with a speed roller coaster car that is moving with a speed of 20.0 m/s.of 20.0 m/s.
If the roller coaster car in the above problem If the roller coaster car in the above problem were moving with twice the speed, then what were moving with twice the speed, then what would be its new kinetic energy?would be its new kinetic energy?
Alison, a platform diver for the Ringling Alison, a platform diver for the Ringling Brother's Circus, has a kinetic energy of Brother's Circus, has a kinetic energy of 15,000 J just prior to hitting a bucket of water. 15,000 J just prior to hitting a bucket of water. If Alison’s mass is 50 kg, then what is her If Alison’s mass is 50 kg, then what is her speed?speed?
9.6 Work-Energy Theorem9.6 Work-Energy Theorem
The work-energy theorem The work-energy theorem describes the relationship between describes the relationship between work and energy: work and energy: whenever work whenever work is done, energy changesis done, energy changes..
Work = Work = ΔΔKEKE
9.6 Work-Energy Theorem9.6 Work-Energy Theorem
Work equals change in KE.Work equals change in KE. If you push on a box and it does not slide, then If you push on a box and it does not slide, then
no work is done on the box.no work is done on the box. If there is no friction, the box will slide. The If there is no friction, the box will slide. The
force and distance of your push will be the KE force and distance of your push will be the KE of the box. (If there is some friction, the net of the box. (If there is some friction, the net force is what is considered.)force is what is considered.)
If the box has a constant speed, your push is If the box has a constant speed, your push is just enough to overcome friction. Since the net just enough to overcome friction. Since the net force is 0, work is 0 and there is no change in force is 0, work is 0 and there is no change in KE of the box. KE of the box.
9.6 Work-Energy Theorem9.6 Work-Energy Theorem
The more KE an object The more KE an object has, the more work has, the more work must be done to stop must be done to stop it.it.
This infrared shot of a This infrared shot of a tire shows that some tire shows that some of the KE of a vehicle of the KE of a vehicle was transferred into was transferred into thermal energy of the thermal energy of the tire when the vehicle tire when the vehicle was stopped.was stopped.
9.6 Work-Energy Theorem9.6 Work-Energy Theorem Recall our skidding Recall our skidding
example from a previous example from a previous slide.slide.
The maximum braking The maximum braking force that brakes supply is force that brakes supply is independent of speed – it independent of speed – it is nearly always the same.is nearly always the same.
Therefore, a car moving at Therefore, a car moving at twice the speed has 4x as twice the speed has 4x as much KE. It will take 4x as much KE. It will take 4x as much distance to stop.much distance to stop.
Distances may be even Distances may be even greater if a driver’s greater if a driver’s reaction time is taken into reaction time is taken into account.account.
9.7 Conservation of Energy9.7 Conservation of Energy Nearly every processNearly every process in nature can bein nature can be analyzed in terms ofanalyzed in terms of transformations oftransformations of energy from one formenergy from one form to another.to another. In this toy car, for example, work is done to In this toy car, for example, work is done to
wind it up. The car then has elastic wind it up. The car then has elastic potential energy. When released, it is potential energy. When released, it is converted into kinetic energy and heat.converted into kinetic energy and heat.
Energy changes from one form to another, Energy changes from one form to another, without a net loss or gain.without a net loss or gain.
9.7 Conservation of Energy9.7 Conservation of Energy
The The law of conservation of energylaw of conservation of energy states that energy states that energy cannot be created or destroyed. It can be cannot be created or destroyed. It can be transformed from one form into another, but the total transformed from one form into another, but the total amount of energy never changes.amount of energy never changes.
9.7 Conservation of Energy9.7 Conservation of Energy
In a swinging pendulum system, there is one quantity that does not change: the total energy
of the system. Due to friction, energy will eventually be transformed into heat.
9.7 Conservation of Energy9.7 Conservation of Energy
Energy changes from one form to another but the total amount of energy remains the same. In this example, the total amount of energy can also be
called the total mechanical energy.
PracticePractice1. As the object moves from1. As the object moves from point A to point D across thepoint A to point D across the surface, the sum of itssurface, the sum of its gravitational potential andgravitational potential and kinetic energies ____.kinetic energies ____.a. decreases onlya. decreases onlyb. decreases and then increasesb. decreases and then increasesc. increases and then decreasesc. increases and then decreasesd. remains the samed. remains the same
2. The object will have a minimum gravitational potential 2. The object will have a minimum gravitational potential energy at point ____.energy at point ____.
a. A b. B c. C d. D e. Ea. A b. B c. C d. D e. E
3. The object's kinetic energy at point C is less than its kinetic 3. The object's kinetic energy at point C is less than its kinetic energy at point ____.energy at point ____.
a. A only b. A, D, and E c. B only d. D and Ea. A only b. A, D, and E c. B only d. D and E
PracticePractice
0 m/s
9.8 Machines9.8 Machines
A A machinemachine is a device that is used to multiply or change is a device that is used to multiply or change the direction of forces.the direction of forces.
Based on the law of conservation of energy, a machine Based on the law of conservation of energy, a machine cannotcannot put out more energy than is put into it. put out more energy than is put into it.
What a machine What a machine cancan do is transfer energy from one do is transfer energy from one place to another or transform it from one form to place to another or transform it from one form to another.another.
LeversLevers A A leverlever is a simple is a simple
machine made up of a machine made up of a bar that turns about a bar that turns about a fixed point.fixed point.
The fixed point is The fixed point is called the called the fulcrum.fulcrum.
A lever A lever changes the changes the direction of a forcedirection of a force – – when the lever is when the lever is pushed down, the pushed down, the load is lifted up.load is lifted up.
LeversLevers If we neglect friction:If we neglect friction:
work input = work outputwork input = work output Since work = Fd,Since work = Fd,
(Fd)(Fd)inputinput = (Fd) = (Fd)outputoutput
On one end of this lever, a On one end of this lever, a smallsmall input force is exerted input force is exerted over a over a largelarge distance. distance.
This produces, on the other This produces, on the other end of the lever, a end of the lever, a largelarge force exerted through a force exerted through a shortshort distance. distance.
This machine has multiplied This machine has multiplied the force (but the force (but notnot the work the work)!)!
Note: in this lever, the fulcrum is relatively close
to the load.
Mechanical AdvantageMechanical Advantage Consider an Consider an idealideal
example: the girl example: the girl pushes with a force of pushes with a force of 50 N and lifts a load of 50 N and lifts a load of 5000 N.5000 N.
The ratio of FThe ratio of Foutputoutput to to FFinputinput for a machine is for a machine is called the called the mechanical mechanical advantage.advantage.
FFoutputoutput = =5000 N5000 N = 100 = 100 FFinputinput 50 N 50 N
Mechanical AdvantageMechanical Advantage MA = MA = FFoutputoutput = 100 = 100 FFinputinput
Notice that dNotice that doutputoutput is is 1/1001/100thth of d of dinput.input.
This means, This means, mechanical advantage mechanical advantage can also be calculated can also be calculated using the distances:using the distances:
MA = MA = ddinputinput
ddoutputoutput
Types (or Classes) of LeversTypes (or Classes) of Levers A type 1 lever has the A type 1 lever has the
fulcrum between the fulcrum between the input and output input and output forces.forces.
Examples include:Examples include:
Types of LeversTypes of Levers In a type 2 lever, the In a type 2 lever, the
output force (the output force (the load) is between the load) is between the fulcrum and the input fulcrum and the input force (the effort).force (the effort).
In this lever, the In this lever, the forces have the same forces have the same direction.direction.
Examples include: Examples include: staplers, bottle staplers, bottle openers, nut openers, nut crackers, and crackers, and
Types of LeversTypes of Levers In a type 3 lever, the In a type 3 lever, the
fulcrum is at one end fulcrum is at one end and the output force and the output force (the load) is at the (the load) is at the other. The input force other. The input force (the effort) is between (the effort) is between them.them.
In this lever, the forces In this lever, the forces have the same have the same direction.direction.
Examples include: your Examples include: your biceps and biceps and
PulleysPulleys A A pulleypulley is a kind of is a kind of
lever that can be used lever that can be used to change the direction to change the direction of a force.of a force.
This single pulley is like This single pulley is like a type 1 lever – the a type 1 lever – the fulcrum (pulley axis) is fulcrum (pulley axis) is between the input and between the input and output forces.output forces.
Someone pulls down on Someone pulls down on the rope and the load is the rope and the load is lifted up. lifted up.
The MA of this type of The MA of this type of pulley is 1: Fpulley is 1: Finputinput = F = Foutputoutput ( & d( & dinputinput = d = doutputoutput))
PulleysPulleys This single pulley acts as a This single pulley acts as a
type 2 lever - the load is type 2 lever - the load is between the input force between the input force and the fulcrum (rope-and the fulcrum (rope-pulley contact point).pulley contact point).
Pulling up on the rope lifts Pulling up on the rope lifts the load up.the load up.
The MA of this pulley is 2: The MA of this pulley is 2: a 2N load can be lifted a 2N load can be lifted with a 1N force; the rope with a 1N force; the rope will be pulled 2 m for will be pulled 2 m for every 1 m the load is every 1 m the load is lifted.lifted.
An easy way to determine An easy way to determine MA is to count the number MA is to count the number of ropes that of ropes that supportsupport the the load: 2!load: 2!
PulleysPulleys The MA for pulley The MA for pulley
systems is the same systems is the same as the number of as the number of strands that support strands that support the load.the load.
In this diagram, there In this diagram, there are 5 strands of rope are 5 strands of rope but only 4 support the but only 4 support the load. The MA, then, is load. The MA, then, is 4.4.
Calculated: MA = Calculated: MA = FFoutputoutput = = 100100 = 4 = 4
FFinputinput 25 25
PracticePractice
If a pulley can lift a 500 N load with a If a pulley can lift a 500 N load with a force of 100 N, what is its MA?force of 100 N, what is its MA?
A lever is pushed downward a A lever is pushed downward a distance of 1m. As a result, a load is distance of 1m. As a result, a load is lifted up 1/8 m. What is the MA of lifted up 1/8 m. What is the MA of this lever?this lever?
John’s biceps contract 1 cm to lift a John’s biceps contract 1 cm to lift a 10 N load a distance of 10 cm. What 10 N load a distance of 10 cm. What is the MA of his biceps?is the MA of his biceps?
9.9 Efficiency9.9 Efficiency
In our IDEAL examples, In our IDEAL examples,
WWinputinput = W = Woutputoutput
IDEAL machines have 100% efficiency.IDEAL machines have 100% efficiency. REAL machines are not 100% REAL machines are not 100%
efficient. In REAL machines, some efficient. In REAL machines, some input energy is used to overcome input energy is used to overcome frictional forces and is converted into frictional forces and is converted into thermal energy.thermal energy.
9.9 Efficiency9.9 Efficiency The The efficiencyefficiency of a machine is the ratio of of a machine is the ratio of
useful energy output to total energy input. It useful energy output to total energy input. It is often expressed as a percentage.is often expressed as a percentage.
E = E = useful Wuseful Woutputoutput x 100 x 100
total Wtotal Winputinput
If you do 50 J of work on a lever and get out If you do 50 J of work on a lever and get out 42 J of work, the efficiency of the lever is: 42 J of work, the efficiency of the lever is: 42J42J x 100 = 0.84 or 84%. x 100 = 0.84 or 84%.
50J50J 8 J of your work is “lost” to heat.8 J of your work is “lost” to heat. The lower the efficiency, the greater the amount of The lower the efficiency, the greater the amount of
energy that is wasted.energy that is wasted.
Practice ProblemsPractice Problems1.1. Using a lever, a person applies 60 N of Using a lever, a person applies 60 N of
force and moves the lever by 1 m. This force and moves the lever by 1 m. This moves the 200 N rock at the other end by moves the 200 N rock at the other end by 0.2 m. What is this machine’s efficiency?0.2 m. What is this machine’s efficiency?
2.2. A person in a wheelchair exerts a force of A person in a wheelchair exerts a force of 25 N to go up a ramp that is 10 m long. 25 N to go up a ramp that is 10 m long. The weight of the person and wheelchair is The weight of the person and wheelchair is 60 N and the height of the ramp is 3 m. 60 N and the height of the ramp is 3 m. What is the efficiency of this machine?What is the efficiency of this machine?
Inclined PlaneInclined Plane This machine is This machine is
generally used to generally used to elevate heavy loads. elevate heavy loads. Less force is required Less force is required (F(Finputinput) to slide a load ) to slide a load up an incline than is up an incline than is required to lift it required to lift it vertically (Fvertically (Foutputoutput).).
Recall that MA = Recall that MA = ddinputinput
ddoutputoutput MA = MA = 6m/3m = 26m/3m = 2 This is an This is an IDEALIDEAL (or (or
theoretical)theoretical) MAMA , the , the IMAIMA..
Inclined PlanesInclined Planes However, inclined However, inclined
planes do not have planes do not have efficiency’s of 100%. efficiency’s of 100%.
Some of the work done Some of the work done to go up a ramp, for to go up a ramp, for example, is “lost” to example, is “lost” to the ramp through the ramp through friction. friction. Actual MAActual MA is is much lessmuch less than than theoretical MA (IMA)theoretical MA (IMA). .
AMA = AMA = FFoutputoutput
FFinputinput
Inclined PlanesInclined Planes
Efficiency can also be expressed as a Efficiency can also be expressed as a ratio of actual MA to ideal MA. ratio of actual MA to ideal MA.
E = E = actual MAactual MA x 100 x 100 ideal MAideal MA Practice:Practice:
1.1. If an inclined plane has a theoretical MA of 2 If an inclined plane has a theoretical MA of 2 but an actual MA of 1.8, what is its efficiency?but an actual MA of 1.8, what is its efficiency?
2.2. A pulley has an IMA of 6 and an AMA of 5. A pulley has an IMA of 6 and an AMA of 5. What is its efficiency?What is its efficiency?
Complex MachinesComplex Machines An auto jack is a An auto jack is a
combination of 2 simple combination of 2 simple machines – a lever and machines – a lever and an inclined plane an inclined plane wrapped around a wrapped around a cylinder. Turning the cylinder. Turning the handle raises the load a handle raises the load a distance of 1 pitch (the distance of 1 pitch (the distance between ridges).distance between ridges).
The theoretical MA of a The theoretical MA of a jack is very high; jack is very high; however, the efficiency is however, the efficiency is only about 20%. Actual only about 20%. Actual MA approximates 100. MA approximates 100.
Inclined plane
Complex MachinesComplex Machines
Automobile engines are also complex Automobile engines are also complex machines.machines.
In the engine, the chemical energy of In the engine, the chemical energy of a fuel is released when the fuel is a fuel is released when the fuel is burned. Much of the energy (65%) burned. Much of the energy (65%) released is transformed into thermal released is transformed into thermal energy; the remainder is transformed energy; the remainder is transformed into the mechanical energy used to into the mechanical energy used to run the engine.run the engine.
9.10 Energy for Life9.10 Energy for Life Cells are machines and need a supply of Cells are machines and need a supply of
energy. They use the energy stored in energy. They use the energy stored in hydrocarbons (like glucose). The energy is hydrocarbons (like glucose). The energy is released during respiration (a reaction of released during respiration (a reaction of the fuel with oxygen).the fuel with oxygen).
There is more energy stored in the There is more energy stored in the molecules of food than is stored in the molecules of food than is stored in the products of food metabolism. This energy products of food metabolism. This energy difference is used to sustain life.difference is used to sustain life.
Metabolism is like the burning of fuel in an Metabolism is like the burning of fuel in an engine. The difference is the rate of engine. The difference is the rate of energy release. Reaction rates in energy release. Reaction rates in metabolism are very slow.metabolism are very slow.
9.11 Sources of Energy9.11 Sources of Energy The The SUNSUN is the source is the source
of nearly all our energy of nearly all our energy on Earth.on Earth.
Plants use sunlight Plants use sunlight during photosynthesis during photosynthesis to produce to produce hydrocarbon hydrocarbon compounds. These compounds. These compounds become compounds become our wood, fossil fuels our wood, fossil fuels and our food.and our food.
Solar PowerSolar Power
SOLAR POWER takes SOLAR POWER takes sunlight and sunlight and converts it converts it directlydirectly into electricity by into electricity by using photovoltaic using photovoltaic cells.cells.
IndirectlyIndirectly, the sun , the sun powers the water powers the water cycle; here falling cycle; here falling water can turn water can turn generator turbines generator turbines and generate and generate electricity.electricity.
Solar PowerSolar Power Wind power is Wind power is
another indirect another indirect form of solar form of solar power. Wind, power. Wind, caused by the caused by the unequal heating of unequal heating of the Earth’s surface, the Earth’s surface, can be used to turn can be used to turn generator turbines generator turbines in special in special windmills. windmills.
Fuel CellsFuel Cells
Much is being done today to use Much is being done today to use hydrogen as a fuel. When it is hydrogen as a fuel. When it is burned, water vapor is the only burned, water vapor is the only product.product.
Hydrogen is NOT a source of energy, Hydrogen is NOT a source of energy, however, because it must be “made” however, because it must be “made” from water and carbon compounds. from water and carbon compounds. This This requiresrequires energy. energy.
Fuel CellsFuel Cells
Electricity is used to split Electricity is used to split water molecules into water molecules into hydrogen and oxygen in hydrogen and oxygen in a process called a process called electrolysis.electrolysis.
Fuel cells makes the Fuel cells makes the electrolysis process run electrolysis process run in reverse. Water is in reverse. Water is produced and electric produced and electric current is generated.current is generated.
Nuclear and Geothermal EnergyNuclear and Geothermal Energy Nuclear fuels, like Nuclear fuels, like
uranium and plutonium, uranium and plutonium, are the most are the most concentrated forms of concentrated forms of usable energy.usable energy.
Radioactivity from such Radioactivity from such fuels keeps the Earth’s fuels keeps the Earth’s interior hot.interior hot.
Geothermal energy is Geothermal energy is held in underground held in underground reservoirs of hot water. reservoirs of hot water. They can be tapped to They can be tapped to provide steam for provide steam for running generator running generator turbines.turbines.
