CHAPTER 5: WORK AND CHAPTER 5: WORK AND MACHINESMACHINES

WORKWORK

WORK IN THE WORK IN THE SENSE OF SCIENCE SENSE OF SCIENCE IS DIFFERENT THAN IS DIFFERENT THAN WHAT MOST WHAT MOST PEOPLE CONSIDER PEOPLE CONSIDER WORK AS BEING.WORK AS BEING.

WorkWork: the transfer of : the transfer of energy when a force energy when a force makes an object makes an object move.move.

TWO CONDITIONS FOR WORKTWO CONDITIONS FOR WORK

1)1) The applied force The applied force must make the must make the object move.object move.

2)2) The movement The movement must be in the must be in the same direction as same direction as the applied force.the applied force.

For ExampleFor Example

When lifting books off the ground, you do When lifting books off the ground, you do work on the books because books move work on the books because books move upward and the force is also upward.upward and the force is also upward.

If you hold the books in your arms and do If you hold the books in your arms and do not move, then no work is being done.not move, then no work is being done.

If you start to move, you will not be doing If you start to move, you will not be doing any work on the books because you are any work on the books because you are moving horizontally, but the force of your moving horizontally, but the force of your arms is still upward.arms is still upward.

ENERGY and WORKENERGY and WORK

When work is done, a transfer of energy When work is done, a transfer of energy always occurs.always occurs.

In other words, ENERGY IS THE ABILITY In other words, ENERGY IS THE ABILITY TO DO WORK.TO DO WORK.

An object can transfer energy to another An object can transfer energy to another object by doing work on that object. object by doing work on that object.

Energy is always transferred from the Energy is always transferred from the object that is doing the work to the object object that is doing the work to the object on which the work is done.on which the work is done.

WORK and POWERWORK and POWER Work is calculated as:Work is calculated as: Work (J) = Force X distance orWork (J) = Force X distance or W = FdW = Fd

Power is the rate at which energy is Power is the rate at which energy is transferred or the amount of work done per transferred or the amount of work done per second and is measured in W or watts.second and is measured in W or watts.

P=Work / time or P =E/tP=Work / time or P =E/t

TWO SYSTEMS OF WORKTWO SYSTEMS OF WORK

The English SystemThe English SystemForce F= poundsForce F= poundsDistance d= feetDistance d= feetTime t = secondsTime t = secondsWork W = Fd = footpounds = ftlbWork W = Fd = footpounds = ftlbPower P = W/t = ftlb/sPower P = W/t = ftlb/sHorsepower 1HP = 550 ftlb/sHorsepower 1HP = 550 ftlb/s

METRIC SYSTEMMETRIC SYSTEM

Force F = Newton = N = 1kgm/sForce F = Newton = N = 1kgm/s²²Distance d = meter = mDistance d = meter = mTime t = secondTime t = secondWORK W = Fd = Newtonmeter = Nm = JWORK W = Fd = Newtonmeter = Nm = JPOWER P= W/s = NM/s = J/s = Watt =WPOWER P= W/s = NM/s = J/s = Watt =WHorsepower 1 HP = 746WHorsepower 1 HP = 746W

WORK – POWER – HORSEPOWER DIAGRAM

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

POWERP = W / t

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

POWERP = W / t

METRIC SYSTEM

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

POWERP = W / t

METRIC SYSTEM

ENGLISH SYSTEM

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

POWERP = W / t

METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

POWERP = W / t

TIM

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METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

Work = Nm = JNewtonmeter = Joule

POWERP = W / t

TIM

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METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

Work = footpoundftlb

Work = Nm = JNewtonmeter = Joule

POWERP = W / t

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METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

Work = footpoundftlb

Work = Nm = JNewtonmeter = Joule

P = Nm/s = J/s = Watt

POWERP = W / t

TIM

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METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

Work = footpoundftlb

P = ftlb/s

Work = Nm = JNewtonmeter = Joule

P = Nm/s = J/s = Watt

POWERP = W / t

TIM

E

METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

WORK – POWER – HORSEPOWER DIAGRAM

WORKW = F d

Work = footpoundftlb

P = ftlb/s

Work = Nm = JNewtonmeter = Joule

P = Nm/s = J/s = Watt

POWERP = W / t

TIM

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METRIC SYSTEM

ENGLISH SYSTEM

F=Newton =N d=meter F=pound d=foot

746 watts = 1 Horse Power = 550 ftlb/s

Example from page 128Example from page 128You push a refrigerator with a force of You push a refrigerator with a force of

100N. If you move the refrigerator a 100N. If you move the refrigerator a distance of 5 m, how much work did you distance of 5 m, how much work did you do?do?

Section 2: Using MachinesSection 2: Using Machines

MachineMachine: a device that makes : a device that makes doing work easier.doing work easier.

Work is Made Easier . . .Work is Made Easier . . . by increasing the force applied.by increasing the force applied.

A hammer used as a lever increases force.A hammer used as a lever increases force. by changing the distance over which the force is by changing the distance over which the force is

applied.applied.An inclined plane increases distance but decreases An inclined plane increases distance but decreases

force applied to do the same work.force applied to do the same work. by changing the direction of an applied force.by changing the direction of an applied force.

A pulley changes the direction of the pull on the A pulley changes the direction of the pull on the rope.rope.

Applied Force = is the force put into the machineApplied Force = is the force put into the machine

Work Input = Effort ForceWork Input = Effort Force

A force is any push or pull.A force is any push or pull.You are doing work when you use a force You are doing work when you use a force

to cause motion.to cause motion.Machines make work easier, but they Machines make work easier, but they

need energy to do work.need energy to do work.A person is usually the source of energy A person is usually the source of energy

for a simple machine.for a simple machine.The force applied to the machine is called The force applied to the machine is called

work inputwork input or or effort force = effort force = WWinin

Work Output = Resistance ForceWork Output = Resistance Force

Machines do work, too.Machines do work, too.The machine exerts a force over a The machine exerts a force over a

distance.distance.This is called output force.This is called output force.The work a machine does is called The work a machine does is called work work

output output = W= Woutout

The work output is used to move a “force” The work output is used to move a “force” you and the machine wish to move or you and the machine wish to move or overcome.overcome.

This is the This is the resistance forceresistance force or the or the loadload..

Machines Work through a DistanceMachines Work through a Distance

The distance that is applied to a machine The distance that is applied to a machine such as a pulley rope or an inclined such as a pulley rope or an inclined plane’s length is called the plane’s length is called the effort distanceeffort distance..

The distance that the machine moves the The distance that the machine moves the object is called the object is called the resistance distanceresistance distance..

Both of these are important when Both of these are important when calculating such things as work, calculating such things as work, mechanical advantage, and efficiency.mechanical advantage, and efficiency.

Recap Recap Work input = work done by the user of the Work input = work done by the user of the

machinemachineEffort Force = the force done by the user of Effort Force = the force done by the user of

the machinethe machineEffort Distance = distance put into machineEffort Distance = distance put into machineWork output = work done by machineWork output = work done by machineResistance Force = he weight of the object Resistance Force = he weight of the object

to be moved by the simple machineto be moved by the simple machineResistance Distance = distance the object is Resistance Distance = distance the object is

movedmoved

Conserving EnergyConserving Energy

Energy is always conserved.Energy is always conserved.WWout out is never greater than W is never greater than W in.in.

A machine does not transfer all of the A machine does not transfer all of the energy it receives to the object. energy it receives to the object.

Some of the energy is transferred to Some of the energy is transferred to heat through friction.heat through friction.

So, WSo, Winin is always greater than W is always greater than Wout.out.

Ideal MachinesIdeal Machines

WWinin = F = Finin x d x dinin

WWoutout = F = Foutout x d x doutout

For an ideal machine, meaning no loss For an ideal machine, meaning no loss to friction, Wto friction, Winin = W = Woutout

OrOrFFinin x d x din in = F= Foutout x d x doutout

See page 135See page 135

Page 135Page 135

A hammer claw moves a distance of A hammer claw moves a distance of 1cm to remove a nail. If the output 1cm to remove a nail. If the output force of 1,500 N is exerted by the force of 1,500 N is exerted by the claw, and you move the handle of the claw, and you move the handle of the hammer 5 cm, find the input force.hammer 5 cm, find the input force.

Mechanical AdvantageMechanical Advantage

The ratio of the output force to the input The ratio of the output force to the input forceforce

MA = FMA = Foutout /F /Finin

Ideal mechanical advantage is MA without Ideal mechanical advantage is MA without frictionfriction

IMA = dIMA = dinin/d/doutout

EfficiencyEfficiency

Efficiency = the measure of how much of Efficiency = the measure of how much of the work put into a machine is changed the work put into a machine is changed into useful output workinto useful output work

Efficiency (%) = output work / input work X Efficiency (%) = output work / input work X 100%100%

Ideal machine = 100%Ideal machine = 100%Real machine = less than 100%Real machine = less than 100%Lubricant reduced friction and increases Lubricant reduced friction and increases

efficiency.efficiency.