05 chapter revised

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5

Chapter 5: Work and Machines

Unit 1: Energy and Motion

5.3: Simple Machines

5.1: Work

5.2: Using Machines

Table of Contents


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To many people, the word work

means something they do to earnmoney.

The word workalso means exerting aforce with your muscles.

What is work?

Work

5.1


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Someone might say they have done workwhen they push as hard as they can againsta wall that doesn't move.

However, in science the word workis usedin a different way.

What is work?

Work

5.1


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Remember that a force is a push or a pull.In order for work to be done, a force mustmake something move.

Work is the transfer of energy that occurswhen a force makes an object move.

Work Makes Something Move

If you push against the desk and nothingmoves, then you haven't done any work.

Work

5.1


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There are two conditions that have to besatisfied for workto be done on an object.

One

is that the applied force must make theobject move,

Two

is that the movement must be in the

same direction as the applied force.

Doing work

Work

5.1


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For example, whenyou lift a stack ofbooks, your armsapply a force upward

and the books moveupward. Because theforce and distance arein the same direction,your arms have donework on the books.

Doing work

Work

5.1


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When you carry bookswhile walking, youmight thinkthat your

arms are doing work.

Force and Direction of Motion

However, in this case,the force exerted by

your arms does no workon the books.

Work

5.1


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The force exerted byyour arms on the booksis upward, but thebooks are movinghorizontally.

The force you exert isat right angles to thedirection the books aremoving.

Work

5.1Force and Direction of Motion


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When workis done, a transfer of energyalways occurs.

Work and Energy

This is easy to understand when you thinkabout how you feel after carrying a heavybox up a flight of stairs.

You transferred energy from your movingmuscles to the box and increased its potentialenergy by increasing its height.

Work

5.1


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You may recall that energy is the abilityto cause change.

Work and Energy

Another way to think ofenergy

is that energy is the ability to do work.

If something has energy, it can transferenergy to another object by doing workon that object.

Work

5.1


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When you do work on an object,

you increase its energy.

Work and Energy

The student carrying the box transferschemical energy in his muscles to the box.

Work

5.1


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Work and Energy

Energy is alwaystransferred fromthe object that is

doing the work tothe object onwhich the work isdone.

Work

5.1

W k


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Calculating Work

The amount of work done depends on theamount of force exerted and the distanceover which the force is applied.

When a force is exerted and an object movesin the direction of the force, the amount ofwork done can be calculated as follows.

Work

5.1

W k


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Calculating Work

One joule is about the amount of work

required to lift a baseball a verticaldistance of 0.7 m.

In this equation, force is measured innewtons and distance is measured inmeters.

Work, like energy, is measured in joules.

Work

5.1

W k


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When is work done?

Suppose you give a book a push and itslides along a table for a distance of 1 mbefore it comes to a stop.

Even though the book moved 1 m, you dowork on the book only while your hand isin contact with it.

Work

5.1

W k


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Power

Suppose you and another student arepushing boxes of books up a ramp and loadthem into a truck.

To make the job more fun, you make agame of it, racing to see who can push abox up the ramp faster.

Work

5.1

W k


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Power

Power is theamount ofwork done in

one second. Itis a ratetherate at whichwork is done.

Work

5.1

W k


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Calculating Power

To calculate power, divide the work doneby the time that is required to do the work.

The SI unit for power is the watt (W).One watt equals one joule of work donein one second.

Work

5.1

W k


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Calculating Power

Because the watt is a small unit, poweroften is expressed in kilowatts.

One kilowatt (kW) equals 1,000 W.

Work

5.1

Work


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Power and Energy Just as power is the rate at which work is

done, power is also the rate at which energyis transferred.

When energy is transferred, the power

involved can be calculated by dividing theenergy transferred by the time needed forthe transfer to occur.

Work

5.1

S ti Ch k


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5.1Section Check

Question 1

__________ is the transfer of energy that

occurs when a force makes an object move.

A. Conversion

B. Energization

C. PowerD. Work

S ti Ch k


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5.1Section Check

Answer

The answer is D. In order for work to be

done, the applied force must make the object

move in the same direction as the appliedforce.

S ti Ch k


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5.1Section Check

Question 2

The amount of work done depends on what

two things?

Answer

The amount of work done depends on the

amount of force exerted and the distance

over which the force is applied.

Section Check


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5.1Section Check

Question 3

Which of the following equations can be

used to calculate power?

A. W = F/d

B. P =Wt

C. t = W/PD. P = t/W

Section Check


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5.1Section Check

Answer

The answer is C. This is a rearrangement of

the equation for calculating power,

P = W/t.

Using Machines


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What is a machine?

A machine is a devicethat makes doing workeasier.

Machines can be simple.

Some, like knives,scissors, and doorknobs,

are used everyday tomake doing work easier.

Using Machines

5.2

Using Machines


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Making Work Easier

Machines can make work easier byincreasing the force that can be appliedto an object.

A second way that machines can makework easier is by increasing the distanceover which a force can be applied.

Machines can also make work easier bychanging the direction of an applied force.

Using Machines

5.2

Using Machines


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Increasing Force

A car jack is anexample of a machinethat increases anapplied force.

The upward forceexerted by the jack isgreater than thedownward force youexert on the handle.

Using Machines

5.2

Using Machines


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Increasing Force

However, the distance you push the handledownward is greater than the distance thecar is pushed upward.

The jack increases the applied force, butdoesn't increase the work done.

Using Machines

5.2

Using Machines


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Force and Distance

The work done in lifting an object dependson the change in height of the object.

The same amount of work is done whetherthe mover pushed the furniture up the longramp or lifts it straight up.

If work stays the same and the distance isincreased, then less force will be needed todo the work.

Using Machines

5.2

Using Machines


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Changing Direction

Some machineschange the directionof the force you

apply.

The wedge-shapedblade of an ax is one

example.

Using Machines

5.2

Using Machines


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The Work Done by Machines

The blade changes thedownward force intoa horizontal force thatsplits the wood apart.

Using Machines

5.2

When you use an axto split wood, youexert a downwardforce as you swing the

ax toward the wood.

Using Machines


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When you use a machine such as a crowbar,you are trying to move something that resistsbeing moved.


If you use a crowbar

to pry the lid off acrate, you areworking against thefriction between thenails in the lid andthe crate.

Using Machines

5.2

Using Machines


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You also could use a crowbar to move alarge rock.


In this case, you would be working againstgravitythe weight of the rock.

Using Machines

5.2

Using Machines


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Two forces are involved when a machineis used to do work.

Input and Output Forces

The force that is applied to the machine is

called the input force.

Fin stands for the effort force.

The force applied by the machine is calledthe output force, symbolized by Fout.

Using Machines

5.2

Using Machines


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Two kinds of work need to be consideredwhen you use a machinethe work doneby you on the machine and the work doneby the machine.

Input and Output Forces

The work done by you on a machine is calledthe input work and is symbolized by Win.

The work done by the machine is called theoutput work and is abbreviated Wout.

Using Machines

5.2

Using Machines


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When you do work on the machine, youtransfer energy to the machine.

Conserving Energy

When the machine does work on anobject, energy is transferred from themachine to the object.

Using Machines

5.2

Using Machines


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The amount of energy the machinetransfers to the object cannot be greaterthan the amount of energy you transfer to

the machine.

Conserving Energy

A machine cannot create energy, so Woutis never greater than Win.

Using Machines

5.2

Using Machines


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When a machine is used, some of the energytransferred changes to heat due to friction.

Conserving Energy

The energy that changes to heat cannot beused to do work, so Wout is always smallerthan Win.

Us g ac es

5.2

Using Machines


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Suppose a perfect machine could be built inwhich there was no friction.

Ideal Machines

None of the input work or output work wouldbe converted to heat.

For such an ideal machine, the input work

equals the output work.

g

5.2

Using Machines


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Suppose the ideal machine increases theforce applied to it.

Ideal Machines

This means that the output force, Fout, isgreater than the input force, Fin.

Recall that work is equal to force times

distance.

g

5.2

Using Machines


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IfFout is greater than Fin, then Win andWout can be equal only if the input force isapplied over a greater distance than the

output force is exerted over.

g

5.2Ideal Machines

Using Machines


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The ratio of the output force to the inputforce is the mechanical advantage of amachine.

The mechanical advantage of a machinecan be calculated from the followingequation.

Mechanical Advantage

g

5.2

Using Machines


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Window blinds are a machine thatchanges the direction of an input force.


A downward

pull on thecord ischanged toan upward

force on theblinds.

g

5.2

Using Machines


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The input and output forces are equal, sothe MA is 1.


g

5.2

Using Machines


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The mechanical advantage of a machinewithout friction is called the idealmechanical advantage, or IMA.

Ideal Mechanical Advantage

The IMA can be calculated by dividingthe input distance by the output distance.

g

5.2

Using Machines


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Efficiency is a measure of how much of thework put into a machine is changed intouseful output work by the machine.

Efficiency

A machine with high efficiency producesless heat from friction so more of the inputwork is changed to useful output work.

g

5.2

Using Machines


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To calculate the efficiency of a machine, theoutput work is divided by the input work.

Calculating Efficiency

Efficiency is usually expressed as a

percentage by this equation:

5.2

Using Machines


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In an ideal machine there is no friction andthe output work equals the input work. Sothe efficiency of an ideal machine is 100

percent.

Calculating Efficiency

The efficiency of a real machine is alwaysless than 100 percent.

5.2

Using Machines


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Machines can be made more efficient byreducing friction. This usually is done byadding a lubricant, such as oil or grease, tosurfaces that rub together.

Increasing Efficiency

A lubricant fills in thegaps between thesurfaces, enabling the

surfaces to slide pasteach other moreeasily.

5.2

Section Check


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5.2

Question 1What do a knife, a doorknob, and a car jack

have in common?

Answer

These are all machines, because they are

devices that make doing work easier.

Section Check


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5.2

Question 2When a machine is used to do work, the

force that is applied to the machine is the

__________.

A. Fulcrum

B. input force

C. mechanical advantageD. output force

Section Check


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5.2

Answer

The answer is B. The input force is applied

to the machine

Section Check


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5.2

Question 3What is the effect of increasing a machines

efficiency?

Answer

Increasing efficiency increases the amount of

input energy converted to useful output.

Simple Machines


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Types of Simple Machines

A simple machine is a machine that doeswork with only one movement of themachine.

There are six typesof simple machines:lever, pulley, wheeland axle, inclined

plane, screw andwedge.

5.3

Simple Machines


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Levers

A lever is a bar that is free to pivot orturn around a fixed point.

The fixed point the lever pivots on iscalled the fulcrum.

5.3

Simple Machines


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Levers

The input arm of the lever is thedistance from the fulcrum to the pointwhere the input force is applied.

The output arm is the distance from thefulcrum to the point where the outputforce is exerted by the lever.

5.3

Simple Machines


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Levers

If the output arm isshorter than theinput arm, then theoutput force isgreater than theinput force.

There are threeclasses of levers.

5.3

Simple Machines


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Ideal Mechanical Advantage of aLever

The IMA of a lever can be calculated fromthis equation:

5.3

Simple Machines


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Pulleys

A pulley is a groovedwheel with a rope, chain,or cable running alongthe groove.

A fixed pulley is amodified first-class lever.

The axle of the pulleyacts as the fulcrum.

5.3

Simple Machines


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Fixed Pulleys

A fixed pulley is attached to somethingthat doesn't move, such as a ceiling orwall.

Because a fixed pulley changes only thedirection of force, the IMA is 1.

5.3

Simple Machines


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Wheel and Axle

A wheel and axleis a simplemachine consistingof a shaft or axleattached to thecenter of a largerwheel, so that the

wheel and axlerotate together.

5.3

Simple Machines


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Mechanical Advantage of theWheel and Axle

The output force is exerted at the rim of theaxle.

So the length of the output arm is the radius

of the axle. The IMA of a wheel and axle is given by this

equation:

5.3

Simple Machines


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Inclined Planes

A slopingsurface, suchas a ramp thatreduces theamount offorce requiredto do work, is

an inclinedplane.

5.3

Simple Machines

5 3


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Mechanical Advantage of anInclined Plane

By pushing a box up an inclined plane, theinput force is exerted over a longer distancecompared to lifting the box straight up.

The IMA of an inclined plane can becalculated from this equation.

5.3

Simple Machines

5 3


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The Screw

A screw is an inclined plane wrapped in aspiral around a cylindrical post.

You apply the input

force by turning thescrew.

The output force is

exerted along thethreads of the screw.

5.3

Simple Machines

5 3


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The Wedge

The wedge is also a simple machine wherethe inclined plane moves through an objector material.

A wedge is an inclined plane with one ortwo sloping sides. It changes the directionof the input force.

5.3

Simple Machines

5 3


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Compound Machines

Two or more simple machines that operatetogether form a compound machine.

A car is a compound machine.

Burning fuel in the cylinders of the enginecauses the pistons to move up and down.

5.3

Simple Machines

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Compound Machines

This up-and-down motion makes thecrankshaft rotate.

5.3


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5 3Section Check


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5.3

Question 1What is the difference between a first-class

lever and a second-class lever?

5 3Section Check


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5.3

AnswerIn a first-class lever the fulcrum is between the

input and output forces; in a second-class lever,

the output force is between the input force andthe fulcrum.

5 3Section Check


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5.3

Question 2Which is a third-class lever?

A. baseball batB. pulley

C. screwdriver

D. wheelbarrow

5 3Section Check


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5.3

AnswerThe answer is A. The output force exerted

by a third-class lever is less than the input

force, but the distance over which theoutput force is applied is increased.

5 3Section Check


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5.3

Question 3A fixed pulley changes only __________.

5 3Section Check


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5.3

AnswerA fixed pulley changes

only the direction of

force, and the IMA is 1.

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