Work and MachinesWork and Machines

Chapter 5Chapter 5

Sections 1-3Sections 1-3

C5- Work & MachinesC5- Work & Machines

Section 1- Work Section 1- Work slides 3-13slides 3-13 Section 2- Using Machines Section 2- Using Machines

slides 14-30slides 14-30 Section 3- Simple Machines Section 3- Simple Machines

slides 31-50slides 31-50

Section 1- WorkSection 1- Work

What You’ll Learn:What You’ll Learn: What work isWhat work is How work & energy are relatedHow work & energy are related How to calculate work & powerHow to calculate work & power

WorkWork

WorkWork is done when a is done when a force causes an force causes an object to move in the object to move in the same direction that same direction that the force is applied.the force is applied.

When a force makes When a force makes an object move, an object move, energy is transferred energy is transferred from one object to from one object to another.another. <www.mcasco.com/p1wke.html>

How do you know if you are How do you know if you are doing work?doing work?

Two things must happen:Two things must happen:– A force must be applied to the objectA force must be applied to the object– The object must The object must movemove in the same in the same

direction as the forcedirection as the force

What does direction have to do What does direction have to do with work?with work?

When you pick up a stack of books, When you pick up a stack of books, your arms apply a force upward. your arms apply a force upward.

You start walking with the books.You start walking with the books. The direction of motion has changed; The direction of motion has changed;

your arms are no longer working, your arms are no longer working, instead your legs supply the forward instead your legs supply the forward directiondirection

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Work & EnergyWork & Energy

Energy is the Energy is the ability to cause ability to cause change or to do change or to do work.work.

When work is done When work is done energy is energy is transferred from transferred from one object to one object to another.another.

<http://www.mnsu.edu/news/article>

Calculating WorkCalculating Work

You multiply You multiply forceforce times times distancedistance to to calculate work.calculate work.

Work (joules)= force (N) X distance (m)Work (joules)= force (N) X distance (m) W=FdW=Fd Calculate the amount of work a painter Calculate the amount of work a painter

does when he lifts a can of paint does when he lifts a can of paint weighing 40 newtons 2 meters.weighing 40 newtons 2 meters.

W=Fd=(40N) (2m)= 80 JW=Fd=(40N) (2m)= 80 J

When is work done?When is work done?

When a pitcher When a pitcher throws a ball to the throws a ball to the catcher, he applies catcher, he applies force to the ball force to the ball only when it is in his only when it is in his hand.hand.

The ball moves 10 The ball moves 10 m after it leaves his m after it leaves his hand but work hand but work occurred only while occurred only while it was in his hand it was in his hand for 1m.for 1m.<http://www.statenews.com/

media/>

PowerPower

PowerPower is the rate is the rate at which work is at which work is done.done.

Something has Something has more power if it more power if it can do the same can do the same amount of work in amount of work in less time.less time.

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How do you calculate How do you calculate power?power?

To calculate power, divide the amount of To calculate power, divide the amount of work done by the time it takes to do it.work done by the time it takes to do it.

Power (in watts)= work (in joules)/time(s)Power (in watts)= work (in joules)/time(s) P=W/t The SI unit for power is the watt.P=W/t The SI unit for power is the watt. Find the power of the machine that can do Find the power of the machine that can do

5,000 joules of work in 20 seconds.5,000 joules of work in 20 seconds. P=W/t=5000J/20 s=250 watts. The power of P=W/t=5000J/20 s=250 watts. The power of

the machine is 250 watts.the machine is 250 watts.

How is power calculated when How is power calculated when energy is transferred?energy is transferred?

You can also calculate power by You can also calculate power by dividing the amount of energy dividing the amount of energy transferred by the time it took to transferred by the time it took to transfer it.transfer it.

Power (in watts)=energy transferred Power (in watts)=energy transferred (j)/ time (s) or P=E/t.(j)/ time (s) or P=E/t.

How is energy transferred How is energy transferred when no work is done?when no work is done?

Suppose a light bulb changes Suppose a light bulb changes electrical energy into light & heat at electrical energy into light & heat at a rate of 100 j /2 s. How many watts a rate of 100 j /2 s. How many watts of power will the light bulb have?of power will the light bulb have?

P=E/t =100j/2s= 50 wattsP=E/t =100j/2s= 50 watts

Section 2- Using MachinesSection 2- Using Machines

What You’ll Learn:What You’ll Learn: How machines make work easierHow machines make work easier How to calculate mechanical How to calculate mechanical

advantageadvantage How to calculate the efficiency of a How to calculate the efficiency of a

machinemachine

What is a machine?What is a machine?

A A machine machine is a is a device that makes device that makes work easier.work easier.

Knives, scissors, Knives, scissors, and doorknobs are and doorknobs are simple machines.simple machines.

Engines are more Engines are more complex machinescomplex machines

<http://www.mshp.dps.missouri.gov/MSHPWeb/PatrolDivisions>

Making Work EasierMaking Work Easier

There are 3 ways machines make There are 3 ways machines make work easier:work easier:

Increasing force on an object;Increasing force on an object; Increasing the distance over which a Increasing the distance over which a

force is applied; orforce is applied; or Changing the direction of an applied Changing the direction of an applied

force.force.

How can force be increased How can force be increased using a machine?using a machine?

If If Work= force X distance, Work= force X distance, then then either force or distance increases.either force or distance increases.

If distance increases, then force If distance increases, then force decreases.decreases.

Machines such as a car jack, increase Machines such as a car jack, increase the distance a force is applied thus the distance a force is applied thus reducing the amount of force needed reducing the amount of force needed to do the same amount of work.to do the same amount of work.

<http://www.dynamicscience.com.au/tester/solutions/hydraulicus/gearrackandpinioncarjack1.gif>

How does increasing distance How does increasing distance decrease force?decrease force?

A rake makes the A rake makes the work easier by work easier by increasing the increasing the distance over which distance over which you apply force.you apply force.

Your hands move a Your hands move a small distance at small distance at the top of the the top of the handle while the handle while the handle moves handle moves across a wider across a wider distance.distance.

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Why do you want a machine Why do you want a machine that will change direction?that will change direction?

Sometimes it’s easier to apply force in Sometimes it’s easier to apply force in a different direction:a different direction:

To raise a flag up a pole, it’s easier to To raise a flag up a pole, it’s easier to pull down on a rope than to climb to pull down on a rope than to climb to the top.the top.

When you use an axe to split wood, a When you use an axe to split wood, a downward force on the handle downward force on the handle changes into a sideways force by the changes into a sideways force by the blade.blade.

The Work Done by MachinesThe Work Done by Machines

Pushing down on Pushing down on the handle raising the handle raising the lid.the lid.

You are doing work You are doing work on the crowbar on the crowbar that opposes the that opposes the friction of the nails friction of the nails in the lid and crate.in the lid and crate.

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What are input forces & output What are input forces & output forces?forces?

A machine changes A machine changes the way you do the the way you do the work making it work making it easier.easier.

The force you apply The force you apply to the machine is to the machine is called the called the input input force or Fforce or Finin..

The force applied by The force applied by the machine is called the machine is called the the output force or output force or FFoutout..<http://img2.timeinc.net/toh/i/a/

solutions/nail-pulling-01.jpg>

What are input work & output What are input work & output work?work?

When you use a machine there are 2 When you use a machine there are 2 kinds of work:kinds of work:– Input work or WInput work or Win in , done by you, done by you

– Output work or WOutput work or Woutout, done by machine, done by machine

How do machines use How do machines use conservation of energy?conservation of energy?

When you do work on a machine, you When you do work on a machine, you transfer your energy to the machine transfer your energy to the machine which then transfers energy to the which then transfers energy to the object.object.

So, WSo, Wout out is never greater than Wis never greater than Winin.. However, a machine does not transfer all However, a machine does not transfer all

its energy to the object. Due to friction, its energy to the object. Due to friction, some of the energy changes to heat.some of the energy changes to heat.

This means that WThis means that Wout out is always smaller is always smaller than Wthan Winin..

What is an ideal machine?What is an ideal machine?

If you could build perfect machine If you could build perfect machine without any friction, the input work without any friction, the input work would equal the output work.would equal the output work.

WWinin=W=Wout out or For FininX dX dinin=F=FoutoutX dX doutout

If the machine could increase the If the machine could increase the input force, then work output would input force, then work output would be greater than the work input.be greater than the work input.

Mechanical AdvantageMechanical Advantage

Some machines make work easier by Some machines make work easier by making the output force greater than making the output force greater than the input force. The number of times the input force. The number of times the applied force is increased by a the applied force is increased by a machine is called the mechanical machine is called the mechanical advantage (MA) of the machine. advantage (MA) of the machine. Mechanical advantage Mechanical advantage is the ratio is the ratio of the output force to the input force. of the output force to the input force. MA = F MA = Foutout/F/Finin

Mechanical AdvantageMechanical Advantage

Using a pulley system you only need Using a pulley system you only need 300 N to lift a piano that weighs 300 N to lift a piano that weighs 1500 N. What is the MA?1500 N. What is the MA?

MA = FMA = Foutout/F/Fin in = 1500 N/300 N= 5= 1500 N/300 N= 5

Notice this ratio cancels out units of Notice this ratio cancels out units of newtons.newtons.

What is ideal mechanical What is ideal mechanical advantage?advantage?

The MA of a The MA of a machine without machine without friction is called the friction is called the ideal MA or IMA.ideal MA or IMA.

You can calculate You can calculate this by dividing the this by dividing the input distance by input distance by the output the output distance.distance.

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EfficiencyEfficiency

Some of the energy put into a real Some of the energy put into a real machine is changed into heat by friction.machine is changed into heat by friction.

So, the output work of a machine is So, the output work of a machine is always less than the work put into it.always less than the work put into it.

Efficiency Efficiency is the comparison of the is the comparison of the amount of work put into a machine to amount of work put into a machine to the amount of work the machine puts the amount of work the machine puts out.out.

High-efficiency means less heat from High-efficiency means less heat from friction!friction!

How do you calculate How do you calculate efficiency?efficiency?

Divide the output work by the input Divide the output work by the input work to get a percentage.work to get a percentage.

Efficiency= WEfficiency= Woutout//inin X 100% X 100% To calculate the efficiency of a To calculate the efficiency of a

machine with Wmachine with Winin of 50 joules & a W of 50 joules & a Woutout of 40 joules: 40/50=0.8, or 80%.of 40 joules: 40/50=0.8, or 80%.

The efficiency of a real machine is The efficiency of a real machine is always less than 100% due to friction.always less than 100% due to friction.

How can machines be made How can machines be made more efficient?more efficient?

Reducing friction by adding oil or grease to Reducing friction by adding oil or grease to the surfaces that rub together fills the the surfaces that rub together fills the gaps between them so the surfaces slide gaps between them so the surfaces slide across each other more easily.across each other more easily.

Section 3- Simple MachinesSection 3- Simple Machines

What You’ll Learn:What You’ll Learn: Six types of simple machinesSix types of simple machines How simple machines make work How simple machines make work

easiereasier How to calculate the ideal How to calculate the ideal

mechanical advantage of simple mechanical advantage of simple machinesmachines

Types of Simple MachinesTypes of Simple Machines

A A simple machine simple machine is a machine that is a machine that does work with only one movement does work with only one movement of the machine.of the machine.

There are six types: lever, pulley, There are six types: lever, pulley, wheel & axle, inclined plane, screw, wheel & axle, inclined plane, screw, and wedge.and wedge.

The screw and wedge are different The screw and wedge are different forms of the inclined plane.forms of the inclined plane.

LeversLevers

A wheelbarrow, a rake & a baseball bat A wheelbarrow, a rake & a baseball bat are all examples of levers.are all examples of levers.

A A leverlever is a bar that pivots, or turns is a bar that pivots, or turns around, a fixed point called the fulcrum.around, a fixed point called the fulcrum.

The input arm is the distance from the The input arm is the distance from the fulcrum to the point where the input fulcrum to the point where the input force is applied; the output arm is the force is applied; the output arm is the distance from the fulcrum to the point distance from the fulcrum to the point where the lever exerts the output force.where the lever exerts the output force.

What are the 3 classes of What are the 3 classes of levers?levers?

The class of a lever is The class of a lever is based on the location of based on the location of the fulcrum, the input the fulcrum, the input force and the output force.force and the output force.

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First-Class LeverFirst-Class Lever

The top figure shows a first-The top figure shows a first-class lever with the fulcrum class lever with the fulcrum located between the input located between the input and output forces.and output forces.

The first-class lever always The first-class lever always changes the direction of the changes the direction of the force.force.

Examples include a Examples include a crowbar, scissors, and a crowbar, scissors, and a seesaw.seesaw.

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Second-Class LeverSecond-Class Lever

The middle figure The middle figure represents a second-class represents a second-class lever with the output force lever with the output force between the input force between the input force and the fulcrum.and the fulcrum.

Both input and output Both input and output forces move in the same forces move in the same direction.direction.

The wheelbarrow is a The wheelbarrow is a classic example.classic example.

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Third-Class LeverThird-Class Lever The bottom figure, a third-The bottom figure, a third-

class lever, shows the output class lever, shows the output force is farther away from the force is farther away from the fulcrum than the input force.fulcrum than the input force.

The output force is always The output force is always less than the input force in a less than the input force in a third-class lever such as a third-class lever such as a baseball bat, but the baseball bat, but the advantage is that it increases advantage is that it increases the distance over which the the distance over which the output force is applied.output force is applied.

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How is ideal mechanical How is ideal mechanical advantage of a lever advantage of a lever

calculated?calculated? To calculate IMA of any machine, To calculate IMA of any machine,

divide the input distance by the divide the input distance by the output distance.output distance.

For a lever, the input distance is the For a lever, the input distance is the length of the input arm & output length of the input arm & output distance is the length of the output distance is the length of the output arm.arm.

IMA= LIMA= Linin/L/Loutout

PulleysPulleys

To raise a sail upward, a To raise a sail upward, a sailor pulls down on a sailor pulls down on a rope wrapped around a rope wrapped around a pulley.pulley.

A A pulley pulley is a grooved is a grooved wheel with a rope, wheel with a rope, chain or cable wrapped chain or cable wrapped around it.around it.

Pulleys may be fixed or Pulleys may be fixed or movable or in systems.movable or in systems.

What is a fixed pulley?What is a fixed pulley?

This modified first-This modified first-class lever changes class lever changes the direction of the the direction of the input force like on input force like on a sail or a flagpole.a sail or a flagpole.

An elevator also An elevator also uses a fixed pulley uses a fixed pulley with a cable.with a cable. <http://www.legoeducation.com/

sharedimages/content/Large/L_Pulley_dia1_fixed.gif>

What is a movable pulley?What is a movable pulley? A movable pulley has A movable pulley has

one end of the rope one end of the rope fixed & the wheel is fixed & the wheel is free to move.free to move.

The movable pulley The movable pulley doesn’t change the doesn’t change the direction of the force, direction of the force, but it does decrease but it does decrease the amount of input the amount of input force needed to lift force needed to lift the object. MA=2the object. MA=2

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What is a block and tackle?What is a block and tackle?

A block and tackle is A block and tackle is a system of fixed & a system of fixed & movable pulleys movable pulleys used together.used together.

The more sections The more sections of the rope a of the rope a system uses to pull system uses to pull up an object, the up an object, the greater the output greater the output force is. IMA=# of force is. IMA=# of sectionssections

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Wheel and AxleWheel and Axle

Simple machine Simple machine with an axle with an axle attached to the attached to the center of a larger center of a larger wheel and both wheel and both turn together.turn together.

Doorknobs and Doorknobs and ferris wheels are ferris wheels are examples.examples.

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What is the IMA of the wheel & What is the IMA of the wheel & axle?axle?

A wheel and axle is a modified lever A wheel and axle is a modified lever with the center of the axle as the with the center of the axle as the fulcrum.fulcrum.

To calculate the IMA of a wheel and To calculate the IMA of a wheel and axle, use this equation:axle, use this equation:

IMA= radius of wheel (m)/radius of axle IMA= radius of wheel (m)/radius of axle (m) or IMA= r(m) or IMA= rww/r/raa

To increase IMA, simply increase the To increase IMA, simply increase the radius of the wheel.radius of the wheel.

How do gears work?How do gears work? A gear is a wheel A gear is a wheel

and axle with teeth and axle with teeth around the rim of around the rim of the wheel.the wheel.

One gear makes One gear makes the other turn with the other turn with the smaller gear the smaller gear turning more times turning more times than the larger than the larger one. Output force one. Output force & direction can be & direction can be changed with a changed with a gear.gear.

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Inclined PlanesInclined Planes

An An inclined plane inclined plane is a sloping surface is a sloping surface that reduces the that reduces the amount of force it amount of force it takes to do work.takes to do work.

Examples include Examples include ramps and ramps and stairways.stairways.<http://www.jaha.org/edu/

inclined_plane/images/IP1_000.jpg>

How does an inclined plane How does an inclined plane make work easier?make work easier?

You do the same work by lifting a box You do the same work by lifting a box straight up or pushing it up a ramp. straight up or pushing it up a ramp.

As the inclined plane becomes longer, As the inclined plane becomes longer, the force needed to move the object the force needed to move the object becomes less.becomes less.

The input force is applied over a longer The input force is applied over a longer distance, so it takes less input force.distance, so it takes less input force.

IMA=length of slope(m)/ height of IMA=length of slope(m)/ height of slope(m)slope(m)

The ScrewThe Screw A A screw screw is an is an

inclined plane inclined plane wrapped in a spiral wrapped in a spiral around a post. around a post.

The inclined plane The inclined plane forms the threads on forms the threads on the screw.the screw.

Apply force by turning Apply force by turning the screw; friction the screw; friction holds it in place.holds it in place.

Examples: jar lid, Examples: jar lid, corkscrew, drill bit, corkscrew, drill bit, light bulb.light bulb.

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The WedgeThe Wedge A A wedge wedge is an is an

inclined plane with inclined plane with one or two sloping one or two sloping sides. sides.

Like the screw, the Like the screw, the inclined plane inclined plane moves through the moves through the object. Knives are object. Knives are wedges.wedges.

The IMA increases The IMA increases as it gets longer & as it gets longer & thinner.thinner.

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Compound MachinesCompound Machines Some machines, Some machines,

like this can opener, like this can opener, are made of several are made of several simple machines.simple machines.

Two or more Two or more working together working together are called a are called a compound compound machinemachine..

The handles are The handles are levers, a wedge levers, a wedge pierces the can, a pierces the can, a wheel & axle turns wheel & axle turns to open the can.to open the can.

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