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What is work?What is work?

In science, the word In science, the word workwork has a has a different meaning than you may different meaning than you may be familiar with.be familiar with.

The scientific definition of work is: The scientific definition of work is: using a force to using a force to move an move an object a distanceobject a distance (when both (when both the force and the motion of the the force and the motion of the object are in the same direction.)object are in the same direction.)

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Work or Not?Work or Not?

According to the According to the scientific definition, scientific definition,

what is work and what is work and what is not?what is not?

A teacher lecturing to A teacher lecturing to

her class?her class?

A mouse pushing a piece A mouse pushing a piece of cheese with its nose of cheese with its nose

across the floor?across the floor?

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What’s work?What’s work?

For work to be done on an object, the For work to be done on an object, the object must move in the same object must move in the same

direction as the force.direction as the force.

So ….2 things are required…So ….2 things are required…

Object Object must MOVEmust MOVE when force is applied when force is applied The The direction of the motiondirection of the motion must be the must be the

same as the forcesame as the force..

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Imagine that you are late for Imagine that you are late for school and are moving quickly to school and are moving quickly to

your locker carrying a heavy your locker carrying a heavy book bag. Because you are book bag. Because you are

making the book bag move, are making the book bag move, are you doing work on it?you doing work on it?

NO – For work to be done on an NO – For work to be done on an object, the object must object, the object must move in move in the SAME direction as the forcethe SAME direction as the force..

You are You are applying a applying a force to hold it force to hold it upup, , but but the the bag is moving bag is moving

forwardforward..

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What’s work?What’s work?

A scientist delivers a speech to an A scientist delivers a speech to an audience of his peers. audience of his peers.

A mother picks up her baby.A mother picks up her baby. A mother carries her baby from room A mother carries her baby from room

to room. to room. A body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head. A woman carries a 20 kg grocery bag A woman carries a 20 kg grocery bag

to her car? to her car?

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What’s work?What’s work? A scientist delivers a speech to an A scientist delivers a speech to an

audience of his peers. audience of his peers. NONO A mother picks up her baby. A mother picks up her baby. YesYes A mother carries her baby from room A mother carries her baby from room

to room. to room. NONO A body builder lifts 350 pounds A body builder lifts 350 pounds

above his headabove his head. . YesYes A woman carries a 20 kg grocery bag A woman carries a 20 kg grocery bag

to her car? to her car? NONO

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Formula for WorkFormula for Work

Work = Force x DistanceWork = Force x Distance(push or pull)(push or pull)

The unit of The unit of forceforce is is NewtonsNewtons The unit of The unit of distancedistance is is metersmeters The unit of The unit of workwork is is Newton-metersNewton-meters One Newton-meter is equal to one One Newton-meter is equal to one

joulejoule

So, the unit of So, the unit of work is a joulework is a joule

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W=FD ~ W=FD ~ Work = Force x Work = Force x DistanceDistance

Calculate: Calculate:

If a man pushes If a man pushes a concrete a concrete block 10 block 10

meters with a meters with a force of 20 N, force of 20 N,

how much work how much work has he done?has he done?

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W=FDW=FD

Work = Force x Work = Force x DistanceDistance

If a man pushes a If a man pushes a concrete block 10 concrete block 10

meters with a meters with a force of 20 N, how force of 20 N, how much work has he much work has he

done? done?

W = 20N x 10m = W = 20N x 10m = 200 joules200 joules

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PowerPower Power is the rate ( speed/ how fast) at Power is the rate ( speed/ how fast) at

which work is done or that energy is which work is done or that energy is transferred. transferred.

It measures It measures how fast work happenshow fast work happens – or how – or how quickly energy is transferredquickly energy is transferred..

Power = Power = WorkWork**

TimeTime **(force x distance)(force x distance)

The The unit of powerunit of power is the is the watt watt (also (also joules/second).joules/second).

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Check for Check for UnderstandingUnderstanding

Two physics students, Ben and Bonnie, are in Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 barbell the same distance over his head 10 times in 10 seconds. times in 10 seconds.

Which student does the most work? Which student does the most work? Which student delivers the most Which student delivers the most

power?power? Explain your answers. Explain your answers.

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Ben and Bonnie do the Ben and Bonnie do the same amount of worksame amount of work; ; they apply the they apply the same forcesame force to lift the same barbell to lift the same barbell the the same distancesame distance above above their heads. their heads.

Yet, Yet, Ben is the most Ben is the most powerfulpowerful since he does since he does the the same work in less same work in less timetime. .

  

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What is a Machine?What is a Machine?

Device or tool that makes Device or tool that makes work work easiereasier

A machine makes work easier by A machine makes work easier by changing the size (magnitude)changing the size (magnitude) or or directiondirection of the force needed of the force needed

What are some machines you What are some machines you

use everyday?use everyday?

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Input vs. OutputInput vs. Output

The The WORKWORK that that you do onyou do on the machine the machine is is work inputwork input.. (W = F x D) (W = F x D)

The The FORCEFORCE you you use onuse on the machine is the machine is called called input forceinput force..

The The workwork done done by the machineby the machine is called is called workwork outputoutput

The The FORCEFORCE the machine applies is called the machine applies is called output forceoutput force..

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Mechanical Advantage Mechanical Advantage (MA)(MA)((Comparing Forces)Comparing Forces) MA = MA = outputoutput force force inputinput force force

MA tells how many times a machineMA tells how many times a machine multiplies forcemultiplies force

If a machine can increase force more than If a machine can increase force more than others, work is generally easier and it has a others, work is generally easier and it has a

greater greater mechanical advantagemechanical advantage

>1 MA -- can help move heavy objects>1 MA -- can help move heavy objects <1 MA -- can <1 MA -- can increase the distance increase the distance an an

object moves (like a hammer) object moves (like a hammer)

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How Machines HelpHow Machines Help

They They don’t increase the don’t increase the amountamount of work of work donedone… Work output can’t be more than work … Work output can’t be more than work inputinput

But, machines allow the force to be applied over But, machines allow the force to be applied over a a greater distancegreater distance – which – which means means less force less force is neededis needed for the same amount of work for the same amount of work..

Machines Machines make work easiermake work easier by by changing the changing the size or direction (or both)size or direction (or both) of theof the input input forceforce..

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Mechanical Efficiency (ME)Mechanical Efficiency (ME)(Comparing (Comparing WorkWork Output and Input)Output and Input)

ME = ME = Work outputWork output x 100 x 100 Work inputWork input

The The work outputwork output of a machine is of a machine is always always lessless than the than the inputinput..

Work has to Work has to overcome frictionovercome friction The less work a machine has to do to The less work a machine has to do to

overcome frictionovercome friction the greater the ME the greater the ME We multiply Xs 100 because it’s expressed We multiply Xs 100 because it’s expressed

as a percentage.as a percentage.

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Machines can’t be 100% efficient Machines can’t be 100% efficient because every machine has because every machine has

moving partsmoving parts. Moving parts have . Moving parts have to use some of the work input to to use some of the work input to

overcome friction.overcome friction.

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Simple MachinesSimple Machines

Ancient people invented simple machines Ancient people invented simple machines that would help them overcome resistive that would help them overcome resistive forces and allow them to do the desired forces and allow them to do the desired

work against those forces.work against those forces.

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Simple Simple MachinesMachines

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The LeverThe Lever

A lever is a rigid bar or A lever is a rigid bar or board board

that rotates around a that rotates around a fixed fixed

pointpoint called the called the fulcrumfulcrum..

The bar may be either The bar may be either straight or curved.straight or curved.

In use, a lever has both In use, a lever has both an an efforteffort (or applied) force (or applied) force and a and a loadload (resistant (resistant force).force).

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Levers can be used to exert a Levers can be used to exert a large force over a small distance large force over a small distance

at one end by exerting only a at one end by exerting only a small force over a greater small force over a greater

distance at the other.distance at the other.

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The 3 Classes of The 3 Classes of LeversLevers

The class of a The class of a lever is lever is

determined by determined by the the location of location of the the effort forceeffort force and the and the loadload

relative to the relative to the fulcrumfulcrum. .

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The 3 Classes of The 3 Classes of LeversLevers

“R” is the resistance force which is the load. “E” is the human effort force applied.

The fulcrum is the fixed point support on which a lever pivots

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First Class LeverFirst Class Lever

In a first-class lever the fulcrum is In a first-class lever the fulcrum is located at some point between the located at some point between the effort and resistance (load) forces.effort and resistance (load) forces.– Common examples of first-class levers Common examples of first-class levers

include include crowbars, scissors, pliers, and crowbars, scissors, pliers, and seesaws.seesaws.

– A first-class lever always changes the A first-class lever always changes the direction of force (I.e. a downward effort direction of force (I.e. a downward effort force on the lever results in an upward force on the lever results in an upward movement of the resistance force). movement of the resistance force).

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Fulcrum is between EF (effort) and RF Fulcrum is between EF (effort) and RF (load)(load)

Effort moves farther than Resistance.Effort moves farther than Resistance. Multiplies EF and changes its directionMultiplies EF and changes its direction

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Second Class LeverSecond Class Lever

With a second-class lever, the With a second-class lever, the load (R) is load (R) is located located between the fulcrum and the effort between the fulcrum and the effort forceforce..

Common examples of second-class levers Common examples of second-class levers include include nut crackers, wheel barrows, doors, nut crackers, wheel barrows, doors, and bottle openersand bottle openers..

A second-class lever does not change the A second-class lever does not change the direction of force. direction of force. When the When the fulcrumfulcrum is is located located closer to the loadcloser to the load than to the than to the

effort force, an increase in output effort force, an increase in output force (mechanical advantage) results.force (mechanical advantage) results.

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RF (load) is between fulcrum and EF RF (load) is between fulcrum and EF Effort moves farther than Resistance.Effort moves farther than Resistance.

Multiplies EF, but does not change its Multiplies EF, but does not change its

directiondirection

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Third Class LeverThird Class Lever

With a third-class lever, the With a third-class lever, the effort effort force is applied between the fulcrum force is applied between the fulcrum and the resistance force.and the resistance force.– Examples of third-class levers include Examples of third-class levers include

tweezers, hammers, shovels, fishing tweezers, hammers, shovels, fishing pole, and about any tool you swing (bat, pole, and about any tool you swing (bat, club)club)..

– A third-class lever does not change the A third-class lever does not change the direction of force; there is an increase direction of force; there is an increase distance with a corresponding decrease distance with a corresponding decrease in force. in force.

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EF is between fulcrum and RF (load) EF is between fulcrum and RF (load) Does not multiply force Does not multiply force

Resistance moves farther than Effort.Resistance moves farther than Effort. Multiplies the distance the effort force Multiplies the distance the effort force

travelstravels

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Lever Mechanical Lever Mechanical AdvantageAdvantage

MA = length of effort arm ÷ length of

resistance arm

If the If the effort arm distanceeffort arm distance (from effort to fulcrum) is greater (from effort to fulcrum) is greater than the than the resistance armresistance arm, then the effort required will be less , then the effort required will be less than the load being moved. This is known as a than the load being moved. This is known as a 'positive 'positive mechanical advantagemechanical advantage'.'.

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Wheel and AxleWheel and Axle

The wheel and axle is a The wheel and axle is a simple machine simple machine consisting of a consisting of a large large wheelwheel rigidly rigidly secured tosecured to a a smaller wheelsmaller wheel or or shaft, called an axle. shaft, called an axle.

When either the wheel or When either the wheel or axle turns, the other part axle turns, the other part also turns. One full also turns. One full revolution of either part revolution of either part causes one full revolution causes one full revolution of the other part. of the other part.

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PulleyPulley

A pulley can be A pulley can be – FixedFixed– MovableMovable

A pulley consists of A pulley consists of a grooved wheel a grooved wheel

that turns freely in that turns freely in a frame called a a frame called a

block.block.

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Fixed PulleyFixed Pulley

A pulley is said to A pulley is said to be a be a fixed pulleyfixed pulley if if it does not rise or it does not rise or fall with the load fall with the load

being moved (only being moved (only spins). A fixed spins). A fixed

pulley changes the pulley changes the direction of a direction of a

force.force.

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Moveable PulleyMoveable Pulley

A moveable pulley A moveable pulley rises and falls with rises and falls with the loadthe load that is being that is being

moved. A single moved. A single moveable pulley does moveable pulley does

not change the not change the direction of a force.direction of a force.

Movable pulleys increase Movable pulleys increase force and distance force and distance

over which the input over which the input force must be exerted.force must be exerted.

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Block and TackleBlock and Tackle(Compound Pulley)(Compound Pulley)

Combines fixed and movableCombines fixed and movable pulley pulley – – sometimes more than one of sometimes more than one of each.each.

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A Pulley’s A Pulley’s Mechanical AdvantageMechanical Advantage

The Mechanical Advantage of a The Mechanical Advantage of a pulley equals the pulley equals the number of rope number of rope segmentssegments that support the load. that support the load.

MA = 4MA = 4 (Don’t count the rope you (Don’t count the rope you

are pulling)are pulling)

Pulley Video

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Inclined PlaneInclined Plane

An inclined plane An inclined plane is an is an even even sloping sloping

surfacesurface. The . The inclined plane inclined plane

makes it easier makes it easier to move a weight to move a weight from a lower to from a lower to

higher elevation.higher elevation.

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Inclined PlaneInclined Plane

The mechanical The mechanical advantage of an inclined advantage of an inclined plane is equal to the plane is equal to the length of the slopelength of the slope divided bydivided by the the height height of the inclined plane.of the inclined plane.

While the inclined plane While the inclined plane produces a mechanical produces a mechanical advantage, it does so by advantage, it does so by increasing the increasing the distancedistance through which through which the force must move. the force must move.

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Although it takes less force for car A to Although it takes less force for car A to get to the top of the ramp, all the cars do get to the top of the ramp, all the cars do the same amount of work.the same amount of work.

A B C

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Inclined PlaneInclined Plane

A wagon trail on a A wagon trail on a steep hill will often steep hill will often traverse back and traverse back and forth to reduce the forth to reduce the slope experienced by slope experienced by a team pulling a a team pulling a heavily loaded wagon.heavily loaded wagon.

This same technique This same technique is used today in is used today in modern freeways modern freeways which travel winding which travel winding paths through steep paths through steep mountain passes. mountain passes.

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WedgeWedge The wedge is a The wedge is a

modification of the modification of the inclined plane. Wedges inclined plane. Wedges are used as either are used as either separating or holding separating or holding devices. devices.

A wedge can either be A wedge can either be composed of one or two composed of one or two inclined planes. A inclined planes. A double wedge can be double wedge can be thought of as two thought of as two inclined planes joined inclined planes joined together with their together with their sloping surfaces sloping surfaces outward.outward.

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The ScrewThe Screw

The screw is also a The screw is also a modified version of modified version of the inclined plane. the inclined plane.

While this may be While this may be somewhat difficult to somewhat difficult to visualize, it may help visualize, it may help to think of the to think of the threads of the screw threads of the screw as a type of circular as a type of circular rampramp (or inclined (or inclined plane). plane).

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A Screw’s MA ~A Screw’s MA ~

The longer and thinner the screw is the greater the MA…Similarly, the longer the spiral on the screw and

closer together the threads are, the greater the MA.