Work, Power, & Machines What is work ? The scientific definition of work is: using a force to...

Work, Power, Work, Power, & Machines& Machines

What is work ?What is work ?

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

Is work being done or Is work being done or not?not?

Mowing the lawnMowing the lawn Weight-liftingWeight-lifting Moving furniture up a Moving furniture up a

flight of stairsflight of stairs Pushing against a Pushing against a

locked doorlocked door Swinging a golf clubSwinging a golf club

YESYES YESYES YESYES

NONO

YESYES

Calculating WorkCalculating Work

All or part of the force must All or part of the force must act in the direction of the act in the direction of the movement.movement.

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Formula for workFormula for work

Work = Force x DistanceWork = Force x Distance The unit of force is newtonsThe unit of force is newtons The unit of distance is metersThe unit of distance is meters The unit of work is newton-metersThe unit of work is newton-meters One newton-meter is equal to one jouleOne newton-meter is equal to one joule So, the unit of work is a So, the unit of work is a joulejoule

Do you do more work Do you do more work when you finish a job when you finish a job quickly?quickly?

Work does NOT involve time, only force Work does NOT involve time, only force and distance.and distance.

No work is done when you stand in place No work is done when you stand in place holding an object.holding an object.

Labeling work: w = F x dLabeling work: w = F x d Newton X meter (N m)Newton X meter (N m)

Which also = Which also = (kg x m(kg x m22)) ss22

The JouleThe Joule 1 newton-meter 1 newton-meter

is a quantity is a quantity known as a known as a joule joule (J).(J).

Named after Named after British physicist British physicist James Prescott James Prescott Joule.Joule.

How quickly work is done.How quickly work is done. Amount of work done per unit time.Amount of work done per unit time. If two people mow two lawns of equal If two people mow two lawns of equal

size and one does the job in half the size and one does the job in half the time, who did more work?time, who did more work?

Same work. Different power exerted.Same work. Different power exerted. POWER = WORK / TIMEPOWER = WORK / TIME

The wattThe watt A unit named after A unit named after

Scottish inventor Scottish inventor James Watt.James Watt.

Invented the steam Invented the steam engine.engine.

P = W/t P = W/t Joules/secondJoules/second 1 watt = 1 J/s1 watt = 1 J/s

wattswatts Used to measure Used to measure

power of light power of light bulbs and small bulbs and small appliancesappliances

An electric bill is An electric bill is measured in measured in kW/hrs.kW/hrs.

1 kilowatt = 1000 W1 kilowatt = 1000 W

Horsepower (hp) = Horsepower (hp) = 745.5 watts745.5 watts

Traditionally associated with engines. Traditionally associated with engines. (car,motorcycle,lawn-mower)(car,motorcycle,lawn-mower)

The term The term horsepower horsepower was developed to was developed to quantify power. A strong horse could quantify power. A strong horse could move a 750 N object one meter in one move a 750 N object one meter in one second.second.

750 N

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History of WorkHistory of Work

Before engines and motors were invented, Before engines and motors were invented, people had to do things like lifting or pushing people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, heavy loads by hand. Using an animal could help, but what they really needed were some clever but what they really needed were some clever ways to either make work easier or faster. ways to either make work easier or faster.

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Simple MachinesSimple MachinesAncient 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.

MachinesMachines A device that makes work A device that makes work easiereasier.. A machine can change the size, the A machine can change the size, the

direction, or the distance over which a direction, or the distance over which a force acts.force acts.

Forces involved:Forces involved:

Input Force Input Force FFII

Force Force applied applied toto a machinea machine

Output ForceOutput ForceFFOO

Force Force applied applied byby a machinea machine

Two forces, thus two Two forces, thus two types of worktypes of work

Work InputWork Input work done work done onon a a

machinemachine=Input force x the =Input force x the

distance through distance through which that force acts which that force acts (input distance)(input distance)

Work OutputWork Output Work done Work done byby a a

machinemachine=Output force x the =Output force x the

distance through distance through which the resistance which the resistance moves (output moves (output distance)distance)

Can you get more work Can you get more work out than you put in?out than you put in?

Work output can never be greater than Work output can never be greater than work input.work input.

Mechanical Advantage (MA) Mechanical Advantage (MA) – expressed in a ratio – expressed in a ratio WITH WITH NO UNITS!!NO UNITS!!

The number of times a machine The number of times a machine multiplies the input force.multiplies the input force.

2 types of mechanical 2 types of mechanical advantageadvantage

IDEALIDEAL Involves no friction.Involves no friction. Is calculated Is calculated

differently for differently for different machinesdifferent machines

Usually input Usually input distance/output distance/output distancedistance

ACTUALACTUAL Involves friction.Involves friction. Calculated the Calculated the

same for all same for all machinesmachines

Different mechanical Different mechanical advantages:advantages:

MA equal to one. MA equal to one. (output force = input (output force = input force)force)

Change the direction Change the direction of the applied force of the applied force only.only.

Mechanical Mechanical advantage less than advantage less than oneone

An increase in the An increase in the distance an object is distance an object is moved (dmoved (doo))

EfficiencyEfficiency Efficiency can never be greater than 100 Efficiency can never be greater than 100

%. Why?%. Why? Some work is always needed to Some work is always needed to

overcome friction.overcome friction. A percentage comparison of work output A percentage comparison of work output

to work input.to work input. work output (Wwork output (WOO) / work input (W) / work input (WII))

1. The Lever1. The Lever A bar that is free to pivot, or move about A bar that is free to pivot, or move about

a fixed point when an input force is a fixed point when an input force is applied.applied.

FulcrumFulcrum = the pivot point of a lever. = the pivot point of a lever. There are three classes of levers based There are three classes of levers based

on the positioning of the effort force, on the positioning of the effort force, resistance force, and fulcrum.resistance force, and fulcrum.

First Class First Class LeversLevers

Fulcrum is located Fulcrum is located between the effort between the effort and resistance.and resistance.

Makes work easier Makes work easier by multiplying the by multiplying the effort force AND effort force AND changing direction.changing direction.

Examples: Examples:

Second Class Second Class LeversLevers

Resistance is found Resistance is found between the fulcrum between the fulcrum and effort force.and effort force.

Makes work easier Makes work easier by multiplying the by multiplying the effort force, but NOT effort force, but NOT changing direction.changing direction.

Examples:Examples:

Third Class Third Class LeversLevers

Effort force is located Effort force is located between the between the resistance force and resistance force and the fulcrum.the fulcrum.

Does NOT multiply Does NOT multiply the effort force, only the effort force, only multiplies the multiplies the distance.distance.

Examples:Examples:

Levers!!!!!!!!!!!Levers!!!!!!!!!!!

Mechanical advantage of Mechanical advantage of levers.levers.

Ideal = input arm Ideal = input arm length/output arm length/output arm lengthlength

input arminput arm = distance = distance from input force to from input force to the fulcrumthe fulcrum

output armoutput arm = = distance from output distance from output force to the fulcrumforce to the fulcrum

2. The Wheel and Axle2. The Wheel and Axle A lever that rotates in A lever that rotates in

a circle.a circle. A combination of two A combination of two

wheels of different wheels of different sizes.sizes.

Smaller wheel is Smaller wheel is termed the axle.termed the axle.

IMA = radius of IMA = radius of wheel/radius of axle.wheel/radius of axle.

3. The Inclined Plane3. The Inclined Plane A slanted surface A slanted surface

used to raise an used to raise an object.object.

Examples: ramps, Examples: ramps, stairs, laddersstairs, ladders

IMA = length of IMA = length of ramp/height of rampramp/height of rampCan Can nevernever be less be less than one.than one.

4. The Wedge4. The Wedge An inclined plane An inclined plane

that moves.that moves. Examples: knife, axe, Examples: knife, axe,

razor bladerazor blade Mechanical Mechanical

advantage is advantage is increased by increased by sharpening it.sharpening it.

5. The Screw5. The Screw An inclined plane An inclined plane

wrapped around a wrapped around a cylinder.cylinder.

The closer the The closer the threads, the greater threads, the greater the mechanical the mechanical advantageadvantage

Examples: bolts, Examples: bolts, augers, drill bitsaugers, drill bits

6. The Pulley6. The Pulley A chain, belt , or rope A chain, belt , or rope

wrapped around a wrapped around a wheel.wheel.

Can either change Can either change the direction or the the direction or the amount of effort forceamount of effort force

Ex. Flag pole, blinds, Ex. Flag pole, blinds, stage curtainstage curtain

Pulley typesPulley types FIXEDFIXED Can only change Can only change

the direction of a the direction of a force.force.

MA = 1MA = 1

MOVABLEMOVABLE Can multiply an Can multiply an

effort force, but effort force, but cannot change cannot change direction.direction.

MA > 1MA > 1

MA = Count # of ropes that MA = Count # of ropes that apply an upward force (note apply an upward force (note the block and tackle!)the block and tackle!)

Fe

A combination of two or more simple A combination of two or more simple machines.machines.

Cannot get more work out of a compound Cannot get more work out of a compound machine than is put in.machine than is put in.