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

What is work ?What is work ?

The product of the force applied to an The product of the force applied to an object and the distance through which object and the distance through which that force is applied.that force is applied.

Work = Force (N) x Distance (m)Work = Force (N) x Distance (m) 1 N*m = Joule (J)1 N*m = Joule (J)

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

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

Work does Work does NOTNOT involve time, only force 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.

Would you do more work if you ran up Would you do more work if you ran up the stairs and skipped 2 at a time?the stairs and skipped 2 at a time?

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 powerpower of light 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 Traditionally associated with enginesengines. . (car,motorcycle,lawn-mower)(car,motorcycle,lawn-mower)

The term The term horsepowerhorsepower 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

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 Involves nono

friction.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 Efficiency can never be greater than

100 %. 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))

Simple MachinesSimple Machines

6 different types of simple machines:6 different types of simple machines:

1.) the lever1.) the lever

2.) the wheel and axle2.) the wheel and axle

3.) the inclined plane3.) the inclined plane

4.) the wedge4.) the wedge

5.) the screw5.) the screw

6.) the pulley6.) the pulley

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.

INPUT FORCE (EFFORT FORCE)

OUTPUT FORCE (Resistance Force)

INPUT ARM (EFFORT DISTANCE)OUTPUT ARM

(RESISTANCE DISTANCE)

FULCRUM

First-Class LeverFirst-Class Lever

• The position of the fulcrum identifies a first-class lever.

- The fulcrum of a first-class lever is ALWAYS located between the input force (effort) and the output force (resistance)

• Depending on position of fulcrum, MA of first-class lever can be greater than 1, equal to 1, or less than 1

Which ones are first-class Which ones are first-class levers?levers?

Second-Class LeversSecond-Class Levers

• Output force is located BETWEEN the input force and the fulcrum

Example of second-class Example of second-class leverlever

When you lift handles of wheelbarrow, it rotates around its fulcrum

Input distance greater than output distance

Increased input distance means it takes LESS force from you to lift the load

MA of a second-class lever is always greater than 1

Third-Class LeversThird-Class LeversInput force is located between the fulcrum and the output force

• The output distance over which the lever exerts its force is always larger than the input distance you move the lever through. Therefore MA for third-class levers is always less than 1

Wheel & AxleWheel & Axle

Consists of 2 discs or cylinders, each one Consists of 2 discs or cylinders, each one with a different radius.with a different radius.

wheel

axle Can have MA greater than or less than 1

Would the MA for the steering wheel be greater than or less than 1?Greater than 1: input distance larger than output distance

Inclined PlanesInclined Planes

Imagine how hard it would be to walk up the side of a Imagine how hard it would be to walk up the side of a steep hill. steep hill.

It would be MUCH easier to follow a gentle slope of a It would be MUCH easier to follow a gentle slope of a winding trail….why is this?winding trail….why is this?

What is happening to input distance if you decide to take the curvy trail instead of going straight up the hill?

Because input distance is greater than output distance, in this case, the input force is decreased…..so it’s easier for you!

Inclined PlanesInclined Planes

Inclined plane: a slanted surface along Inclined plane: a slanted surface along which a force moves an object to a which a force moves an object to a different elevationdifferent elevation

Input distanceOutput distance

Inclined PlanesInclined Planes

What is the MA of the following inclined What is the MA of the following inclined plane?plane?

6m1 m

Mechanical Advantage = 6

MA for inclined plane will NEVER be less than 1

Wedges & ScrewsWedges & Screws

Similar to inclined planes BUT sloping Similar to inclined planes BUT sloping surfaces can move.surfaces can move.

Wedge: a V-shaped object whose sides Wedge: a V-shaped object whose sides are two inclined planes sloped toward are two inclined planes sloped toward each other.each other.

A thin wedge of a given length has a greater MA than thick wedge of the same length. Why is this so?

Length of wedge is divided by width of the opening. Smaller the opening (thickness), the larger your MA will be

Examples: knife, axe, razor blade

WedgeWedge

Why did Samurai’s feel it was so important to keep their blade sharp?

They wanted to have the highest MA possible!

Sharpening the sword increases MA (just like having a thinner wedge in the wood worked better than a thicker one)

ScrewsScrews

Screw: an inclined plane wrapped around Screw: an inclined plane wrapped around a cylindera cylinder

threads

Which screw would be easier to put into a piece of wood?-A screw with the threads very close together or a screw with the threads somewhat far apart-The screw with threads CLOSE together!-Though it would take you less turns for the screw with fewer threads, it would require you to exert a greater force

-Screws with threads that are closer together have a greater MA

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.