Chapter 12 – Work and Machines Work is when a force is used to make something move. Work is when a force is used to make something move. Work = Force (Distance)

Chapter 12 – Work and MachinesChapter 12 – Work and Machines

►Work is when a force is used to make Work is when a force is used to make something move.something move.

Work = Force (Distance)Work = Force (Distance)

►Work unitsWork units English units will be footEnglish units will be foot..poundspounds Metric units will be newtonMetric units will be newton..meters (which meters (which

is called a joule) is called a joule)

Example problem:Example problem:

Use the four step method to determine Use the four step method to determine how much work is done if a 50 how much work is done if a 50 newton force is used to drag a rock 30 newton force is used to drag a rock 30 meters.meters.

The force used in this equation is the The force used in this equation is the force in the force in the direction of motiondirection of motion..

30 newtons Force

40 newtons Force

30 newtons Force used to move box

MachinesMachines► A machine is a device with which you A machine is a device with which you

can do work in a way that is easier can do work in a way that is easier or more effectiveor more effective

► A machine can be simple or complexA machine can be simple or complex► A machine makes work easier by:A machine makes work easier by:

a.a. Changing the amount of force you exert orChanging the amount of force you exert or

b.b. Changing the distance over which you exert Changing the distance over which you exert the force the force

c.c. The direction in which you exert your forceThe direction in which you exert your force

► Input force – force you exert on the machineInput force – force you exert on the machine► Output force – force exerted by the machineOutput force – force exerted by the machine

Output ForceOutput Force Mechanical advantage = Mechanical advantage =

Input ForceInput Force

► Multiplying ForceMultiplying Force (mechanical advantage (mechanical advantage greater than 1) greater than 1) If the input force is less than the output If the input force is less than the output force, the input force must be exerted over force, the input force must be exerted over a greater distance.a greater distance. Examples: a car jack, pulling nails with a hammerExamples: a car jack, pulling nails with a hammer

►Multiplying distanceMultiplying distance (mechanical (mechanical advantage less than 1) advantage less than 1)

If the input force is greater than the If the input force is greater than the output force, the output end of the output force, the output end of the machine will move a greater distance.machine will move a greater distance. Examples: a fan, a hockey stick, high Examples: a fan, a hockey stick, high

gear on a bicycle or car gear on a bicycle or car

►Changing Direction of ForceChanging Direction of Force Example: a pulley, a leverExample: a pulley, a lever

►Efficiency Efficiency

Efficiency is the total work or energy Efficiency is the total work or energy input into a machine compared to the input into a machine compared to the useful work or energy output of the useful work or energy output of the machinemachine

Output WorkOutput Work

Efficiency = ------------------ x 100Efficiency = ------------------ x 100

Input WorkInput Work

► If there was no energy loss through If there was no energy loss through friction or other causes, the efficiency friction or other causes, the efficiency of a machine would be 100% of a machine would be 100%

► No machine is 100% efficient, No machine is 100% efficient, although some simple machines are although some simple machines are very close to 100% efficient very close to 100% efficient

For simple machinesFor simple machines

Input work ≈ Output workInput work ≈ Output work

20 lbs

50 lbs

Push down a distance of 5 ft.

Lift a distance of 2 ft.

Input work = F (D)

Input work = 20 lb (5 ft.)

Input work = 100 ft-lbs

Output work = F (D)

Output work = 50 lb (2 ft.)

Output work = 100 ft-lbs100 ft-lbs ≈≈ 100 ft-lbs

Use the efficiency equation to determine Use the efficiency equation to determine the approximate efficiency of the the approximate efficiency of the lever system.lever system.

output workoutput work

EfficiencyEfficiency == x 100 x 100 ==

input workinput work

100 ft-lbs100 ft-lbs

x 100x 100 efficiency = 100% efficiency = 100%

100 ft-lbs100 ft-lbs

Example efficiency problems: Use the Example efficiency problems: Use the four step method to solve these four step method to solve these problemsproblems

1.1. Determine the efficiency of a Determine the efficiency of a machine that requires a work machine that requires a work input of 224 foot pounds for a work input of 224 foot pounds for a work output of 200 foot pounds.output of 200 foot pounds.

1.1. A hydraulic jack requires thirty ½ A hydraulic jack requires thirty ½ foot down ward strokes of 20 foot down ward strokes of 20 pounds each to lift an 1150 pounds each to lift an 1150 pound object a distance of .25 feet. pound object a distance of .25 feet. Calculate the efficiency of the jack.Calculate the efficiency of the jack.

► Ideal Mechanical Advantage – the Ideal Mechanical Advantage – the mechanical advantage of a machine mechanical advantage of a machine if there was no friction. This can if there was no friction. This can be calculated.be calculated.

►Actual Mechanical Advantage – the Actual Mechanical Advantage – the real mechanical advantage of a real mechanical advantage of a machine. This can be different for machine. This can be different for each machine and must be measured.each machine and must be measured.

Part 2 – Simple MachinesPart 2 – Simple Machines

The 6 types of simple machines are:The 6 types of simple machines are:

1.1. The inclined planeThe inclined plane

2.2. The wedgeThe wedge

3.3. The screwThe screw

4.4. The leverThe lever

5.5. The wheel and axleThe wheel and axle

6.6. The pulleyThe pulley

The Inclined PlaneThe Inclined Plane

► Mechanical advantage (MA)= output Mechanical advantage (MA)= output force/input forceforce/input force

► Ideal MA = length of the incline/height of Ideal MA = length of the incline/height of inclineincline

► If there was no friction the actual MA would If there was no friction the actual MA would equal ideal MA. In real life it is always less.equal ideal MA. In real life it is always less.

Incline height

Incline length

WedgeWedge

►A wedge is like a moving inclined A wedge is like a moving inclined plane to split or cut things apart.plane to split or cut things apart.

ScrewScrew► A screw is also related to the inclined plane.A screw is also related to the inclined plane.► The threads of a screw are like a spiral inclined The threads of a screw are like a spiral inclined

plane.plane.► The closer the threads, the greater the MA of the The closer the threads, the greater the MA of the

screw.screw.

Levers (3 types)Levers (3 types)► First Class Lever (a First Class Lever (a

pry or teeter-totter)pry or teeter-totter)

► Second class Lever Second class Lever (wheel barrel)(wheel barrel)

► Third class LeverThird class Lever

Output force

Input force

Output force

Output force

Input force

Input force

Distance from fulcrum to input force

Ideal MA =

Distance from fulcrum to output force

Wheel and AxleWheel and Axle

►The wheel and axle are fastened The wheel and axle are fastened together so they rotate together. together so they rotate together. (door knob, screw driver, (door knob, screw driver, steering wheel of a car)steering wheel of a car)

radius of wheelradius of wheel

mechanical advantage =mechanical advantage = radius of axle radius of axle

PulleyPulley► A single fixed pulley has a mechanical A single fixed pulley has a mechanical

advantage of one.advantage of one.► A single movable pulley has a mechanical A single movable pulley has a mechanical

advantage of twoadvantage of two► We can add more movable pulleys to add We can add more movable pulleys to add

more mechanical advantage. more mechanical advantage.

Compound Machine Compound Machine

• A machine that uses two or more simple machines

• Identify the simple machines in each of the following.