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Work and Simple Work and Simple MachinesMachines

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What is work?What is work?

WorkWork is done on an is done on an objectobject when the when the object object movesmoves in the same in the same directiondirection in in which the which the forceforce is exerted. is exerted.

To do To do workwork, the , the objectobject MUST MUST movemove some DISTANCE as a result of your some DISTANCE as a result of your force.force.

The The forceforce you exert MUST be in the you exert MUST be in the SAME SAME directiondirection as the object’s as the object’s motion.motion.

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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 a teacher lecturing

to her classto her class a mouse pushing a a mouse pushing a

piece of cheese piece of cheese with its nose across with its nose across the floorthe floor

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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 a teacher lecturing

to her classto her class a mouse pushing a a mouse pushing a

piece of cheese piece of cheese with its nose across with its nose across the floorthe floor

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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 body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head. A mother carries her baby from room A mother carries her baby from room

to room. to room. A father pushes a baby in a carriage.A father pushes a baby in a carriage. 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 body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head. YesYes A mother carries her baby from room A mother carries her baby from room

to room. to room. NoNo A father pushes a baby in a carriage. A father pushes a baby in a carriage. YesYes A woman carries a 20 km grocery A woman carries a 20 km grocery

bag to her car? bag to her car? NoNo

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Calculating WorkCalculating Work

The amount of The amount of workwork you do depends on you do depends on both the both the amountamount of force you exert and of force you exert and the the distancedistance the object the object movesmoves..

To calculate the amount of work done To calculate the amount of work done on an object, use the following formula:on an object, use the following formula:

Work = Force X DistanceWork = Force X Distance The SI unit for work is the The SI unit for work is the newtonnewton X X

meter.meter. This unit is called the This unit is called the joulejoule..

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W=FDW=FD

Work = Force x Work = Force x DistanceDistance

Calculate: If a Calculate: If a man pushes a man pushes a concrete block 10 concrete 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

Calculate: If a Calculate: If a man pushes 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? 200 joules200 joules(W = 20N x 10m)(W = 20N x 10m)

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Check for UnderstandingCheck for Understanding

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

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

Explain your answers. Explain your answers.

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

  

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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 Machines

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

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Simple MachinesSimple Machines

The six simple machines are:The six simple machines are: Lever Lever Wheel and Axle Wheel and Axle Pulley Pulley Inclined Plane Inclined Plane Wedge Wedge Screw Screw

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Simple MachinesSimple Machines

A A machinemachine is a device that allows you is a device that allows you to do to do workwork in a way that is in a way that is easiereasier..

Using a Using a machinemachine DOES NOT mean you DOES NOT mean you do less do less workwork..

A A machinemachine makes makes workwork easier by easier by changing one of three things:changing one of three things: changing the amount of force exerted changing the amount of force exerted changing the direction the force is exerted changing the direction the force is exerted Changing the distance the force is exertedChanging the distance the force is exerted

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Mechanical AdvantageMechanical Advantage

It is useful to think about a machine It is useful to think about a machine in terms of the in terms of the input forceinput force (the (the force you apply)force you apply) and the and the outputoutput forceforce (force which is applied to the (force which is applied to the tasktask).).

When a machine takes a small input When a machine takes a small input force and increases the magnitude of force and increases the magnitude of the output force, a the output force, a mechanical mechanical advantageadvantage has been produced. has been produced.

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Mechanical AdvantageMechanical Advantage

Mechanical advantage is Mechanical advantage is the number of times a the number of times a machine increases a force.machine increases a force.

MA = output/input MA = output/input

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Inclined PlaneInclined Plane

An inclined plane is An inclined plane is a sloping surface a sloping surface connecting lower connecting lower level to a higher level to a higher level. level. The inclined The inclined plane makes it plane makes it easier to easier to move a move a weight from a weight from a lower to higher lower to higher elevation.elevation.

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Inclined PlaneInclined Plane While the inclined While the inclined

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

Longer Longer inclines=less force; inclines=less force; shorter shorter inclines=more forceinclines=more force

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Although it takes less force for car A to get to the top of the ramp, Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work.all the cars do 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 slope experienced by a team pulling a by a team pulling a heavily loaded heavily loaded wagon.wagon.

Examples: ramp, Examples: ramp, stairs, ladder, slide, stairs, ladder, slide, escalator, mountain escalator, mountain roadroad

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WedgeWedge The wedge is a The wedge is a

modification of the modification of the inclined plane. inclined plane. Wedges are used Wedges are used as either cutting, as either cutting, splitting, or splitting, or separating. separating.

A wedge can be a A wedge can be a single or double single or double incline plane incline plane placed together. placed together.

WedgeWedge

The longer and thinner a wedge The longer and thinner a wedge the greater the mechanical the greater the mechanical advantage.(longer distance)advantage.(longer distance)

Examples: knife, axe, plow, Examples: knife, axe, plow, chisel, tip of nail or pin, door stopchisel, tip of nail or pin, door stop

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ScrewScrew The screw is an The screw is an

incline plane incline plane wrapped around a wrapped around a cylinder. cylinder.

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

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MA of an screw can be calculated by dividing the number of turns per inch.

ScrewScrew

Screws are used to hold or fasten Screws are used to hold or fasten things together.things together.

The closer the threads are on a screw The closer the threads are on a screw the greater the mechanical the greater the mechanical advantage. (longer distance)advantage. (longer distance)

Examples: drill bit, bottle cap, screws, Examples: drill bit, bottle cap, screws, base of light bulb, jar lidsbase of light bulb, jar lids

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The LeverThe Lever

A lever is a rigid bar A lever is a rigid bar that rotates or pivots that rotates or pivots around a fixed point around a fixed point called the fulcrum.called the fulcrum.

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 effort (or applied) an effort (or applied) force and a load force and a load (resistant force) and a (resistant force) and a fulcrum.fulcrum.

LeverLever

Levers are used to lift objectsLevers are used to lift objects.. Levers increase your input force and Levers increase your input force and

change the direction of your input force.change the direction of your input force. Examples: scissors, pliers, branch cutters, Examples: scissors, pliers, branch cutters,

oars, crowbar, see saw, nut cracker, bottle oars, crowbar, see saw, nut cracker, bottle opener, wheel barrow, tweezers broom, opener, wheel barrow, tweezers broom, fishing polefishing pole

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The 3 Classes of LeversThe 3 Classes of Levers

The class of a lever The class of a lever is determined by is determined by the location of the the location of the effort force and the effort force and the load relative to the load relative to the fulcrum. fulcrum.

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To find the MA of a lever, divide the output force by the input force, or divide the length of the resistance arm by the length of the effort arm.

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First Class LeverFirst Class Lever

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

include crowbars, scissors, pliers, tin include crowbars, scissors, pliers, tin snips and seesaws. snips and 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 (load)Fulcrum is between EF (effort) and RF (load)Effort moves farther than Resistance.Effort moves farther than Resistance. Multiplies EF and changes its direction Multiplies EF and changes its direction

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Second Class LeverSecond Class Lever

With a second-class lever, With a second-class lever, the load is the load is located between the fulcrum and the effort located between the fulcrum and the effort force.force.

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

A second-class lever does not change the A second-class lever does not change the direction of force. When the fulcrum is direction of force. When the fulcrum is located closer to the load than to the effort located closer to the load than to the effort force, an increase in force (mechanical force, an increase in force (mechanical advantage) results. 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 directionMultiplies EF, but does not change its direction

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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, and shovels.tweezers, hammers, and shovels. A third-class lever does not change the A third-class lever does not change the

direction of force; third-class levers direction of force; third-class levers always produce a gain in speed and always produce a gain in speed and distance and a corresponding decrease distance and 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 travelsMultiplies the distance the effort force travels

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Wheel and AxleWheel and Axle The wheel and axle is The wheel and axle is

a simple machine a simple machine consisting of a consisting of a large large wheel rigidly secured wheel rigidly secured to a smaller wheel or to a smaller wheel or shaft, called an axle. shaft, called an axle.

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

Wheel and AxleWheel and Axle

When the larger wheel or handle When the larger wheel or handle turns the smaller wheel/axle turns.turns the smaller wheel/axle turns.

The wheel and axle increases your The wheel and axle increases your force, but you must exert your force force, but you must exert your force over a longer distance.over a longer distance.

Examples: door knob, steering wheel, Examples: door knob, steering wheel, screw driverscrew driver

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PulleyPulley A pulley consists A pulley consists

of a grooved of a grooved wheel with a wheel with a rope or chain rope or chain through the through the groove.groove.

Pulleys are used Pulleys are used to move heavy to move heavy objects over a objects over a distance. distance.

Types of PulleysTypes of Pulleys A pulley is said to be a A pulley is said to be a fixed pulleyfixed pulley if it does not if it does not

rise or fall with the load being moved. A fixed rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, pulley changes the direction of a force; however, it does not create a mechanical advantage. it does not create a mechanical advantage.

A A moveable pulleymoveable pulley rises and falls with the load rises and falls with the load that is being moved. A single moveable pulley that is being moved. A single moveable pulley creates a mechanical advantage; however, it creates a mechanical advantage; however, it does not change the direction of a force. does not change the direction of a force.

Pulley system or compound pulley- Pulley system or compound pulley- Fixed and a Fixed and a moveable pulley woarking together.moveable pulley woarking together.

The mechanical advntage of a moveable pulley is The mechanical advntage of a moveable pulley is equal to the number of ropes that support the equal to the number of ropes that support the moveable pulley. moveable pulley.

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PulleysPulleys

The mechanical advantage of a The mechanical advantage of a pulley is = to the number of rope pulley is = to the number of rope that support the object.that support the object.

Examples: flag pole, window Examples: flag pole, window blinds, elevatorblinds, elevator

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GearsGears

A device with toothed wheeled that fit into A device with toothed wheeled that fit into one another.one another.

Gears are used toGears are used to: multiply or reduce : multiply or reduce speed and force, change the direction of speed and force, change the direction of motion, transmit a force over a distancemotion, transmit a force over a distance

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GearsGears

Low gear ratio= more speed=less Low gear ratio= more speed=less forceforce

High gear ratio=less speed=more High gear ratio=less speed=more forceforce

Examples: car engines, clocks, Examples: car engines, clocks, VCR, power meters, bikesVCR, power meters, bikes

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EfficiencyEfficiency We said that the input force times the distance equals We said that the input force times the distance equals

the output force times distance, or:the output force times distance, or:

Input Force x Distance = Output ForceInput Force x Distance = Output Force x Distancex Distance

However, some output force is lost due to friction.However, some output force is lost due to friction.

The comparison of work input to work output is called The comparison of work input to work output is called efficiency. efficiency.

No machine has 100 percent efficiency due to friction. No machine has 100 percent efficiency due to friction.

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Practice QuestionsPractice Questions1. Explain who is doing more work and why: a bricklayer 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and being constructed, or a project supervisor observing and recording the progress of the workers from an observation recording the progress of the workers from an observation booth. booth.

2. How much work is done in pushing an object 7.0 m 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back across a floor with a force of 50 N and then pushing it back to its original position? to its original position?

3. Using a single fixed pulley, how heavy a load could you 3. Using a single fixed pulley, how heavy a load could you liftlift??

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Practice QuestionsPractice Questions1. Explain who is doing more work and why: a bricklayer carrying 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. progress of the workers from an observation booth. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work.

2. How much work is done in pushing an object 7.0 m across a 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original floor with a force of 50 N and then pushing it back to its original position? position? Work = 7 m X 50 N X 2 = 700 N-m or J

3. Using a single fixed pulley, how heavy a load could you lift?3. Using a single fixed pulley, how heavy a load could you lift?Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.