Work And Simple Machines

Work And Simple Machines - Transcript

Work and Simple Work Machines Machines
From Jefferson labs http www jlab org

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What is work
In science the word work has a different In work

meaning than you may be familiar with meaning The scientific definition of work is using a The force to move an object a distance when both the force and the motion of the object are in the same direction the

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Work or Not


According to the According scientific definition what is work and what is not is




a teacher lecturing to her teacher class class a mouse pushing a piece mouse of cheese with its nose across the floor across

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Work or Not


According to the According scientific definition what is work and what is not is




a teacher lecturing to her teacher class class a mouse pushing a piece mouse of cheese with its nose across the floor across

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What s work
A scientist delivers a speech to an audience of scientist

his peers A body builder lifts 350 pounds above his body head A mother carries her baby from room to room mother


A father pushes a baby in a carriage

A woman carries a 20 kg grocery bag to her woman

car
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What s work
A scientist delivers a speech to an audience of scientist

his peers No No A body builder lifts 350 pounds above his body head Yes Yes A mother carries her baby from room to room mother No No


A father pushes a baby in a carriage Yes father Yes

A woman carries a 20 km grocery bag to her woman

car No No
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Formula for work Work Force x Distance
The unit of force is newtons The unit of distance is meters The unit of work is newton meters One newton meter is equal to one joule So the unit of work is a So

joule joule
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W FD
Work Force x Work Distance Distance
Calculate If a man Calculate pushes a concrete block 10 meters with a force of 20 N how much work has he done done
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W FD
Work Force x Work Distance Distance
Calculate If a man Calculate pushes a concrete block 10 meters with a force of 20 N how much work has he done 200 joules 200 W 20N x 10m
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Power
Power is the rate at which work is done Power Power Work Time


force x distance

The unit of power is the watt

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Check for Understanding
1 Two physics students Ben and Bonnie are in the Two weightlifting room Bonnie lifts the 50 kg barbell over her head approximately 60 m 10 times in one minute Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds Which student does the most work Which student delivers the most power power Explain your answers Explain

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Ben and Bonnie do the Ben same amount of work they apply the same force to lift the same barbell the same distance above their heads Yet Ben is the most powerful since he does the same work in less time Power and time are inversely proportional inversely

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2 How much power will it take to move a 10 2 kg mass at an acceleration of 2 m s s a distance of 10 meters in 5 seconds This problem requires you to use the formulas for force work and power all in the correct order force
Force Mass x Acceleration Force Mass Work Force x Distance Work Force Power Work Time
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2 How much power will it take to move a 10 kg mass at an 2 acceleration of 2 m s s a distance of 10 meters in 5 seconds This problem requires you to use the formulas for force work and power all in the correct order and
Force Mass x Acceleration Force Mass Force 10 x 2 Force 10 Force 20 N Work Force x Distance Work 20 x 10 Work 200 Joules Power Work Time Power 200 5 Power 40 watts

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History of Work

Before engines and motors were invented people had to do Before things like lifting or pushing heavy loads by hand Using an animal could help but what they really needed were some clever ways to either make work easier or faster
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Simple Machines
Ancient people invented simple machines that Ancient would help them overcome resistive forces and allow them to do the desired work against those forces

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Simple Machines


The six simple machines are


Lever Lever Wheel and Axle Wheel Pulley Pulley Inclined Plane Inclined Wedge Wedge Screw Screw

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Simple Machines
A machine is a device that helps make work machine

easier to perform by accomplishing one or more of the following functions


transferring a force from one place to another transferring changing the direction of a force changing increasing the magnitude of a force or increasing increasing the distance or speed of a force increasing

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Mechanical Advantage
It is useful to think about a machine in terms It

of the input force the force you apply and input the output force force which is applied to the output force task task When a machine takes a small input force and When increases the magnitude of the output force a mechanical advantage has been produced mechanical

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Mechanical Advantage






Mechanical advantage is the ratio of output force divided by Mechanical input force If the output force is bigger than the input force a machine has a mechanical advantage greater than one machine If a machine increases an input force of 10 pounds to an output If force of 100 pounds the machine has a mechanical advantage MA of 10 In machines that increase distance instead of force the MA is In the ratio of the output distance and input distance the MA output input MA

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No machine can increase both the No both magnitude and the distance of a force at the same time force

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The Lever




A lever is a rigid bar that lever rotates around a fixed point called the fulcrum called The bar may be either The straight or curved straight In use a lever has both an In effort or applied force and a load resistant force

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The 3 Classes of Levers


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


In a first class lever the fulcrum is located at some point between the effort and resistance forces forces


Common examples of first class levers include crowbars scissors pliers tin snips and seesaws A first class lever always changes the direction of first class force I e a downward effort force on the lever results in an upward movement of the resistance force

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Fulcrum is between EF effort and RF load Effort moves farther than Resistance Multiplies EF and changes its direction

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Second Class Lever


With a second class lever the load is located between With the fulcrum and the effort force the Common examples of second class levers include nut Common crackers wheel barrows doors and bottle openers crackers A second class lever does not change the direction of second class force When the fulcrum is located closer to the load than to the effort force an increase in force mechanical advantage results

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RF load is between fulcrum and EF RF Effort moves farther than Resistance Effort Multiplies EF but does not change its direction Multiplies
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Third Class Lever
With a third class lever the effort force is With

applied between the fulcrum and the resistance force force


Examples of third class levers include tweezers Examples hammers and shovels hammers A third class lever does not change the direction of third class force third class levers always produce a gain in speed and distance and a corresponding decrease in force

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EF is between fulcrum and RF load EF Does not multiply force Resistance moves farther than Effort Resistance Multiplies the distance the effort force travels Multiplies
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Wheel and Axle




The wheel and axle is a The simple machine consisting of a large wheel rigidly secured to a smaller wheel or shaft called an axle When either the wheel or When axle turns the other part also turns One full revolution of either part causes one full revolution of the other part

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Pulley








A pulley consists of a grooved wheel that turns pulley freely in a frame called a block freely A pulley can be used to simply change the direction of a force or to gain a mechanical advantage depending on how the pulley is arranged A pulley is said to be a fixed pulley if it does not pulley fixed rise or fall with the load being moved A fixed pulley changes the direction of a force however it does not create a mechanical advantage A moveable pulley rises and falls with the load moveable that is being moved A single moveable pulley creates a mechanical advantage however it does not change the direction of a force The mechanical advantage of a moveable pulley The is equal to the number of ropes that support the moveable pulley

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Inclined Plane


An inclined plane is an An even sloping surface The inclined plane makes it easier to move a weight from a lower to higher elevation higher

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Inclined Plane




The mechanical advantage The of an inclined plane is equal to the length of the slope divided by the height of the inclined plane While the inclined plane While produces a mechanical advantage it does so by increasing the distance through which the force must move

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

A

B

C
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Inclined Plane


A wagon trail on a steep hill wagon will often traverse back and forth to reduce the slope experienced by a team pulling a heavily loaded wagon wagon This same technique is used This today in modern freeways which travel winding paths through steep mountain passes
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Wedge


The wedge is a modification of The the inclined plane Wedges are used as either separating or holding devices A wedge can either be composed wedge of one or two inclined planes A double wedge can be thought of as two inclined planes joined together with their sloping surfaces outward



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Screw




The screw is also a The modified version of the inclined plane While this may be While somewhat difficult to visualize it may help to think of the threads of the screw as a type of circular ramp or inclined plane
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MA of an screw can be calculated by dividing the number of turns per inch

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Efficiency


We said that the input force times the distance equals the output We force times distance or force Input Force x Distance Output Force x Distance Distance However some output force is lost due to friction



The comparison of work input to work output is called efficiency The No machine has 100 percent efficiency due to friction No

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

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 position How much power is used if this work is done in 20 sec power

3 Using a single fixed pulley how heavy a load could you lift

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Practice Questions
4 Give an example of a machine in which friction is both an advantage and a disadvantage advantage 5 Why is it not possible to have a machine with 100 5 efficiency efficiency

6 What is effort force What is work input Explain the relationship between effort force effort distance and work input
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Practice Questions
1 Explain who is doing more work and why a bricklayer carrying bricks and 1 placing them on the wall of a building being constructed or a project supervisor observing and recording the 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 floor with a force of 50 N and then pushing it back to its original position How much power is used if this work is done in 20 sec Work 7 m X 50 N X 2 700 N m or J Power 700 N m 20 sec 35 W 3 Using a single fixed pulley how heavy a load could you lift Since a fixed pulley 3 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

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Practice Questions
4 Give an example of a machine in which friction is both an advantage and a 4

disadvantage One answer might be the use of a car jack Advantage of friction It allows a car to be raised to a desired height without slipping Disadvantage of friction It reduces efficiency

5 Why is it not possible to have a machine with 100 efficiency Friction lowers the efficiency of a machine Work output is always less than work input so an actual machine cannot be 100 efficient

6 What is effort force What is work input Explain the relationship between effort force effort distance and work input The effort force is the force applied to a machine Work input is the work done on a machine The work input of a machine is equal to the effort force times the distance over which the effort force is exerted

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