Chapter 12: Work and Machines

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• Chapter 12 Work and Machines

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What is Work ?

• Force acting over a distance
• Motion and Direction
• For work, the object must move in the direction
  of the applied force
• Examples of WORK Pushing a grocery cart, picking
  up a backpack, throwing a football.
• Is this WORK? Pushing against a wall.
• Why?

NO
Object was not moved by the force.
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Calculation of Work

• Work Force x Distance
• Units Joules Newtons meters
• (J) (N) (m)
• A high jumper weighs 700 N. What work does he do
  when he jumps over a bar 2 m high?
• W F x D
• W 700 N x 2 m
• W 1400 joules

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What is a Machine?

• A device that can be used to do work in an easier
  way or more efficient way
• Input Force force you exert on a machine
• Output Force force exerted by the machine

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How Machines Make Work Easier

• 1. Reduce effort force
• Some machines reduce the amount of effort needed
  to move something. They do this by multiplying
  your effort force.
• The tradeoff you have to apply the smaller
  effort force over a longer distance in order to
  do the same amount of work.
• Ex. Lifting a box vs. pushing it up a ramp

Small Force, Long Distance
Large Force, Short Distance
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How Machines Make Work Easier

• 2. Multiply distance
• Some machines allow you to move one part of it a
  short distance, causing another part of the
  machine to move a greater distance.
• The tradeoff You must use a lot of effort force
  to do the work.

Move the broom a small distance here
The broom moves a longer distance across the floor
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How Machines Make Work Easier

• 3. Change Direction of the Object
• Some machines do not reduce effort force or
  multiply distance. Instead, they change the
  direction of the object
• Ex. pull down, and an object moves up
• This can make work easier, because it may be
  easier to pull down than it is to pull up

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What is Mechanical Advantage?

• Mechanical advantage (MA) refers to how much a
  machine multiplies your effort force (also called
  input force)
• MA output force (force the machine applies)
• input force (force you apply)
• Example A bar is used to lift a 400 N rock with
  a force of 200 N. What is the mechanical
  advantage?
• MA

400 N 200 N
Output force Input force
MA 2
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Mechanical Advantage in each situation

• Reducing effort force
• MA is greater than 1
• Output force is greater than input force
• The machine multiplies your input force (or
  reduces effort)
• Multiplying distance
• MA is less than 1
• Input force is greater than output force
• The machine exerts your input force over a larger
  distance
• Changing direction
• MA is equal to 1
• Output force and input force are equal
• It is easier to exert input force from a
  different direction

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Actual vs. Ideal Mechanical Advantage

• Ideal MA the MA provided by a machine without
  taking work done to overcome friction
  (theoretical)
• Actual MA the MA provided by a machine taking
  friction into consideration

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What is Efficiency?

• Efficiency is the comparison of output work to
  input work
• It is calculated as a percent ()
• The efficiency can never be greater than 100
  because work output cannot be greater than work
  input
• No machine is 100 efficientsome of the work the
  machine does is used to overcome friction.
• Because of friction, work output is always less
  than work input.

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Calculation of Efficiency

• Efficiency can be calculated using the following
• Efficiency
• Example What is the efficiency of a machine
  where work input is 200 joules, and work output
  is 100 joules ?
• Efficiency

Output work Input work
X 100
100 joules 200 joules
X 100
.5 X 100
50
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What are the 6 Simple Machines?

• 1. Inclined Plane
• An inclined plane is a flat surface set at an
  angle
• It takes less force to raise the object, but the
  object must move through a longer distance
• As the slant of an inclined plane decreases, the
  mechanical advantage increases

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• 2. Wedge
• A wedge is two inclined planes joined together
  back to back
• A wedge reduces effort force
• The longer and thinner a wedge is, the less
  effort is needed, and the more MA it has
• A sharp knife cuts easier than a dull one!

Less effort force needed more MA
More effort force needed less MA
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• 3. Screw
• An inclined plane wrapped around a cylinder
• The closer together the threads of a screw are,
  the longer the distance over which the effort is
  exerted, and less effort force needed
• Closer the threads, greater the MA

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• 4. Lever
• A rigid bar that can pivot or rotate about a
  fixed point
• This fixed point is called a fulcrum
• There are 3 classes of levers
• First Class Lever
• Reduces effort force
• MA gt 1
• Ex. seesaw, hammer claws
• used to pull out nails

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• Second Class Lever
• Also reduces effort force
• MA gt 1
• Ex. bottle opener, nut cracker, wheel barrow
• Third Class Lever
• Multiplies distance
• MA is less than 1
• Ex. fishing rod, broom, tweezers (two third class
  levers)

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• Common Examples of the Classes of Levers

(Two second class levers)
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• 5. Wheel and Axle
• A wheel connected to a rigid pole (called an
  axle)
• The wheel and axle are attached and spin together
• Reduces effort force, much like a lever
• A small effort force is used to turn a large
  wheel a great distance, which causes the smaller
  axle to turn a shorter distance but with much
  force
• Ex. door knobs, screw driver, steering wheel

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• 6. Pulley
• A pulley is a rope, belt, or chain wrapped around
  a grooved wheel
• Three Types of Pulleys
• A. Fixed Pulley
• This pulley changes the direction that the object
  moves, but does not reduce effort force
• MA 1
• To lift a 120 N object, you would need to use 120
  N of effort force

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• B. Movable Pulley
• Pulley is attached to the object
• Reduces effort force needed, but you need to reel
  in more rope (pull for a longer distance) to move
  the object
• MA gt 1
• To lift a 120 N object, you would need less than
  120 N of effort force

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• C. Pulley System
• Combination of fixed and movable pulleys (also
  called block and tackle)
• Reduces effort force even more, because it uses
  more rope (effort force spread over longer
  distance)
• Also changes the direction of the object
• MA gt 1
• The MA is to the number of supporting ropes in
  the pulley system