Chapter 8 Work and Machines

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Chapter 8Work and Machines

• Section 1
• Work and Power

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To Work or Not to Work

• If you had a choice what would you prefer to do,
  work or not work?
• Give some examples of work.

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The Scientific Meaning of Work

• Work occurs when a force causes an object to move
  in the direction of the force.
• The object must move for work to occur!

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Force and Motion in the Same Direction

• Youll know work is being done on an object if
  two things occur
• 1. the object moves as the force is applied
• 2. the direction of the objects motion is the
  same as the direction of the force applied.

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Force and Motion in the Same Direction
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How do we calculate work?

• Work force x distance
• Force expressed in Newtons
• Distance is expressed in meters
• So the expression for work is the Newton meter
• or simply the joule
• Work Newtons x meters Joules

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Practice

• A man applies a force of 500N to push a truck
  100m down the street. How much work does he do?
• In which situation do you do more work?
• a. You lift a 75 N bowling ball 2m off the
  floor.
• b. You lift two 50 N bowling balls 1m off
  the floor

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Power-How Fast Work Is Done

• Power is the rate at which work is done
• To calculate power (P) divide the amount of work
  done (W) by the time (t) it takes to do that
  work or
• P W/ t
• Unit for work is joule
• Unit for time is second
• So the unit for power is joules / second or J/s
• Or more commonly called a watt (W)

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Chapter 8Work and Machines

• Section 2
• What is a Machine?

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Machines - Making Work Easier

• A machine is something that makes work easier by
  changing the size or direction of a force

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Machines - Making Work Easier

• Work input the work you do on a machine
• Work output the work done by the machine
• Output force the force that the machine applies
  that opposes the forces of you and the machine
• Machines do not increase the amount of work that
  is done because work output can never be more
  than work input

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Machines - Making Work Easier

• Machines make work easier because they change the
  size or direction of the input force
• If force decreases then distance must increase
  and vice versa

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Mechanical Advantage

• A machines mechanical advantage tells how many
  times a machine multiplies forceit compares the
  input force to the output force
• Mechanical Advantage (MA) output force
• input force

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Mechanical Advantage
The larger the mechanical advantage, the easier
the machine makes your work!
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Mechanical Advantage

1. The output force is the same as the input force
   if the machine changes direction of a force and
   the MA is 1.
2. The input force is greater than the output force
   allows lower force over a longer distance and has
   a MA less than 1.
3. The output force is greater than the input force
   gives a MA of greater than 1.

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Mechanical Efficiency
While output cannot be greater than input the
input is always greater than the output because
some of the work done by the machine is to
overcome gravity. The less work a machine has to
do to overcome friction the more efficient it
is. Mechanical Efficiency work output x
100 work input
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Mechanical Efficiency
Mechanical Efficiency work output x
100 work input Mechanical efficiency tells
what percentage of the work input gets converted
into work output. So reducing friction in a
machine makes it more efficient.
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Chapter 8Work and Machines

• Section 3
• Types Of Machines

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Types of Machines Levers

• Lever a simple machine consisting of a bar that
  pivots at a fixed point called a fulcrum
• There are three classes of levers
• First class levers
• Second class levers
• Third class levers

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Types of Machines Levers

• First class levers always change the direction
  of the input forces

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Types of Machines Levers

• Second class levers do not change the direction
  of the input forces output force is greater than
  the input force

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Types of Machines Levers

• Third class levers do not change the direction
  of the input forces and do not increase the
  input force

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Types of Machines Inclined Planes

• An inclined plane is a simple machine that is a
  straight, slanted surface
• A ramp is an example of an incline plane
• MA of an inclined plane

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Types of Machines Wedges

• A wedge is a double inclined plane that moves
• A wedge applies an output force that is greater
  than the input force
• MA of wedges
• The longer and
• thinner the
• wedge, the better.

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Types of Machines Screws

• A screw is an inclined plane that is wrapped in a
  spiral
• MA of screws same advantage as a long inclined
  plane

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Types of Machines Wheel and Axle

• A wheel and axle is a simple machine consisting
  of two circular objects of different sizes

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Types of Machines Wheel and Axle

• The MA of this machine can be determined by
  dividing the radius of the wheel by the radius of
  the axle

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Types of Machines Pulleys

• A pulley is a simple machine consisting of a
  grooved wheel that holds a rope or a cable. A
  load is attached to one end of the rope and an
  input force is applied to the other end.
• There are two types of pulleys
• 1) fixed
• 2) movable

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Types of Machines Pulleys
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Types of Machines Pulleys
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Compound Machines

• Compound machines are made of two or more simple
  machines.
• The block and tackle we just talked about is a
  compound machine

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Compound Machines
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Mechanical Efficiency of Compound Machines
Generally, the more moving parts a machine has,
the lower its mechanical efficiency What would
happen if the mechanical efficiency of a car was
too high?