Foundations of Physical Science

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Foundations of Physical Science

• Unit 2 Work and Energy

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Chapter 4 Machines and Mechanical Systems

• 4.1 Force and Machines
• 4.2 The Lever
• 4.3 Designing Gear Machines

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Learning Goals

• Describe and explain a simple machine.
• Apply the concepts of input force and output
  force to any machine.
• Determine the mechanical advantage of a machine.
• Construct and analyze a block and tackle machine.
• Describe the difference between science and
  engineering.
• Understand and apply the engineering cycle to the
  development of an invention or product.

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Learning Goals (continued)

• Describe the purpose and construction of a
  prototype.
• Design and analyze a lever.
• Calculate the mechanical advantage of a lever.
• Recognize the three classes of levers.
• Build machines with gears and deduce the rule for
  how pairs of gears turn.
• Design and build a gear machine that solves a
  specific problem.

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Vocabulary

• engineering gear
• engineering cycle
• engineers
• force
• fulcrum
• gear
• input
• input arm
• input force
• input gear

• lever
• machine
• mechanical advantage
• mechanical systems
• output
• output arm
• output force
• output gear
• prototype
• simple machine

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4.1 Forces in Machines
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Mechanical Systems and Machines

• The world without machines
• Technology of today
• Sowhat is a machine?

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Machine

• A device with moving parts that work together to
  accomplish a task.
• A device for multiplying forces or simply
  changing the direction of forces
• All machines employ the conservation
  
  of energy
• A bicycle is a good example.
• Input everything you do to
  
  make the machine work, like
  
  pushing on the pedals
• Output what the machine does
  
  for you, like going fast

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Simple Machines

• An unpowered mechanical device, such as a
• Lever
• Wheel and axle
• Block and tackle
• Gear
• Ramp

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Simple Machines Input and Output

• Lever
• Input force what you apply
• Output force what the lever applies to what you
  are trying to move
• Block and Tackle
• Input force what you apply to the rope
• Output force what gets applied to the load you
  are trying to lift
• Most machines we use today are made up of
  combinations of different simple machines

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Mechanical Advantage

• The ratio of output force to input force
• If the mechanical advantage is gt 1, the output
  force is greater than the input force
• If the mechanical advantage is lt 1, the output
  force is smaller than the input force
• Mechanical engineers people who design machines

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Mechanical Advantage
Output force (N)
MA Fo Fi
mechanical advantage
Input force (N)
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How a Block and Tackle Works

• The forces in ropes and strings
• Ropes and strings carry tension forces along
  their lengtha pulling force (not a pushing
  force!)
• Every part of a rope has the same tension
• If friction is very small, the force in a rope is
  the same everywhere
• The forces in a block and tackle
• More rope, easier to pull (see diagram slide)

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How a Block and Tackle Works

• Mechanical advantage
• More ropes, more output force than input
  forceeasier to lift!
• Multiplying force with the block and tackle
• Input force can be much less with more ropes
• If the mechanical advantage is 4, the input force
  for the machines is ¼ the output force

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4.2 The Lever
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GIVE ME A PLACE TO STAND AND I WILL MOVE THE
EARTH

• Archimedes
• Greek scientist
• 3rd century BC

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What is a lever?

• Another simple machine
• Pliers, wheelbarrow, human biceps, forearm
• Your bones and muscles work as levers to perform
  everything from chewing to throwing a ball

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What is a lever?

• A stiff structure that rotates around a fixed
  point called the fulcrum
• We can arrange the fulcrum and the lengths of the
  input and output arms to make almost any
  mechanical advantage we need

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How a Lever Works

• Fulcrum in the middle input and output forces
  are the same
• Input arm is longer output force is larger than
  the input force
• Input arm is 10x longer than the output arm, the
  output force will be 10x bigger than the input
  force
• Input arm is shorter output force is smaller
  than the input force
• Input arm is 10x shorter than the output arm, the
  output force will be 10x less than the input
  force

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4.3 Designing Gear Machines
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Science and Engineering

• Engineering/Technology The application of
  science to solve problems
• Scientists study the world to learn the basic
  principles behind how things work
• Engineers use scientific knowledge to create or
  improve inventions that solve problems

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Sample Engineering Problem

• Conceptual design
• Prototype
• Testing the prototype
• Changing the design and testing again

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Gears and Rotating Machines

• Many machines require that rotating motion be
  transmitted from one place to another.
• Gears change force and speed.
• Gears are better than wheels because they have
  teeth and dont slip as they turn together.

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What is the Gear Ratio?

• Gears have input and output
• Input gear the one you turn, or apply forces to
• Output gear the one that is connected to the
  output of the machine
• Gear ratio the ration of output turns to input
  turns
• Smaller gears turn faster the gear ratio is the
  inverse of the ratio of teeth in two gears

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Gear Ratios
Number of teeth on input gear
To Ni Ti No
Turns of output gear
Turns of input gear
Number of teeth on output gear
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Designing Machines

• Machines are designed to do specific things
• Simple machines can be combined to solve more
  complex problems
• Two pairs of gears with a 21 ratio can be
  combined to make a machine with a ratio of 41

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Designing Machines

• Design involves trade offs
• Even the best designs are always being improved