Chapter 9 - Simple Machines

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Chapter 9 - Simple Machines

• 9.2 Mechanical Advantage
• pp. 210-216

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9.2 Mechanical advantage

• Mechanical advantage is the ratio of output force
  to input force.

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9.2 Mechanical advantage

• Machines multiply forces.
• One person could lift an elephantquite a heavy
  loadwith a properly designed system of ropes and
  pulleys!

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9.2 Simple Machines and Mechanical Advantage

• Simple machines are all about using mechanical
  advantage to make what ordinarily would be a
  complex task into a much simpler task.

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9.1 Mechanical Advantage
Output force (N)
MA Fo Fi
mechanical advantage
Input force (N)
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9.2 The Lever

• A lever includes a stiff structure (the lever)
  that rotates around a fixed point called the
  fulcrum.

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9.2 The Lever

• Levers are useful because you can arrange the
  fulcrum and the input arm and output arm to
  adjust the mechanical advantage of the lever.

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9.2 The Lever

• Each class of levers is defined by the location
  of the input and output forces relative to the
  fulcrum.

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9.2 Gears

• Many machines require that rotating motion be
  transmitted from one place to another.
• Gears change force and speed.

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9.2 Designing Gear Machines

• The gear ratio is the ratio of output turns to
  input turns.
• You can predict how force and speed are affected
  when gears turn by knowing the number of teeth
  for each gear.

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9.2 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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9.2 Tension

• Ropes and strings carry tension forces along
  their length.
• If the rope is not moving, its tension is equal
  to the force pulling on each end.

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9.2 Ramps

• A ramp is a simple machine that allows you to
  raise a heavy object with less force than you
  would need to lift it straight up.

• The mechanical advantage of a ramp is the ramp
  length divided by the height of the ramp.

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9.2 Screws

• A screw is a rotating ramp.
• You find the mechanical advantage of a screw by
  dividing its circumference by the lead.

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9.2 Wedge

• A wedge is like a ramp that can work while in
  motion (a ramp is always stationary).
• A wedge has a side that slopes down to a thin
  edge.
• The mechanical advantage for a wedge is inversely
  related to the size of the wedge angle.

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9.2 Wheel and axle

• A wheel rotates around a rod called an axle.
• The mechanical advantage is the ratio of the
  radius of the wheel to the radius of the axle.
• The wheel and axle move together to move or lift
  loads.

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Solving Problems

• A crowbar is a type of lever that you use to pull
  a nail out of a piece of wood.

• If the handle of a crowbar is 40 centimeters and
  the foot is 2 centimeters, what is its mechanical
  advantage?

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Solving Problems

• Looking for
• mechanical advantage of lever
• Given
• input arm 40 cm output arm 2 cm
• Relationships
• M.A. Length of input arm
• Length of output arm
• Solution
• M.A. 40 cm 2 cm 20

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MECHANICAL ADVANTAGE

• Complete Your Turn problems on p. 215
• Complete Skill Sheet