Modern Technology uses Compound Machines

1
Modern Technology uses Compound Machines

• What simple machines do you recognize in a tow
  truck?
• A machine that is made up of two or more simple
  machines is a compound machine.
• How are simple machines related to compound
  machines?

2
Gears

• Gears are based on the wheel and axle.
• The gear that turns another gear applies the
  input force and the gear that is turned applies
  the output force.

3
Mechanical Advantage of Compound Machines

• The mechanical advantage of any compound machine
  is equal to the product of the mechanical
  advantage of all the simple machines that make up
  the compound machine.
• For example , the ideal machine advantage of a
  pair of scissors would be the product of the
  ideal mechanical advantage of its two levers and
  two wedges.

4
Compound Machines and Friction

• Compound machines typically must overcome more
  friction than simple machines, because they have
  more moving parts.
• Scissors for example have a lower efficiency than
  one lever because they have more moving parts.

5
Mechanical Advantage

• When a machine helps you work, there is an
  advantage to them. The number of times a machine
  multiples the input force is called the machines
  mechanical advantage.
• To find a machines mechanical advantage, divide
  the output force by the input force.
• Mechanical advantage Output force/Input force

6
Mechanical advantage of a machine

• For machines such as a rake the output force is
  less than the input force.
• If the input force of the rake is 10 newtons and
  the output force is 5 Newtons mechanical
  advantage is .5

Input force
Output force
7
Output Work is always less than input work

• The work you do on a machine is called the input
  work, and the work the machine does in turn is
  called the output work.
• A machines efficiency is the ratio of its output
  work to the input work.
• An ideal machine would be 100 efficient.
• Actual machines lose input work to friction.

8
Calculating Efficiency

• You can calculate a machines efficiency by
  dividing the machines output work by its input
  work and multiplying that number by 100.
• Efficiency () Output work/Input work x 100

9
Calculating Efficiency

• Work is measure in Joules, suppose you do 600 J
  of work using a rope system to lift a box. The
  work done is 540J.
• Efficiency 540 J/600 J x 10090

10
Increasing Efficiency

• All machines lose input force to friction, one
  way to improve efficiency is by reducing
  friction.
• Oil is used to reduce friction between the moving
  parts of a car engine and a bicycle.