Science 1: Simple Machines

Big Idea

• Simple machines are the simplest objects that can be used to magnify and/or change the direction of a a force when doing work.

Demonstration

• Examples of the Six Simple Machines:

  1. Lever
  2. Wheel and Axle
  3. Pulley
  4. Inclined Plane
  5. Wedge
  6. Screw

Background

• You should know about forces, torque, and work to fully appreciate this lesson.

Main Explanation

• Simple machines are used to change the magnitude and direction of applied forces.

• We represent the change in magnitude of a force by the mechanical advantage.
  The mechanical advantage is the ratio of the load force that is moved to the effort force that is applied.

• We talk about two kinds of mechanical advantage: real mechanical advantage and ideal mechanical advantage.

• Real mechanical advantage is simply RMA = F_L/F_E, and is based on real measurements of a real simple machine.

• Ideal mechanical advantage uses the symbol IMA, and is calculated based on the ideal model of the machine. It is always a function of the dimensions of the machine.

• Ideal machines are massless, frictionless, and inelastic (in other words, they do not bend, stretch, twist, squish, or otherwise deform.

• The IMA can be calculated using one of three methods:

1. Use force or torque balance to calculate what F_L/F_E is in terms of the geometry of the machine.
2. Use energy conservation to calculate F_L/F_E.
3. If the forces remain parallel to the direction of displacement, then energy conservation says:
   W_in = W_out
   F_Ed_E = F_Ld_L
   F_L/F_E = d_E/d_L

• Efficiency is the ratio of the useful work you get out of a machine to the work you put into it:
  Efficieny = W_out / W_in * 100.

Related Topics

• Why real-world machines have less than ideal performance.

Links for further information