Editing Lever (section)

== Types of levers ==
[[File:Lever (PSF).png|thumb|right|Three classes of levers]]
[[File:Levers of the Human Body.svg|thumb|The three classifications of levers with examples of the human body]]
Levers are classified by the relative positions of the fulcrum, effort, and resistance (or load). It is common to call the input force "effort" and the output force "load" or "resistance". This allows the identification of three classes of levers by the relative locations of the fulcrum, the resistance and the effort:<ref>{{cite book
  |title=Physics in Biology and Medicine |edition=3rd
  |first1=Paul
  |last1=Davidovits
  |publisher=Academic Press
  |year=2008
  |isbn=978-0-12-369411-9
  |page=10
  |chapter-url=https://books.google.com/books?id=e9hbt3xisb0C&pg=PA10
  |chapter=Chapter 1
  |access-date=2016-02-23
  |archive-date=2014-01-03
  |archive-url=https://web.archive.org/web/20140103072006/http://books.google.com/books?id=e9hbt3xisb0C&pg=PA10
  |url-status=live
  }}</ref>
*{{anchor|Class 1 lever}} '''Class I''' – Fulcrum is located between the effort and the resistance: The effort is applied on one side of the fulcrum and the resistance (or load) on the other side. For example, a [[seesaw]], a [[Crowbar (tool)|crowbar]], a pair of [[scissors]], a [[balance scale]], a pair of [[pliers]], and a [[claw hammer]] (pulling a nail). With the fulcrum in the middle, the lever's mechanical advantage may be greater than, less than, or even equal to 1.
*{{anchor|Class 2 lever}} '''Class II''' – Resistance (or load) is located between the effort and the fulcrum: The effort is applied on one side of the resistance and the fulcrum is located on the other side, e.g. a [[wheelbarrow]], a [[nutcracker]], a [[bottle opener]], a [[wrench]], a pair of [[bellows]], and the [[brake]] [[Automobile pedal|pedal]] of a car. Since the load arm is smaller than the effort arm, the lever's mechanical advantage is always greater than 1. It is also called a force multiplier lever.
*{{anchor|Class 3 lever}} '''Class III''' – Effort is located between the resistance and the fulcrum: The resistance (or load) is applied on one side of the effort and the fulcrum is located on the other side, e.g. a [[Hoe (tool)|hoe]], a pair of [[tweezers]], a [[hammer]], a pair of [[tongs]], a [[fishing rod]], and the [[mandible]] of a human skull. Since the effort arm is smaller than the load arm, the lever's mechanical advantage is always less than 1. It is also called a speed multiplier lever.

These cases are described by the mnemonic ''fre 123'' where the ''f'' fulcrum is between ''r'' and ''e'' for the 1st class lever, the ''r'' resistance is between ''f'' and ''e'' for the 2nd class lever, and the ''e'' effort is between ''f'' and ''r'' for the 3rd class lever.