Lever

This article is about the simple machine. For other uses, see Lever (disambiguation).

Lever
Levers can be used to exert a large force over a small distance at one end by exerting only a small force (effort) over a greater distance at the other.
Classification	Simple machine
Components	fulcrum or pivot, load and effort
Examples	see-saw, bottle opener, etc.

A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or fulcrum. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is divided into three types. It is one of the six simple machines identified by Renaissance scientists. A lever amplifies an input force to provide a greater output force, which is said to provide leverage, which is mechanical advantage gained in the system, equal to the ratio of the output force to the input force. As such, the lever is a mechanical advantage device, trading off force against movement.

Etymology

The word "lever" entered English around 1300 from Old French: levier. This sprang from the stem of the verb lever, meaning "to raise". The verb, in turn, goes back to Latin: levare,^[1] itself from the adjective levis, meaning "light" (as in "not heavy"). The word's primary origin is the Proto-Indo-European stem leg^wh-, meaning "light", "easy" or "nimble", among other things. The PIE stem also gave rise to the English word "light".^[2]

History of the lever

The earliest evidence of the lever mechanism dates back to the ancient Near East c. 5000 BC, when it was first used in a simple balance scale.^[3] In ancient Egypt c. 4400 BC, a foot pedal was used for the earliest horizontal frame loom.^[4] In Mesopotamia (modern Iraq) c. 3000 BC, the shadouf, a crane-like device that uses a lever mechanism, was invented.^[3] In ancient Egypt, workmen used the lever to move and uplift obelisks weighing more than 100 tons. This is evident from the recesses in the large blocks and the handling bosses which could not be used for any purpose other than for levers.^[5]

The earliest remaining writings regarding levers date from the 3rd century BC and were provided, by common belief, by the Greek mathematician Archimedes, who famously stated "Give me a lever long enough and a fulcrum on which to place it, and I shall move the world."

Autumn Stanley argues that the digging stick can be considered the first lever, which would position prehistoric women as the inventors of lever technology.^[6]

Force and levers

A lever in balance

A lever is a beam connected to ground by a hinge, or pivot, called a fulcrum. The ideal lever does not dissipate or store energy, which means there is no friction in the hinge or bending in the beam. In this case, the power into the lever equals the power out, and the ratio of output to input force is given by the ratio of the distances from the fulcrum to the points of application of these forces. This is known as the law of the lever.

The mechanical advantage of a lever can be determined by considering the balance of moments or torque, T, about the fulcrum. If the distance traveled is greater, then the output force is lessened.

$${\displaystyle {\begin{aligned}T_{1}&=F_{1}a,\quad \\T_{2}&=F_{2}b\!\end{aligned}}}$$

where F₁ is the input force to the lever and F₂ is the output force. The distances a and b are the perpendicular distances between the forces and the fulcrum.

Since the moments of torque must be balanced, ${\displaystyle T_{1}=T_{2}\!}$. So, ${\displaystyle F_{1}a=F_{2}b\!}$.

The mechanical advantage of a lever is the ratio of output force to input force.

$${\displaystyle MA={\frac {F_{2}}{F_{1}}}={\frac {a}{b}}.\!}$$

This relationship shows that the mechanical advantage can be computed from ratio of the distances from the fulcrum to where the input and output forces are applied to the lever, assuming a weightless lever and no losses due to friction, flexibility or wear. This remains true even though the "horizontal" distance (perpendicular to the pull of gravity) of both a and b change (diminish) as the lever changes to any position away from the horizontal.

Types of levers

Three classes of levers

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:^[7]

• 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, 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.
• 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, and the brake 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.
• 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 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.

Compound lever

Main article: Compound lever

A compound lever comprises several levers acting in series: the resistance from one lever in a system of levers acts as effort for the next, and thus the applied force is transferred from one lever to the next. Examples of compound levers include scales, nail clippers and piano keys.

The malleus, incus and stapes are small bones in the middle ear, connected as compound levers, that transfer sound waves from the eardrum to the oval window of the cochlea.

Law of the lever

See also: Mechanical advantage § Lever

The lever is a movable bar that pivots on a fulcrum attached to a fixed point. The lever operates by applying forces at different distances from the fulcrum, or a pivot.

As the lever rotates around the fulcrum, points further from this pivot move faster than points closer to the pivot. Therefore, a force applied to a point further from the pivot must be less than the force located at a point closer in, because power is the product of force and velocity.^[8]

If a and b are distances from the fulcrum to points A and B and the force F_A applied to A is the input and the force F_B applied at B is the output, the ratio of the velocities of points A and B is given by a/b, so we have the ratio of the output force to the input force, or mechanical advantage, is given by: $${\displaystyle MA={\frac {F_{B}}{F_{A}}}={\frac {a}{b}}.}$$

This is the law of the lever, which was proven by Archimedes using geometric reasoning.^[9] It shows that if the distance a from the fulcrum to where the input force is applied (point A) is greater than the distance b from fulcrum to where the output force is applied (point B), then the lever amplifies the input force. On the other hand, if the distance a from the fulcrum to the input force is less than the distance b from the fulcrum to the output force, then the lever reduces the input force.

The use of velocity in the static analysis of a lever is an application of the principle of virtual work.

Virtual work and the law of the lever

A lever is modeled as a rigid bar connected to a ground frame by a hinged joint called a fulcrum. The lever is operated by applying an input force F_A at a point A located by the coordinate vector r_A on the bar. The lever then exerts an output force F_B at the point B located by r_B. The rotation of the lever about the fulcrum P is defined by the rotation angle θ in radians.

Archimedes lever, Engraving from Mechanics Magazine, published in London in 1824

Let the coordinate vector of the point P that defines the fulcrum be r_P, and introduce the lengths

$${\displaystyle a=|\mathbf {r} _{A}-\mathbf {r} _{P}|,\quad b=|\mathbf {r} _{B}-\mathbf {r} _{P}|,}$$

which are the distances from the fulcrum to the input point A and to the output point B, respectively.

Now introduce the unit vectors e_A and e_B from the fulcrum to the point A and B, so

$${\displaystyle \mathbf {r} _{A}-\mathbf {r} _{P}=a\mathbf {e} _{A},\quad \mathbf {r} _{B}-\mathbf {r} _{P}=b\mathbf {e} _{B}.}$$

The velocity of the points A and B are obtained as

$${\displaystyle \mathbf {v} _{A}={\dot {\theta }}a\mathbf {e} _{A}^{\perp },\quad \mathbf {v} _{B}={\dot {\theta }}b\mathbf {e} _{B}^{\perp },}$$

where e_A^⊥ and e_B^⊥ are unit vectors perpendicular to e_A and e_B, respectively.

The angle θ is the generalized coordinate that defines the configuration of the lever, and the generalized force associated with this coordinate is given by

$${\displaystyle F_{\theta }=\mathbf {F} _{A}\cdot {\frac {\partial \mathbf {v} _{A}}{\partial {\dot {\theta }}}}-\mathbf {F} _{B}\cdot {\frac {\partial \mathbf {v} _{B}}{\partial {\dot {\theta }}}}=a(\mathbf {F} _{A}\cdot \mathbf {e} _{A}^{\perp })-b(\mathbf {F} _{B}\cdot \mathbf {e} _{B}^{\perp })=aF_{A}-bF_{B},}$$

where F_A and F_B are components of the forces that are perpendicular to the radial segments PA and PB. The principle of virtual work states that at equilibrium the generalized force is zero, that is

$${\displaystyle F_{\theta }=aF_{A}-bF_{B}=0.\,\!}$$

Simple lever, fulcrum and vertical posts

Thus, the ratio of the output force F_B to the input force F_A is obtained as

$${\displaystyle MA={\frac {F_{B}}{F_{A}}}={\frac {a}{b}},}$$

which is the mechanical advantage of the lever.

This equation shows that if the distance a from the fulcrum to the point A where the input force is applied is greater than the distance b from fulcrum to the point B where the output force is applied, then the lever amplifies the input force. If the opposite is true that the distance from the fulcrum to the input point A is less than from the fulcrum to the output point B, then the lever reduces the magnitude of the input force.

See also

• Applied mechanics – Practical application of mechanics
• Balance lever coupling
• bascule
• Linkage (mechanical) – Assembly of systems connected to manage forces and movement
• Mechanism (engineering) – Device which converts input forces and motion to output forces and motion
• On the Equilibrium of Planes – Mechanical treatise by Archimedes
• Simple machine – Mechanical device that changes the direction or magnitude of a force

References

1. Chisholm, Hugh, ed. (1911). "Lever" . Encyclopædia Britannica. Vol. 16 (11th ed.). Cambridge University Press. p. 510.
2. "Etymology of the word "lever" in the Online Etymological". Archived from the original on 2015-05-12. Retrieved 2015-04-29.
3. Paipetis, S. A.; Ceccarelli, Marco (2010). The Genius of Archimedes -- 23 Centuries of Influence on Mathematics, Science and Engineering: Proceedings of an International Conference held at Syracuse, Italy, June 8-10, 2010. Springer Science & Business Media. p. 416. ISBN 9789048190911.
4. Bruno, Leonard C.; Olendorf, Donna (1997). Science and technology firsts. Gale Research. p. 2. ISBN 9780787602567. 4400 B.C. Earliest evidence of the use of a horizontal loom is its depiction on a pottery dish found in Egypt and dated to this time. These first true frame looms are equipped with foot pedals to lift the warp threads, leaving the weaver's hands free to pass and beat the weft thread.
5. Clarke, Somers; Engelbach, Reginald (1990). Ancient Egyptian Construction and Architecture. Courier Corporation. pp. 86–90. ISBN 9780486264851.
6. Stanley, Autumn (1983). ""Women Hold Up Two-Thirds of the Sky: Notes for a Revised History of Technology."". In Rothschild, Joan (ed.). Machina Ex Dea: Feminist Perspectives on Technology. Pergamon Press.
7. Davidovits, Paul (2008). "Chapter 1". Physics in Biology and Medicine (3rd ed.). Academic Press. p. 10. ISBN 978-0-12-369411-9. Archived from the original on 2014-01-03. Retrieved 2016-02-23.
8. Uicker, John; Pennock, Gordon; Shigley, Joseph (2010). Theory of Machines and Mechanisms (4th ed.). Oxford University Press USA. ISBN 978-0-19-537123-9.
9. Usher, A. P. (1929). A History of Mechanical Inventions. Harvard University Press (reprinted by Dover Publications 1988). p. 94. ISBN 978-0-486-14359-0. OCLC 514178. Archived from the original on 26 July 2020. Retrieved 7 April 2013.

External links

• Lever at Diracdelta science and engineering encyclopedia
• A Simple Lever by Stephen Wolfram, Wolfram Demonstrations Project.
• Levers: Simple Machines at EnchantedLearning.com