Simple Tools

A lever is a mechanism that can be used to exert a large force over a small distance at one end of the lever by exerting a small force over a greater distance at the other end of the lever.

levers first-class

In general the effort force can be expressed as

Fe = Fl dl / de         (1)
where
Fe = effort force (N, lb)
Fl = load force (N, lb) (note that weight is a force)
dl = distance from load force to fulcrum (m, ft)
de = distance from effort force to fulcrum (m, ft)

Orders of Levers

First-order levers

  • the fulcrum is positioned between the effort and the load
  • the effort is smaller than the load
  • the effort moves further than the load
  • the lever can be considered as a force magnifier

Second-order Levers

  • the effort and the load are postioned are positioned on the same size of the fulcrum but applied in oposite directions
  • the load lies between the effort and the fulcrum
  • the effort is smaller than the load
  • the effort moves further than the load
  • the lever can be considered as a force magnifier

Third-order Levers

  • the effort lies between the load and the fulcrum
  • the effort is greater than the load
  • the load moves further than the effort
  • the lever can be considered as a distance magnifier

Example – First-Class (Order) Lever – A force (weight) of 1 pound is exerted at the end of a lever at distance 1 ft from the fulcrum

The effort force at a distance of 2 ft from the fulcrum can be calculated as

Fe = (1 lb) (1 ft) / (2 ft)
    = 0.5 (lb)

The formula (1) can be modified to express required load if you know the effort, or required distance from fulcrum if load and effort forces are known and so on.
The level above where the fulcrum located between the load and effort force is often characterized as a first-class level mechanism.
A level where the load and effort force are located on the same side of the fulcrum is often characterized as a second-class level mechanism.

levers second-class

Example – Second-Class (Order) Lever

A force (weight) of 1 pound is exerted at a distance of 1 ft from the fulcrum.
The effort force at a distance of 2 ft from the fulcrum can be calculated as

Fe = (1 lb) (1 ft) / (2 ft)
    = 0.5 (lb)

Example – Lever calculation with SI-units – weight of 1 kg mass acting 1 m from the fulcrum

The effort force at a distance of 2 m from the fulcrum can be calculated as

Fe = (1 kg) (9.81 m/s2) (1 m) / (2 m)
    = 4.9 N

A lever mechanism where the input effort is higher than than the output load is often characterized as a third-class lever mechanism.

third-class levers

Example – Third-Class (Order) Lever

A force (weight) of 1 pound is exerted at a distance of 2 ft from the fulcrum.
The effort force at a distance of 1 ft from the fulcrum can be calculated as

Fe = Fl dl / de
    = (1 lb) (2 ft) / (1 ft)
    = 2 (lb)

One or more forces acting on a lever

A lever with two acting load forces and one effort force is indicated in the sketch below:
levers with more than two forces
The generic equation for one effort force with one or more acting load forces can be expressed as

Fe = (FlA dlA +  FlB dlB + .. + FlN dlN ) / de         (2)

This equation is modified for three acting loads below.

Example – A Lever with three acting loads and one effort force

A weight A of 1 pound is exerted at a distance of 1 ft from the fulcrum. A weight B of 2 pound is exerted at a distance of 2 ft from the fulcrum, and a weight C of 3 pound is exerted at a distance of 3 ft from the fulcrum.
The effort force at a distance of 2 ft from the fulcrum can be calculated as

Fe = (FlA dlA +  FlB dlB + FlC dlC ) / de
    = ((1 lb) (1 ft) + (2 lb) (2 ft) + (3 lb) (3 ft)) / (2 ft)
    = 7 (lb)

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