The Physics
Hypertextbook
Opus in profectus

# Simple Machines

## Discussion

In the most general sense, a machine is any device that can be used to perform a task. In the mechanical sense, a machine is a device for transmitting work from one location to another.

• A bicycle is a machine. The rider does work on the pedals, which in turn do work on the front crank, which does work on the chain, which does work on the rear sprocket, which does work on the wheel, which does work on the axle, which does work on the frame, which does work on the rider.
• A bicycle rider is a machine — that is, people and other animals with skeletons are machines. Our muscles do work on our bones which in turn do work on the world around us.
• A doorknob is a machine. Work is done on the outer diameter of the knob, which in turn does work on the spindle, which does work on the cylinder, which does work on the latch.
• A hammer is a machine. Work is done on the handle, which in turn does work on the head, which does work on a nail.

Work is defined as the product of force and displacement…

W = Δs

Machines are devices by means of which a primary force (called the effort) exerted over some displacement at one location gives rise to a secondary force (called the load) and displacement at a different location. The work done on the machine at the effort end is called the work in and the work done by the machine at the load end is called the work out.

 work done on the machine Win = FeffortΔseffort
 work done by the machine Wout = FloadΔsload

The load and effort usually differ in magnitude and direction as well as location. In the ideal world, mechanical energy is never lost to other forms and work in equals work out. (Oh happy day!) In the real world, however, mechanical energy is always lost, so work in is strictly greater than work out. (Curses, foiled again!)

 ideal machines Win = Wout FeffortΔseffort = FloadΔsload
 real machines Win > Wout FeffortΔseffort > FloadΔsload

efficiency, Well, I like this part.

 η = Wout Ein

except for the Greek eta (η) symbol. It looks too much like the Latin letter n.

The human body is exceptionally efficient at extracting energy from food. Feces retain only about 5% of the chemical energy originally present in the food consumed.

Whad'ya think of that?

mechanical advantage, ho hum

 M.A. = Feffort Fload

velocity ratio, hum ho.

 V.R. = vload veffort

classical machines, simple machines. Is this topic even worth teaching anymore?

The classical list counts 6 devices, but 3 of them are variations of the same thing.

1st use in english 1545, for a different purpose

1545 in J. Schäfer Early Mod. Eng. Lexicogr. (1989) II. (at cited word), The hole machyne of this world is divided in .2. parte. That is to saye, in the celestiall and into the elementall regions.

More OED

1704 J. Harris Lexicon Technicum Machine, or Engine, in Mechanicks, is whatsoever hath Force sufficient either to raise or stop the Motion of a Body... Simple Machines are commonly reckoned to be Six in Number, viz. the Ballance, Leaver, Pulley, Wheel, Wedge, and Screw... Compound Machines, or Engines, are innumerable.

simple machine n. one in which there is no combination of parts, e.g. a lever, or any other of the so-called mechanical powers.

• lever
• seesaw
• bascule bridge
• arms, legs, fingers, toes, jaw
• crowbar
• hand jack
• hammer
• sword
• cricket/baseball bat
• golf club
• wheel and axle
• is the passive wheel on a cart or trailer a machine? is the wheel by itself a machine? are ball bearings machines?
• are sleds machines?
• is a sail a machine?
• windlass, winch
• crossbow crank
• train, bike, car, truck wheels
• gears
• doorknob
• screwdriver
• pulley
• block and tackle
• chain hoist
• inclined plane
• ramps and stairs and escalators
• screw
• archimedes screw
• wood metal screw
• worm gear
• press
• oil press
• printing press
• wedge
• axe

Why is the inclined plane on this list? In my opinion, it doesn't belong here because we shouldn't be using the word "machine". On an inclined plane, there is no distinction between load an effort locations, unless a machine is…

• a device for altering the magnitude or direction of a force.
• any device that transmits a force or directs its application.
• an instrument designed to transmit or modify the application of power, force or motion
• any device that transmits a force or directs its application
• Anything that transmits force or directs its application.
• transferring a force from one place to another
• changing the direction of a force
• changing the magnitude of a force
• changing the distance or speed of a force

Effort is the unaltered force. Load is the altered force.