Multiplying forces

 

Terms, units, and abbreviations

These metric units are used: force, effort or load in newtons (N) work in joules (j) power in watts (W) The equivalent customary units and conversion factors are as follows: 1 newton — 7.23 poundals 1 joule = 23.73 foot poundals 1,000 watts or 1 kW = 1.34 horsepower

A machine is defined in physics as a device for applying or transmitting mechanical power. Its function may be to overcome resistance to motion or to change of shape or size at one point of an object by applying a force, often at some other point The multiplication of forces is possible in mechanical devices, enabling work to be performed that human strength alone could not do. Two terms commonly used in describing machines are work and power; both of which have been given precise, technical meanings. In a simple machine, the work done is equal to the product of the load (a force) and the distance it moves in the direction of the force, irrespective of the time taken; it is usually measured in joules (abbreviation j—1 joule = 1 newton meter). Power is the rate of doing work, and is expressed in watts (abbreviation W).

Levers

Levers are among the most commonly used means of multiplying a force, but doing so usually involves a greater movement at the point of effort than at the point of work. A class one lever has the effort and load points on opposite sides of the pivot, or fulcrum. A pair of scissors (A) consists of two class one levers with a common fulcrum. A class two lever, as in a wheelbarrow (B), has the points of effort and work on the same side of the fulcrum but with the work point closer to it. A class three lever, as in the tongs (C), also has the effort and work points on the same side of the fulcrum, but with the effort closer. As a result, the work point moves farther than the effort point.

Pulleys

Hiero II, the King of Syracuse, is reported to have challenged Archimedes to demonstrate the power of the simple machines (pulleys) that he was studying. In response, Archimedes arranged a system of pulleys so that single-handedly he dragged a fully-laden ship out of the water and onto dry land.

Other simple machines

The wheel-and-axle arrangement, and gearing systems, also supply mechanical advantage. In a wheel and axle, one turn of the (larger) wheel results in several turns of the connected axle. The velocity ratio (and therefore the mechanical advantage, if the system is totally efficient) is equal to the radius of the wheel divided by the radius of the axle.

Hydraulics

Hydraulics is the study of the flow of fluids and how they can be used in machines capable of giving a mechanical advantage This is possible because of the property of liquids described by Pascal’s principle, which states that the force per unit area (pressure applied to an enclosed liquid is transmitted without change to every part of it, whatever the shape of its container.

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