Simple machines Compound machines Changing the direction of a force Changing the magnitude of a force Mechanical advantage Loss of energy through a system

Mechanics is the study of machines. Machines, in their various forms, are everywhere in our modern society and used in most aspects of everyday life. Machines vary from simple through compound and complex mechanical systems using a combination of different machines in combination. Machines are used to change the direction and magnitude of forces. For example, a single person would be hard pressed to directly lift a car, but with a jack one person can, with a lot of effort, lift a car enough to change a tyre.

Simple machines
These are considered under separate topics but, in summary, the six simple machines are levers, pulleys, wedges, screws, inclined planes and wheels and axles.

Compound machines
These are machines made up from a combination of simple machines.

A corkscrew, for example, has a screw device for pushing into the cork, and a pair of levers to lift the cork out of the bottle neck. A car jack raises a load by use of a screw, and the screw is turned by a handle that uses the wheel and axle principle.

The keys on a piano drive a series of levers that result in a wooden hammer hitting a string, to produce a note of the frequency at which that string vibrates.

Changing the direction of a force
In an internal combustion engine, the piston moves up and down in the cylinder. The direction of the piston changes at a very fast rate, but to do work and drive the car, the up and down motion of the cylinders is changed to the circular motion of the tyres by turning a crankshaft that then spins a flywheel and axle.

Gears can also be used to change direction. When one gear meshes with another the second wheel turns in the opposite direction. This can be used to change the direction of circular motion.

Gears

Changing the magnitude of a force
Levers are examples of force magnifiers and can be used to move objects much heavier than any single force available. The wheel and axle is also a force magnifier, and a combination of the two can be used in a compound machine such as the steering mechanism in a car.

Mechanical advantage

Most machines multiply the force of your effort or force. For example, you can use a lever to lift a rock that normally you would not have been able to pick up. Machines that increase your ability to apply a force on an object are said to have a "mechanical advantage" greater than 1. Mechanical advantage (MA) is measured as output force (load)/input force (effort). Thus, a lever with an MA of 5 will allow you to pick up a 100N rock with a force of 20N.

Note: Weight is really measured in Newtons (N) and not kilograms, which is the unit of mass. On Earth, 1 kg is approximately equal to 10N. A 100N rock has an approximate mass of 10kg.

Machines with an MA greater than 1 are said to be "force multipliers". Hammers, wheel barrows, spanners and car jacks are examples of "force multipliers levers". Axle and wheels, some gear systems and multiple pulley systems are also "force multipliers".

Machines with a mechanical advantage of less than 1 are not used to apply a strong force or lift heavy objects, but are used to increase the distance over which a force is applied. These machines are called "distance magnifiers". Distance magnifiers apply an effort force over a longer distance. A human arm and bikes with gears are two examples of distance magnifying machines.

Loss of energy through a system
No mechanical system is 100% efficient. Transformation of energy into less useful forms occurs through friction, producing thermal energy, sound and vibration. A measure of how much energy is actually successfully transferred for the task is called 'efficiency'. The efficiency of a machine is the ratio of the energy transfer to the energy supplied to the system. The useful energy output from a machine with 80% efficiency will be 8/10 or four fifths of the energy input. The best machines are those designed for maximum efficiency, so that as little energy as possible is wasted.

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