Key words: Machines, resistance, friction, heat, simple machine, energy, work, mechanical advantage, efficiency, lever, windmill, automobile, physics, Physical Science, Ron Kurtus, School for Champions. Copyright © Restrictions
Overview of Machines
by Ron Kurtus (revised 3 November 2015)
A machine is a device that converts an input into a greater output. The input and output could be forces, distances, or speeds. A machine can change the magnitude or direction of the input.
Machines usually consist of interrelated parts that combine to perform some kind of work, converting input energy into useful work. The input energy may be from humans, animals, engines or other sources.
Simple machines use a very basic configuration to do work. An example of a simple machine is a lever. A more complex machine is a bicycle.
Machines are used to create a mechanical advantage of the output force over the input force. Since there are losses in all motion, the efficiency of a machine is the output work over the input energy.
Questions you may have include:
- What does a machine do?
- What is mechanical advantage?
- What is the efficiency of a machine?
This lesson will answer those questions. Useful tool: Units Conversion
What a machine does
Machines can be used to cause the output force, output distance moved or output speed to be increased over the input.
There are situations where you want to move an object, but the resistance to that movement is greater than the force you have available. In such a situation, you might be able to use a machine to transform that force into one great enough to overcome the resistance.
A simple example of a machine is using a lever to lift a weight and overcome the force of gravity. According to the placement of the lever fulcrum, the input or effort force would be less the than that required to life the load.
Sometimes you may want to move an object a greater distance than your input distance. A different configuration of a lever can increase the distance that the load is moved over the input distance.
Lever configuration results in load moving further than effort moves
Wind turbines used for generating electrical energy are huge in size, but they rotate at a speed that is much slower than required to operating the electrical generator. Machinery consisting of shafts and gearing transform the input speed of rotation into a higher speed needed for the generator.
Mechanical advantage of a machine
The mechanical advantage of a machine relates to the increase in output over the input.
Typically, mechanical advantage concerns the increase in force through the configuration of the machine. In such a case,
MAF = Fo/Fi
- MAF is the force mechanical advantage
- Fo is the output force
- Fi is the input force
Note: MA is not M times A. Rather, it is an abbreviation for mechanical advantage.
There are situations where you may want to increase the distance or the speed that you move an object. In such cases, the distance mechanical advantage is:
MAd = Do/Di
and the speed mechanical advantage is:
MAs = So/Si
A catapult is a good example of a machine that increases the output speed over the input speed of the force you apply.
Efficiency of a machine
The work done by a machine is the distance the force moves the object against the resistive force. This output work is designated as:
Wo = FoDo
- Wo is the output work
- Fo is the output force to move the object
- Do is the distance the object moves
Since work is equivalent to energy expended and following the Law of Conservation of Energy, the output work should equal the input work:
Wo = Wi
But that is true only in an ideal machine. In reality, there are losses to the output from internal friction, heat or other things that take away from the efficiency of the machine.
Efficiency = Wo/Wi
Efficiency = work done/energy used
As an illustration of the losses in all machines, a simple lever loses about 2% of the input energy to internal friction at its fulcrum, such that its efficiency is 98%. If 100 joules of work is input, 98 joules of work is the output.
On the other hand, the efficiency of an automobile is only around 15%. About 75% of the energy is lost through wasted heat from the engine and another 10% is lost due to internal friction, including losses from the tires.
A machine is a device that make moving an object against some resistance easier. It does this by changing the magnitude or direction of the input force. Most machines are complex, although there are machines consisting of a very basic configuration to do work. Machines create a mechanical advantage of the output force over the input force. Since there are losses in all motion, the efficiency of a machine is the output work over the input energy.
Use science to make things more efficient
Resources and references
Machine - Wikipedia
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Overview of Machines