Mechanical Energy Summary

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Human Body as a Machine
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Summary
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The human body can be seen as a machine that consumes food and converts it to mechanical work and heat (See Chapter 5). Power is transmitted through the action of forces on the muscles, bones, and joints that make body’s lever system. As muscles contract, they pull a series of tendons and bones (levers) across joints (fulcrums, pivots, or hinges) to perform various tasks. Muscles always act as pairs, so when one contracts, its
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Mechanical energy is the total energy an object has excluding the thermal and internal energies associated with the structure of its atoms and molecules. In other words, it is the sum of its potential, kinetic, and rotational energies. Mechanical energy is what it takes to lift, move, turn, and twist an object. Examples of mechanical energy are energy possessed by a fast ball, energy stored in a spring or a rubber band, and the energy it takes to move a heavy object up a stairs. Simple machines are devices that are at our disposal to carry out the difficult tasks with less effort; they allow us to do the same amount of work by applying a smaller force (effort) but over longer distances. In performing any task there are always some frictional forces that we must overcome. For this reason, some mechanical energy is always converted to heat. In other words, in real systems, mechanical efficiencies are always smaller than “one”.
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V
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e = Vr = 5 = 25 cm/sdedr1.00.2..MA = = = = 5.0FrFe(200) (9.8)3921.00.2
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EX 2-8
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Figure 2-4
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A human elbow acts as a third class lever
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during lifting a weight.
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BicepsTricepsBone40 cm4 cm30 cmMuscle100¼
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(a) (b)
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EX 2-7
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WW/2WW
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39
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Chapter 2 - Mechanical energy
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antagonist extends. Examples of the first, second and third class levers are the joint between skull and vertebrae (neck joints), the Achilles tendons, and the elbow joints, respectively. Most of the movements of the body are produced by third class levers, where the force is between fulcrum and weight. This design lends itself to speed of movement rather than force. For example, when we lift a weight, the biceps and triceps work as a pair; when the biceps flex the arm (effort) or lift a weight (load or resistance), the triceps relax to extend the arm (Figure 2-4).
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Question: A man is lifting a heavy weight by contracting the bicep muscle, and at the same time relaxing his triceps muscle. The elbow acts as the fulcrum. What is the force needed to lift a 10 pound weight?
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Answer: Force needed is higher by ratio of the arms (40 to 4); we need ten times the weight of the object to support the weight. In the discussions above we assumed that there are no losses involved. When losses are present, mechanical advantage will be reduced by a factor equal to the mechanical efficiency.
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==References==
==References==

Revision as of 23:15, 28 June 2010

Summary Mechanical energy is the total energy an object has excluding the thermal and internal energies associated with the structure of its atoms and molecules. In other words, it is the sum of its potential, kinetic, and rotational energies. Mechanical energy is what it takes to lift, move, turn, and twist an object. Examples of mechanical energy are energy possessed by a fast ball, energy stored in a spring or a rubber band, and the energy it takes to move a heavy object up a stairs. Simple machines are devices that are at our disposal to carry out the difficult tasks with less effort; they allow us to do the same amount of work by applying a smaller force (effort) but over longer distances. In performing any task there are always some frictional forces that we must overcome. For this reason, some mechanical energy is always converted to heat. In other words, in real systems, mechanical efficiencies are always smaller than “one”.

References

Further Reading

External Links