Quick return mechanism

Viscoelasticity illustrates materials that exhibit both viscous and elastic characteristics. Viscous materials tike honey resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Viscoelasticity is the result of the diffusion of atoms or molecules inside an amorphous material. Rubber is highly elastic, but yet a viscous material. This property can be defined by the term viscoelasticity. Viscoelasticity is a combination of two separate mechanisms occurring at the same time in mbber. A spring represents the elastic portion, and a dashpot represents the viscous component (Figure 28.7). 

Neural Excitation of Muscle. Voluntary contraction of human muscle initiates in the frontal motor cortex of the brain, where impulses from large pyramidal cells travel downward through corticospinal tracts that lead out to peripheral muscles. These impulses from the motor cortex are called action potentials, and each impulse is associated with a single motor neuron. The principle structure of a motor neuron is shown in Fig. 6.IS. The action potential initiates in the cell body, or soma, and travels down a long efferent trunk, called the axon, at a rate of about 80 to 120 m/s. The action potential waveform is the result of a voltage depolarization-repolarization phenomenon across the neuron cell membrane. The membrane ionic potential at rest is distuibed by a surrounding stimulus, and Na ions are allowed to momentarily rush inside. An active transport mechanism, called the Na —K+ pump, quickly returns the transmembrane potential to rest This sequence of events, which lasts about 1 ms, stimulates a succession of nerve impulses or waves that eventually reach muscle... 

The reverse process, i.e., mechanically induced changes in shape of a cross-linked weak polyelectrolyte, should cause a change in its ionization state. When a piece of weak polyelectrolyte gel was placed in a cell and compressed from the vertical direction using a glass piston (Fig. 8a), the pH of the gel changed as shown in Fig. 9. When the gel was unloaded, the pH quickly returned to its original value with some overshoot. 

In addition to the classical trajectory calculations, a variety of other theoretical studies have been done. Accurate quantum mechanical scattering results for zero total angular momentum have been reported for reaction (40). The reaction has also been studied by using transition-state theory (TST). Transition-state theory overestimates (compared to experiment) the thermal rate by a factor of about two. This may be caused by errors in the PES or to recrossings , that is, trajectories which pass through the transition state but quickly return to reactants rather than going on to form products (TST incorrectly counts these as reactive events). Studies are also being done to understand the spectroscopy of the transition state of this reaction. ... 

Another type of horizontal belt vacuum filter uses reciprocating vacuum trays mounted under a continuously traveling filter cloth. The trays move forward with the cloth as long as the vacuum is appHed and return quickly to their original position after the vacuum is released. This overcomes the problem of friction between the belt and the trays because there is no relative movement between them while the vacuum is being appHed. The mechanics of this filter are rather complex, and the equipment is expensive and requires intensive maintenance. A range of solvents can be used. Widths up to 2 m and areas up to 75 m are available. The cloth can be washed on both sides. 

The employees returned on Sunday to complete the job. They decided to strengthen this poorly-constructed box by welding a 2-inch (5 cm) angle iron stiffener around the middle of the ill-designed box. After the angle iron improvement was completed, the mechanic closed the valve and moved away. It was a good thing he did move away rapidly, because the box quickly exploded. 

Fast response is possible with swelling-based sensing mechanisms. For example, the mapping of a chemical plume caused by a release into the air could employ such a sensor positioned on an unmanned aerial vehicle (UAV). With this vehicle traveling at 40 miles/h, subsecond response times would be required to locate and map out the released analyte. An example of this is shown in Fig. 16, which describes a thin film of poly(2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole-co-tetrafluoroethylene), Teflon AF placed on an interferometer which in turn was placed in the nose cone of an UAV, which was positioned in a wind tunnel with the wind moving at 40 mph. Small amounts of toluene vapor were introduced into the air stream to get an idea of whether the sensor would work in such an application. The sensor responded quickly, established equilibrium in seconds, and reversibly returned to baseline after the material passed. 

The kidneys receive a large blood flow (approximately a quarter of the total cardiac output of 5 litres per minute) and from this volume of blood approximately 170 litres of filtrate are produced every day Clearly, the body would quickly become dehydrated if this volume of fluid were lost to the sewage system, so most of it is reabsorbed from the kidney tubule and returned to the bloodstream. Small molecules that are dissolved in the glomerular filtrate are also reabsorbed back into the bloodstream, either by passive diffusion (which obeys Fick s law) or by the utilisation of energy in an active transport process similar to the mechanisms for gut absorption discussed previously. It should be realised that reabsorption from the glomerular filtrate and return to the bloodstream are involved in the duration of action of many drugs, and a drug molecule may be filtered and reabsorbed many times before it is finally excreted from the body. 

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