Backward Jumps Research Articles

Stress relaxation in polymers: a modified site model analysis

The site model theory is used to derive an expression for stress relaxation in a polymer governed by a single thermally activated process. Backward jumps of the structural units are taken into account and it is further assumed that when a structural unit executes a jump it does not regenerate a configuration suitable for another jump of a similar kind in the same direction. A limiting “internal stress” is predicted, as obtained in practice, and it is found that the formulation corresponds exactly to the standard linear solid of group I. This correspondence extends to creep, with the retention of the same parametric identities. Consideration is given to the influence of residual stresses and to the aging characteristics of fabricated polymers.

The movements of jumping ciliates

Jumping ciliates fall into 3 movement groups. The movements of Halteria, Mesodinium and Uronychia resemble those of typical motile ciliates. The backward jumps of their usual avoidance reactions are due to the high development of the motor organelles. In Askenasia volvox, Cyclidium and Strobilidium gyrans forward jumps and darts take the place of the avoidance reaction's backward movement and rapid forward spiraling, over a wide range, replaces reversal. However, these species can move backward. Urotricha farcta (group 3) also responds with forward jumps and darts to brief repellent stimuli and with rapid forward spiraling to intense, continuous noxious stimuli. But U. farcta is incapable of backward movement. The mechanisms and nature of the movements are described and discussed. The avoidance reaction is re-examined.

The jump of Petrobius (Thysanura, Machilidae)

The jump of the sea‐shore bristletails, Petrobius brevistylis and P. maritimus, has been investigated. A brief study of jumping distance and orientation showed that jumping behaviour was partly random, particularly with regard to the direction of a jump. A high speed cine camera was used to analyse both forward and backward jumps. The jumping action consists of a tail beat which reflects the strong depression of the posterior abdomen. This swings up the anterior part of the body, including the centre of gravity. The model suggested to explain this action proposes two movements spreading from either end of the bristletail: 1. A straightening of the abdomen starting posteriorly, 2. An increasing ventral curvature of the thorax starting anteriorly. Movement 1 will account for the raising of the centre of gravity, and the rapidity of movement 2 will determine the final direction of the jump.The muscular basis of these movements has been discussed using the data of Manton (1972), Barlet (1967) and Bitsch (1973). A catapult system was proposed which depended on the dynamic balance between two abdominal muscular systems. It was suggested that the dorsal longitudinal muscles were antagonised by the deep oblique muscles. A short study of the energetics of jumping was made in order to estimate the times needed for sufficient energy to be stored for particular jumps. These times were then compared with known times spent by Petrobius in the cocked (energy storing) pre‐jump position.

Theory of quasielastic neutron scattering by hydrogen in metals considering finite jump times

A method is presented of calculating the quasielastic neutron scattering by hydrogen diffusing in metals, taking into consideration finite jump times τ 1, in contrast to earlier theories in which only the case of τ 1 being small compared to the mean residence time τ 0 had been treated. Compared to the case τ 1 = 0, finite jump times cause a considerable change in the shape and half-width of the scattering function. For τ 1/τ 0 ≈ 10 −1 with increasing jump time a broadening of the scattering function results. For τ 1/τ 0 ≈ 1 at certain values of the momentum transfer the central quasielastic peak disappears and side peaks occur which reflect the backward and forward jumping between two interstitial sites.