Rebound Speed Research Articles

The difference of dynamic elasticity characteristics and stroke effect between two types of new material seamed plastic table tennis ball

The diameter and weight of different brands of table tennis ball will affect the ball’s elasticity and stroke. The purpose of this study is to analyze the difference of the dynamic elasticity and stroke effect between the two brands of new plastic ball. A self-made experiment was designed to test the dynamic elasticity characteristics of DHS D40 + and Nittaku 40+. Table tennis players (N = 18) were randomly selected from the China Table Tennis College (Mage = 15.16 ± 2.41; Mheight = 1.59 ± 0.32 m; Mweight = 45.72 ± 5.17 kg). Each participant was righthand shake-hands grip. A speedometer was used to record the ball speed and a high-speed camera was used to measure the spin speed. Data normality was verified by using the Kolmogorov-Smirnov test. The independent t-test was conducted to assess the differences of the dynamic elasticity and stroke effect between the two types of plastic ball. Results showed that the rebound speed and decrement rate of DHS D40 + and Nittaku 40 + both increased with the increased falling speed, respectively. When falling at high speed, there was a significant difference of dynamic elasticity between DHS D40 + and Nittaku 40+ (p < 0.01). There was also a significant difference in the ball speed and spin speed between the two types of new material seamed plastic ball during the backhand backspin stroke (p = 0.041, p = 0.022, respectively), and the ball speed and spin speed of DHS D40 + were higher than that of Nittaku 40 + ball. Compared with the DHS D40+, the Nittaku 40 + has a faster rebound speed, higher rebound height, and better dynamic elasticity. Therefore, under same striking conditions, when hitting the Nittaku 40 + ball, players need to increase the swing distance and hit the ball with more strength to improve the ball speed and rotation speed; increase the spin and decrease the ball’s rebound height of the serve.

Nature of storage iron turnover.

Despite recent advance in the study of the nature of storage iron turnover, a comprehensive analysis remains lacking. This study aimed to clarify the nature of storage iron turnover. Ferritin-hemosiderin iron transformation rate and the standard normal storage iron turnover rate were utilized in this study to describe the mechanism of iron absorption in relation to ferritin and hemosiderin iron turnover. The synchronization of radioiron uptake peaks by bone marrow and liver indicates that the distribution of radioiron is proportional to the pre-existing iron levels in organs at 24 h after radioiron injection. Moreover, the synchronization indicates the independence of iron mass from red cell precursors in acquiring plasma iron. Thus, the erythron does not dominate the radioiron uptake process. The inverse correlation between transformation rate and the amount of pre-existing iron storage implies that the intra-storage iron turnover is active in iron deficiency, but inactive in iron overload. The decreased ferritin/hemosiderin iron ratio in chronic hepatitis C (CHC) with normal iron storage suggests a trend of iron transformation from ferritin into hemosiderin. The correlation between the pretreatment iron storage and the speed of rebound in CHC implies that the vacant iron-storing rooms in iron-removed cells have a potential to increase iron absorption. This study presents new insights into the turnover of stored iron to enhance our understanding of iron metabolism in various hematologic disorders.

A comprehensive analysis of low velocity impact response of [0/±45/90]s thin woven GFRP composites at room temperature

This study is motivated by the need to explore the low-velocity impact behavior of angle-ply thin woven GFRP composites with stacking sequence of [0/±45/90]s, especially the lack of literature concerning damage analysis correlation with a comprehensive range of measurements. The damage diameter, area, and perimeter length are characterized in this study by a novel AutoCAD image processing technique. The test results revealed that specimens subjected to impact energies in the range of 2–30 J are free from edge delamination compared to those tested at 40–50 J. Decreasing the contact duration at impact energy of 60 J is due to perforation accompanied with damage propagation toward the specimen edges and thus, the contact force is sharply dropped from its maximum value of about 4.48kN to 1.3kN. Consequently, both rebound speed and coefficient of restitution dropped to zero. The direct measurement of the energy threshold is increased in the following order: energy threshold at Fmax (2.3–14.9 J), first penetration (load drop) after peak load (23.9–25.6 J), penetration energy threshold (37.9 J), perforation energy threshold (31.1 J), and perforation and crack propagation energies (43.3 J). The perimeter length represents a novel damage parameter, exhibiting a strong correlation with impact energy, as evidenced by a high coefficient of determination of 0.97.

Versatile Stretchable Conductor with Exceptional Resilience and Rapid Rebound Capabilities: Toward Sustainable and Damage‐Resistant Soft Electronics

AbstractStretchable conductors (SCs) have garnered significant attention in both academia and industry, owing to their promising applications in flexible electronics. However, achieving a highly resilient and rapidly rebounding SC that enables prompt electrical response to applied strain, while also being fully recyclable and self‐healable, remains a challenge. Here, the design and preparation of a high‐performance SC based on a small bio‐molecule (α‐Lipoic acid) are reported, using a straightforward, cost‐effective, and scalable method. The engineered SC exhibits a ≈100% rebound ratio and fast rebound speed (≈0.4 s) under repeated high strains, along with outstanding electrical sensitivity to both deformation and temperature variation. This ensures that the SC possesses high precision and real‐time sensing attributes. Moreover, the SC features high optical transparency (&gt;97%), robust reversible adhesion to diverse surfaces (up to 5.0 MPa), efficient room‐temperature self‐healing properties, and full recyclability. The superior properties of the SC are fundamentally attributed to the installed hierarchical dynamic bonding at the molecular level, which enables continuous energy dissipation and is analyzed through DFT calculations. The developed SC holds great potential for applications in soft electronics and human–machine interfaces.

The effect of special exercises in strength and rebound speed on inclined surfaces on some basic skills of young volleyball players

The effect of special exercises in strength and rebound speed on inclined surfaces on some basic skills of young volleyball players

High rate response of elastomeric coatings for wind turbine blade erosion protection evaluated through impact tests and numerical models

The high strain rate (above ∼104/s) behavior of an elastomer is characterized using low strain rate (below ∼101/s) dynamic mechanical thermal analysis and time–temperature superposition. This approach is validated using high strain rate ball impact experiments and finite element predictions of ball deformations and rebound speeds. The ball impact experiments are performed by shooting 6 mm rubber balls with a controlled impact speed of 50–170 m/s at steel and polyurethane targets. An explicit axisymmetric finite element model of the ball impact experiment is established in the commercial code Abaqus. The validated material properties are used to model the viscously dissipated energy and the corresponding temperature increase in the polyurethane target. The region with the largest dissipated energy in the target material is predicted to be in a ring around the impact center with a radius of approximately 1 mm. This region corresponds well to locations of early damage initiation observed in the impact fatigue experiments for similar materials. The predictions are confirmed by the observed temperature distributions using thermal camera imaging of the rubber ball impact experiment. The experimental setup was developed for impact fatigue testing of anti-erosion coatings for the leading-edges of wind turbine blades.

One-year clinical observation of the effect of internal bleaching on pulpless discolored teeth.

This study aimed to observe the color rebound and rebound rates of non-pulp discolored teeth within 1 year after routine internal bleaching to guide clinical practice and prompt prognosis. In this work, the efficacy of bleaching was observed in 20 patients. The color of discolored teeth was measured by using a computerized colorimeter before bleaching; immediately after bleaching; and at the 1st, 3rd, 6th, 9th, and 12th months after bleaching. The L*, a*, and b* values of the color of cervical, mesial, and incisal parts of the teeth were obtained, and the color change amounts ΔE*, ΔL*, Δa*, and Δb* were calculated. The overall rebound rate (P*) and the color rebound velocity (V*) were also analyzed over time. In 20 patients following treatment, the average ΔE* of tooth color change was 14.99. After bleaching, the neck and middle of the teeth ΔE* and ΔL* decreased in the 1st, 3rd, 6th, 9th, and 12th months, and the differences were statistically significant. Meanwhile, from the 9th month after bleaching, the rebound speed was lower than that in the 1st month, and the difference was statistically significant. The incisal end of the tooth ΔE* and ΔL* decreased in the 6th, 9th, and 12th months after bleaching, and the differences were statistically significant. No significant difference was found in the rebound speed between time points. However, this rate settled after the 9th month, with an average color rebound rate of 30.11% in 20 patients. The results indicated that internal bleaching could cause a noticeable color change on pulpless teeth. The color rebound after bleaching was mainly caused by lightness (L*), which gradually decreased with time, and it was slightly related to a* and b*. The color of the teeth after internal bleaching rebounded to a certain extent with time, but the color rebound speed became stable from the 9th month. Clinically, secondary internal bleaching can be considered at this time according to whether the colors of the affected tooth and the adjacent tooth are coordinated and depending on the patient's needs.

Energy Loss for Droplets Bouncing Off Superhydrophobic Surfaces.

A water droplet can bounce off superhydrophobic surfaces multiple times before coming to a stop. The energy loss for such droplet rebounds can be quantified by the ratio of the rebound speed UR and the initial impact speed UI; i.e., its restitution coefficient e = UR/UI. Despite much work in this area, a mechanistic explanation for the energy loss for rebounding droplets is still lacking. Here, we measured e for submillimeter- and millimeter-sized droplets impacting two different superhydrophobic surfaces over a wide range of UI (4-700 cm s-1). We proposed simple scaling laws to explain the observed nonmonotonic dependence of e on UI. In the limit of low UI, energy loss is dominated by contact-line pinning and e is sensitive to the surface wetting properties, in particular to contact angle hysteresis Δ cos θ of the surface. In contrast, e is dominated by inertial-capillary effects and does not depend on Δ cos θ in the limit of high UI.

An implementation of viscous pressure-force (‘soft-sphere’) model in REBOUND for local ring simulations

ABSTRACT Numerical simulations are an essential tool for studying dynamics of dense, self-gravitating planetary rings. Here, we present a new implementation of a soft-sphere collision model into the parallel gravitational N-body code REBOUND. Specifically, our module is designed for simulations done in the local shearing sheet approximation, and includes a simple viscoelastic model for the contact forces between colliding particles. The soft-sphere treatment is particularly important in regimes where collisions cannot be treated as independent binary impacts, as is done in the hard-sphere method already implemented in the code. The new module is fully parallelized with MPI and it can be used in conjunction with the available gravity octree method, also fully parallelized, to perform self-gravitating simulations. We check the validity of our soft-sphere model by successfully reproducing the main steady-state properties of non-gravitating and self-gravitating systems reported in earlier literature. Moreover, we show that it gives realistic results also in the high optical depth and strong self-gravity regimes, where the hard-sphere method fails due to particle overlaps. In addition, we test the CPU-scaling and efficiency of the MPI-parallelization. The algorithm proves to retain much of the original REBOUND speed without sacrificing the accuracy of the results. Our new tool can thus be applied with confidence to planetary ring studies, such as those aimed at understanding the onset and evolution of viscous overstability in dense self-gravitating rings.

TO CALCULATE THE LOADING OF KNITTING NEEDLES OF AUTOMATIC HALF-HOSE MACHINES

An important link in the analysis of the longevity of details by the criterion of fatigue strength is the calculation of parameters that take into account the peculiarities of the technological regime in terms of load and the corresponding number of cycles that affect the accumulation of fatigue failure. The final range of loads of knitting needles of automatic half-hose machine was based on the systematization of amplitudes of forces depending on the frequency of their implementation. Previous works has assumed the assumption of a one interaction of the needle heel with the cams, which is experimentally refuted by the example of modern high-speed automatic half-hose machine. The aim of the work is to adjust the provisions of calculating the load of knitting needles of half-hose machines for design estimate of longevity, taking into account the specific features of the impacts of needles with the cams of the knitting system, which affect the total number of load cycles. The condition of rupture of the contact in the pair of the needle heel and the inclined cam after their impact is considered, taking into account the inertial and rigid parameters of the system, including the resisting force of the needle in the groove of the cylinder. According to the results of the dynamic analysis, the expressions of the minimum speed at which the needle can rebound from the cam and the vertical component of the needle rebound speed at the moment of its separation from the cam are obtained. The analysis of the influence of the force of resistance of the needle movement in the race on the vertical speed of the needle is performed. The issue of the bounce of the heels of unselected needles from the lower edge of the inclined cam due to their inertial run-out was studied separately. It is established that at certain circular speeds of the needle cylinder leads to additional impacts of the needles with the horizontal limiting cams and increases their destruction. The results of studies of the possible rebound of the needle heel from the inclined cam and the interaction of the needle with the limiting horizontal cams are a supplement to the method of calculating the loading of needles and are aimed at improving the accuracy of design longevity of needles.

Study on cushioning and energy absorption of sports mouthguard

PURPOSE Using impact test to study cushioning and energy absorption of sports mouthguard of different thickness, material and combination. METHODS According to the thickness, material and combination, the mouthguards were divided into 7 groups with 6 testing pieces in each group. A pendulum device was used to apply impact energy to the mouthguard and dentition model, and the magnitude of the impact energy was measured by changing the release height of the pendulum. The force sensor and the laser vibrometer were respectively connected to the data acquisition and signal analyzer to obtain the incident speed, rebound speed, maximum collision force, collision start time and collision end time of the hammer head. Collision time, energy absorption and energy absorption ratio of the mouthguard were calculated, which reflected different mouthguard cushioning and energy absorption characteristics. One-way analysis of variance and t test in SPSS 22.0 software package was used to analyze the difference between different groups. RESULTS No matter what type of mouthguard was worn by the dentition model, the impact force was significantly reduced than without mouthguards. At the same height, there was significant difference between the average maximum collision force and the average collision time among soft mouthguards with thicknesses of 2, 3, 4, and 5 mm(P<0.05); among which the average maximum collision force of the 5 mm was the smallest and the average collision time was the shortest. There was no significant difference in the average energy absorption and the average energy absorption ratio among these 4 groups. There was significant difference in the average maximum collision force, average collision time, average energy absorption and average energy absorption ratio between the two groups of mouthguards with the same thickness of splints and different materials(P<0.05). Among them, the average maximum impact force of the 4 mm soft mouthguard was smaller and the average collision time was longer. CONCLUSIONS Wearing sports mouthguard can reduce the risk of tooth fracture during collision. Soft mouthguard has the best cushioning performance and it is positively related to thickness. Locally strengthening hard materials can enhance energy absorption and provide better protection.

Proof of concept of a novel frictional shock absorber; analytical model and experimental analysis

Recurrent impact protection devices usually need to dissipate large amounts of energy to prevent damage to the infrastructure they protect and to make efficient use of them. This requires that protection devices consider some type of damping and have some mechanism to recover their original form. In this study a novel device is proposed, capable of absorbing a large part of the energy imposed by impact loads and recovering its original form autonomously. The device proposed in this article is composed of rigid parts with articulated joints, an elastic element that allows the recovery of its shape and an element that dissipates energy by friction. The necessary equations were developed to describe the non-linear behavior of the device and parametric simulations of the proposed model were performed to describe the dynamic interaction between the device and a mass that impacts. Additionally, a scale model of the device was constructed to be experimentally tested, which allowed to verify the effectiveness in the dissipation of energy, the reduction in the force transmitted to the support structures and the decrease in the rebound speed of the impacting mass. An error parameter was defined for a load-unload cycle between experimental results and analytical calculations. The error considers both the impact force and the dissipated energy, obtaining values of up to 6%. The energy absorption capacity of the device –between 83% and 93% of the impact energy– was verified experimentally, as well as the reduction of the impact speed –between 91% and 96%–.

Impact of a ball with a flexible beam

The impact of a ball with a flexible beam is analysed both experimentally and theoretically. The impact generates a bending wave that may or may not return to the impact point before the ball bounces off the beam. If the ball rebounds before the bending wave returns, then the rebound speed of the ball is determined by localised bending of the beam in the vicinity of the impact point. For a short duration impact, the rebound speed is therefore independent of whether the beam is freely supported or clamped at one or both ends, unless the impact occurs close to an end of the beam. Two different values of the coefficient of restitution can be defined, depending on whether the speed of the beam is measured when the ball rebounds or at a later stage. Experimental results were obtained using a small rubber ball impacting light wood and steel beams. A video camera and two piezoelectric disks were used to capture the relevant ball speeds and vibration response of the beam. The experiment would be suitable for undergraduate students.

Speed running is determined by strength and power in young football players

Introduction: For years researchers and trainers have been looking for determinants of sports talent in the youngest and also in the relationship between individual motor features relevant in given sport disciplines. There is a fundamental consensus in the opinion that the strength and power of the lower limbs are correlated with the speed of running. However, this relationship also among the youngest is not so clear. The purpose of the work was to show the relationship between strength and power generated during vertical jump and achieved height jump and the speed of flat running in the youngest footballers. Material and methods: The research covered 75 children training football at the age of 6–14 years divided into two age groups. The subjects made a countermovement jump on a dynamometric platform measuring strength, power, speed of rebound and the height obtained, after which they performed a flying run over a distance of 20 m. Results: A strong, positive correlation was demonstrated between all dynamic and kinetic parameters of the jump and running speed. This correlation turned out to be clearly stronger in the older group of football players than in the younger group. Conclusions: The obtained results suggest that strength and power are good predictors, such an important feature in football as the player’s motion speed, already at the basic level of training.

Collision of a ball with the edge of a step

An experiment is described where a rubber ball was incident on the edge of a step to simulate the impact of a sports ball with a sharp edge. Measurements of the rebound speed, angle and spin indicated that the impact was equivalent to one with an inclined plane. The incline angle depends not only on the impact point on the ball but also on the rotation angle of the ball during the impact. Results are presented for cases where the ball was incident vertically on the step and also obliquely.

Predictions of time to HIV viral rebound following ART suspension that incorporate personal biomarkers.

Antiretroviral therapy (ART) effectively controls HIV infection, suppressing HIV viral loads. Suspension of therapy is followed by rebound of viral loads to high, pre-therapy levels. However, there is significant heterogeneity in speed of rebound, with some rebounds occurring within days, weeks, or sometimes years. We present a stochastic mathematical model to gain insight into these post-treatment dynamics, specifically characterizing the dynamics of short term viral rebounds (≤ 60 days). Li et al. (2016) report that the size of the expressed HIV reservoir, i.e., cell-associated HIV RNA levels, and drug regimen correlate with the time between ART suspension and viral rebound to detectable levels. We incorporate this information and viral rebound times to parametrize our model. We then investigate insights offered by our model into the underlying dynamics of the latent reservoir. In particular, we refine previous estimates of viral recrudescence after ART interruption by accounting for heterogeneity in infection rebound dynamics, and determine a recrudescence rate of once every 2-4 days. Our parametrized model can be used to aid in design of clinical trials to study viral dynamics following analytic treatment interruption. We show how to derive informative personalized testing frequencies from our model and offer a proof-of-concept example. Our results represent first steps towards a model that can make predictions on a person living with HIV (PLWH)’s rebound time distribution based on biomarkers, and help identify PLWH with long viral rebound delays.

Rotation of a rebounding-coalescing droplet on a superhydrophobic surface

Droplet impact and droplet coalescence are two very common phenomena. When these two processes occur on a superhydrophobic surface in an appropriate sequence, an interesting but little-noticed phenomenon will occur with rotation of the rebounding-coalescing droplet. When a droplet impacts another stationary droplet on a superhydrophobic surface with an appropriate velocity and position, the reaction force produced by the impact and the moment arm formed by the liquid bridge produces a reversed torque. This reverse torque causes the droplet to rotate after rebounding. The liquid bridge in the early stage of the coalescence process is the key to the rotation, and the relative development speed of coalescence and rebound determines whether rotation can occur and its relative strength.

Effect of topspin on the apparent speed of a tennis court

Experimental results are presented concerning the impact of a tennis ball incident obliquely with topspin on a rigid, horizontal surface. The ball was launched at speeds between 12 and 17 m/s onto P800 emery paper attached to a heavy block of granite. The angle of incidence to the surface was varied from $$14^{\circ }$$ to $$65^{\circ }$$ and the spin of the ball was varied from 0 to 360 rad/s to examine the effects on the horizontal rebound speed. The ratio of the rebound to the incident horizontal speed increased from 0.5 to 0.95 as the incident spin was increased. The classification or perception of a tennis court as being fast or slow is therefore of limited value, given that most players strike the ball with topspin.

Bounce of a perfectly elastic ball

If a perfectly elastic ball collides vertically with the floor, the rebound speed is the same as the incident speed. If the floor remains at rest, then kinetic energy is conserved but momentum is not conserved. To conserve momentum as well as energy, the floor must recoil at finite speed, even if it is extremely heavy.

Trial of Low-Temperature and High-Temperature Hardness Testing Using the Small-Ball Rebound Test eNM3A10

The small-ball rebound hardness test measures the coefficient of restitution (rebound hardness) of a sample by shooting a 3 mm diameter alumina ball indenter at a speed of 10 m/s and measuring the rebound speed. The obvious advantages of this test are a remarkably small “mass effect” and satisfaction of the similarity rule of hardness. In addition, the test is also considered suitable for measuring high- and low-temperature hardness because the indenter comes into contact with the specimen only for an extremely short time.For this application, samples made of steel were cooled and heated to various temperatures ranging from –196ºC to 1000ºC. It was found that there was no damage to the tester or the indenter, making it possible to easily perform reproducible low- and high-temperature rebound hardness tests.