Wind tunnel measurements in ski jumpers and simulation of the jumps

Ski jumping performance can be divided into different phases as follows: gliding, takeoff, transient flight, actual flight, flare-out and landing. It has been suggested that in the phase of transient flight the drag should be at its minimum, during actual flight the lift-to-drag ratio a tits maximum, and in the flare-out phase the lift at its maximum. The purpose of the tunnel measurements was to investigate ski jumpers attempting to reach their minimal drag of the gliding position in the up-hill and individual maximal lift-to-drag ratio and achieve the optimal flying position with a low pitching moment coefficient for the said phase according to the feeling from their sensor motor feedback system. The jumpers were attached to an overhead three-component platform-balance with a modified "seat" and belts on the abdominal side of the hip. The system was pivoted close to the center of gravity in order to allow the jumper to adjust the angle of attack. A 3 dimensional flight posture model with sixteen points was created for the measurements. In the flight phases when the lift-to-drag ratio was at its maximum, the individual averaged positions for selected sample times were calculated for the angle of attack of skis, lower legs and upper body. The sweep angle of the skis (V - angle ) and the distance of the ankle joints were also calculated. The deviation was large in all measured variables. The height and "a real mass ratio" of the jumper had a positive effect on the lift-to-drag ratio. Positive regression coefficients for the lift-to-drag ratio in respect to the average values were found in the angle of attack of the skis, trunk and V-angle. The low pitching moment indicated that it was relatively easy to find a stable flying position in the wind tunnel. Therefore, wind tunnel conditions could be useful in training basic flying technique for a longer period of time. A modified Aquila simulation program for the center of gravity of the jumper-equipment system was developed to work in Excel. The input variables were as follows: air density, initial velocity at starting gate, mass of the jumper-equipment system, friction coefficient, aerodynamic reference area, takeoff force, aerodynamic parameters during gliding phases on the track and in the air, sensitivity of lift-to-drag ratio for head and side winds, profile of the jumping hill starting gate position and head and side wind velocity. The output variables were as follows: the tangential and normal component of the velocity at the takeoff table, resultant velocity, release angle, instantaneous x-, y- and z- coordinates, v x-, v y- and v z- components and resultant velocity as a function of time. The length of a jump was defined as the distance travelled when the path of the jumper's center of gravity intersected the profile the of the hill. In a hill of K = 120 m, the relative difference between the simulated and measured jumps was on average 2.8 %. The largest difference between the measured jump and simulated one could be 10 - 20 m, if the wind velocity was between 1.0 m s-l following and 3.0 m s-1 head wind. It can be concluded that the simulation model of center of gravity was in agreement with real jumps

RELATIONSHIPS OF SUCCESSFUL MANOEUVRES IN SELECTED MATCH AND TEST CONDITIONS IN JUNIOR ICE HOCKEY PLAYERS

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KINEMATICS OF CLEAR IN JUNIOR BADMINTON PLAYERS

Introduction the motion of rotating body segments of different shots in badminton can be described in terms of angular position, displacement, velocity or acceleration. The linear velocity of the rotating racket hitting the shuttle is directly proportional to the sum of both the angular velocity and the radius of rotation of the consecutive body segments in badminton strokes (Lee 1993). The timing of these consecutive rotational movements is important in relation to the hit of the racket with the shuttle (Gowitzke 1979).The linear momentum of the clearing arm and racket transfers to the shuttle according to the analogy of the force impulse and the change of linear momentum. The purpose of the present preliminary research was to' study the release velocity of the shuttle in maximal clear placing the shuttle on the opponent scourt in junior badminton players, to explain the produced angular and linear velocities and accelerations in the racket, hand, forearm, upper arm and trunk. METHODS Ten volunteer junior badminton player strained to perform maximal clears on the court. For motion analysis subjects performed a minimum of five successful clears. The fastest clear of every subject was selected for detailed analysis. Anthropometric data (Mean+S.D.) for the subjects was as follows: age 12.223.3years, height 1.562k0.137 m and mass46.85 11.0 kg. On average, they had training 1.851.7 years in badminton. Each clear was recorded for 3 D analysis with NAC 400 (side view) high speed video(100 fps) and Magnavox (back view)camcorder (60 fps). The optical axis of the cameras were perpendicular. The calibration scaling frame was rectangular with the dimensions of 2.0 m x 2.0-m x3.0 m. The same racket and shuttle were used in all measurements. An APAS was used to process frame crabbing, digitizing, smoothing (DLT) and transformation. The mechanical model of trunk, head, upper arm, lower arm and hand (twelwe points) was combined with the racket (four points) and shuttle. A descriptive analysis was performed on the differences in instantaneous positions, linear and rotational velocities. RESULTS The maximal release velocities of the shuttle were in the youngest subject (8years) and oldest one (19 years) 20.2and 56.0 ms-', respectively. The maximal linear velocities in the youngest and oldest subject were as follows: racket head 15.9 and 44.0 ms-' , wrist 4.7 and10.8 ms-', elbow 2.5 and 5.9 ms-' and shoulder 1.6 and 2.9 ms-', respectively. CONCLUSION In conclusion, the most experienced subject produced high speed for the shuttle through the kinematic chain of body segments with high angular and linear velocity through the impact of the shuttle and racket. The correct timing pattern of the consecutive body segments was also observed. The lengths and high angular velocities of the body segments were advantageous features for badminton players. REFERENCES Gowitzke, B.A. (1979) In: Science in Racquet Sports. J.Terauds (ed.) pp. 7-1 5.Del Mar, CA: Academic Publishers.Lee, K.B. (1993) In: Biomechanics inSports XI, J. Hamill. T.R. Derrick & E.H.Elliott (eds), pp. 239-242, University of Masschusetts, Amherst, MA

Aerodynamics Of Soccer Balls And Volleyballs

The purpose of this study was to investigate range, flight time, speed and trajectory of selected official soccer balls and volleyballs in a constant release angle (18 degree) shot by a ball gun with a hydraulic shooting steel leg mechanism in an indoor soccer hall. Five soccer balls (m=0.422 ± 0.010 kg, d=0.108 + 0.002 rn, p=(0.69 + 0.01) x 105 Nm-2) and five volleyballs (m=0.278 ± 0.006 kg, d=0.105 + 0.002 rn, p=(0.69 ±0.01) x 105 Nm-2) were used. Three speeds of the kicking steel leg were applied to the stationary and rotating balls with two constant angular velocities (45.6 and 65.0 radls). All shots of the balls were recorded with two JVC 707 5-VHS camcorders (50 Hz) for kinematic and kinetic motion analysis. An APAS performance analysis system was used for detailed 3D analysis of the release phase of the shot. The range of the ball was measured in x-z coordinates on the Astroturf ground. A Panasonic camcorder with timer was used to measure the flight time of the ball. A 3- way ANOVA was applied in order to study the differences of the balls and their behaviour in the placid air. The release velocities of the stationary soccer balls with three speed categories .were 18.8 + 0.1 ms·1, 23.1 ± 0.1 msand 26.5 ± 0.1 ms-1 and in volleyballs 20.5 ± 0.2 ms-I, 25.6 ± 0.2 ms-1 and 29.4 ± 0.2 ms-1, respectively. The range of the fastest shot off the soccer balls was on average 44.8 m with the lateral deviation of 3.3 degrees. The average range of the volleyballs in the fastest shots without spin was 45.2 m with the lateral deviation of 4.8 degrees. The average ground speeds in the fastest shots of the soccer ball and volleyball were 20.6 ms-1 and 24.7 ms-1, respectively. With the fastest spin and shot the range and deviation of soccer balls and volleyballs were 34.4 m and 38.6 m and 19.7 and 21.4 degrees, respectively. The main effects of the ball type, shot speed and spin were significant (

MANAGEMENT SYSTEM FOR QUALITY COACHING IN TEAM GAMES

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Social Role in Organizational Management - Understanding People Behavior and Motivation

The aim of this work is to respond to the need to rethink the behavior and motiva-tion of employees in their relationship with managers and social groups, i.e., one`s main goal is based on increasing engagement in order to reach organiza-tional goals and job workers satisfaction, a complex concept that is influenced by different causes. Indeed, in this work it is analyzed the impact of working condi-tions on job satisfaction. This is where attention is drawn to the concept of entro-py, since we are not focusing on the value a variable can take, but on the effort that has been expended to obtain it. The idea of entropy comes from a principle of thermodynamics dealing with energy. It usually refers to the idea that everything in the universe eventually moves from order to disorder, and entropy is the meas-urement of that change, that is used here to understand and assess the workers behavior and motivation. The subsequent formal model is based on a set of logi-cal structures for knowledge representation and reasoning that conform to the above entropic view, then leading to an Artificial Neural Network approach to computation, an archetypal that considers the motive behind the action

Ethnic In-Group Favoritism Among Minority and Majority Groups: Testing the Self-Esteem Hypothesis Among Preadolescents

The self-esteem hypothesis in intergroup relations, as proposed by social identity theory (SIT), states that successful intergroup discrimination enhances momentary collective self-esteem. This hypothesis is a source of continuing controversy. Furthermore, although SIT is increasingly used to account for children’s group attitudes, few studies have examined the hypothesis among children. In addition, the hypothesis’s generality makes it important to study among children from different ethnic groups. The present study, conducted among Dutch and Turkish preadolescents, examined momentary collective self-feelings as a consequence of ethnic group evaluations. The results tended to support the self-esteem hypothesis. In-group favoritism was found to have a self-enhancing effect among participants high in ethnic identification. This result was found for ethnic majority (Dutch) and minority (Turkish) participants.

From 2D leg kinematics to 3D full-body biomechanics-the past, present and future of scientific analysis of maximal instep kick in soccer

Biomechanics investigation on soccer kicking has a relatively long history, yet the body of knowledge is still small. This paper reviews articles published from 1960s to 2011, summarizing relevant findings, research trends and method development. It also discusses challenges faced by the field. The main aim of the paper is to promote soccer kicking studies through discussions on problem solving in the past, method development in the present, and possible research directions for the future