Читать книгу Tennis Method - Defined Timing - Siegfried Rudel - Страница 8

If it proves to be meaningful to consider the ball as the object of perception, what shall and can be perceived of this object must be considered. The possibilities of perception are infinite, even if perception is focussed on the ball. For example, the player can concentrate on the colour, the round form, the shadow of the ball, or on how the ball seems to become bigger or smaller during its flight, or on the rotation of the ball, etc. These are possible perceptions whose selection is, among other things, determined by the wish to find a describable relationship between the movement of the racket face and the movement of the ball. The ball moves in a curve which is a spatial presentation in time and a physical fiction, an imagined line by means of which movements can be can be described. If the player lets his actions be guided by this ball curve, the question occurs whether he is able to do so. Can he perceive the curve of the ball?

V.v, Weizsäcker has shown that the movements which man perceives are not always correspondent with reality, i.e., it is not possiblefor man to perceive arbitrary movements objectively. But, "the.perceiving eye behaves as if it were aware of this law - one could allegorically say - as if it were a mathematician or a physicist". - "We call this behaviour nomophily or nomotrophy ..." (V.v. Weizsäcker 1973, 13). Elsewhere we read: "Perception behaves as if there were a world existing of only two bodies in an empty room which follow the law of gravitation. The eye perceives what physically would be possible" (V.v. Weizsäcker 1973, 264). This possibility theorem means that it is useful to folow the physical principle because this represents one possible way of perception. Since human perception even behaves in such a way if the movement does not follow this principle, it is necessary to examine in how far the real movement, i.e. in this case the movement of the ball, fulfills or follows the law of gravitation. The objective must be to discover the law of gravitation in reality in order to establish a connection between a fact and the perception of this fact. In order to do this, the objective ball behaviour must be looked at. By objective the presentation of certain phenomena under physical conditions is meant. Newton formulates the following laws:

K = m x b bzw. G:= m x g

This means that in a vacuum all things, even if their weight is different, fall to the ground with identical velocity (see the comparison between a feather and a ball). This law holds valid independent of an inertial system moving at a constant speed. What does this mean? A ball which is dropped from a certain height falls with the same velocity as a ball which is dropped from the same hight inside a moving train. An outside observer, however, does not see the vertical fall, but a throwing/flying parabola if the train moves at a constant speed. This is the principle of the independence of translation movements. The importance of the invariance of gravitation and the independence of translation movements can be illustrated by a further example. In figure 2, three ballistic ball curves with different horizontal velocities are shown. The balls are shot off horizontally at the same time, the curves having an identical maximum.

Figure 2: Ballistic ball curves of different horizontal velocities

The first ball is only dropped (VH1 = 0), the second ball is shot off at a slow horizontal velocity (VH2) and the third ball at a higher horizontal velocity (VH3). The silhouettes of these three balls, which are created on a screen which is placed perpendicularly to the level of the ball curve by parallel light coincide with each other. The pictures of the two balls shot off at different horizontal velocities are identical with the movement of the ball which only falls. The balls are at the same time at identicaI heights! The time available to the player is only dependent on the vertical distance covered by the ball.

This phenomenon is expressed in the fall law

H = g/2 t2

and means that the falling movement takes place independently of the respective horizontal velocity.Thus the horizontal component can be looked at separately from the vertical component, Another finding is that a body which is only acted upon by small forces or no forces at all, remains in a state of uniform movement. Since the gravitation force only acts vertically, i.e. perpendicularly to the centre of the earth, and there is no influence of any horizontal force, the horizontal velocity remains constant with such throwing/flying curves (definition of a parabola).

What do these laws mean for the movement of the tennis ball ? If one compares two balls, the one being smoothly struck vertically into the air, the other being hit hard across the field, one finds that with the flight height of the balls being identical the duration of the flight - from striking until landing -is also identical. This means that the time available or striking the ball can be determined by the vertical distance covered by the ball.

This is not the place to explain in detail what deviations the ball is submitted to under consideration of the distance-time behaviour. However, basically it can be said of the so-called ballistic curves (taking into account of friction losses in the air and on the ground with a given form of the ball) that the horizontal velocities of the tennis ball are high as compared with the vertical velocities (exception: high lobs). The reason for this is that the length of the tennis court is very great, whereas the height of fall is not very considerable. A consequence of this is that friction hardly influences the vertical movement of the ball, whereas the horizontal velocity of the ball is considerably reduced by the air resistance (R v2). Although friction proportionally grows with the square of velocity, the decrease in ball velocity is insignificant (up to 4 %). The loss in vertical height, however, is almost identical. Since, however, only the vertical movement is interesting for time orientation, the 'ideal' law of gravitation is almost fulfilled.

Things are different with balls with forward spin (drive/topspin) or backward spin (slice). The so-called ´Magnus effect´, which is characterised by a combination of very high horizontal velocities and high rotational velocities (identical level of direction), changes the behaviour of the ball. In this case the vertical distance-time-law deviates from this phenomenon, and with the simultaneous appearance of spin, the horizontal velocity is responsible for the ball falling to the ground faster or slower. A physical explanation for this is that gravitation is superimposed upon by another force which has its origin in the spin. This means as far as perception is concerned that one sees the ball rising or falling as if in a slow-motion or fast-motion film. Such a ball with 'higher gravitation' has to be corresponded to by the movement of the racket face with a changed vertical velocity. It must be mentioned here that e,g. the topspin, which falls down faster and thus helps to save time, later loses this time again by reaching a higher maximum during bounding (later as far as time is concerned). The importance of this physical fact is that it makes possible for the player to let his movements as far as time and space are concerned be guided by the vertical movements of the ball.

In accordance with the possibility theorem of perception, which implies the idealised movement, and the invariance of gravitation, which even exists under ballistic conditions, there is the possibility to determine the relationship between object and subject as regards time and space. 'Timing' can be defined. Perception and movement can be related to each other. The structural problem of form is solved with the example of a concrete movement. The required unit of perception and movement (see Rudel 1977, teaching film) can be expressed in a 'graspable' relationship.

In order to explain once again the connecting character of gravitation as far as perception and movement are concerned, it must once more be stated which perception and which movement are meant. The movement is the flight of the ball with its invariance in the vertical aspect and the movement af the racket face. They are connected in the 'guiding-beam movement' of the racket face, in the 'sticking', in the simultaneous 'drawing', or whatever image one uses.