LITMIR.BIZ

      Figure 1: The ball as the object of perception

      1.1 The ball as the object of perception

      It needs no further consideration to identify the ball as the object which the tennis player must focus his eyes on and which he must observe (figure 1). in spite of this, the teaching method which is still used today pretends that the ball does not exist. Although all the player's movement sequences are described in detail, he is kept 'blind'. The instruction "Look at the ball!" only shows the uselessness of this method, which does not make it possible to relate the player's movement form to his perception, which takes place simultaneously. Only if it were possible to establish this relation between the movement of the racket face and the movement of the ball, would the demand to consider perception and movement always as a unit be fulfilled. This means that the movement of the racket face must be related to the movement of the ball, which is the object of perception as far as space and time are concerned. Thus, timing is placed at the centre of attention!

      1.2 Ball behaviour - gravitation - possibility theorem of perception

      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.