Preparation in classical physics

 
Investigations in classical physics require that the physical to be investigated must be prepared of brought to a definite state. The experimenter must produce well-defined initial conditions. Only then are the results of measurements reproducible.
 

If, for example, the law of motion for the horizontal throw of a ball is to be tested a device has to be set up which makes it possible to throw balls with definite initial values of position and velocity. The device shown in the figure prepares balls with the two properties "is located at the position (x0, y0)" and "has the initial velocity (vx0,vy0)".

This example demonstrates a main difference between classical and quantum mechanics. In classical physics, there is no principle obstacle for preparing simultaneously position and velocity (momentum) in the same direction.In quantum mechanics, there are fundamental limits of nature placed upon the simultaneous preparation of certain pairs of properties. This is described by Heisenberg`s uncertainty principle.

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