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In order to model natural phenomena, a faithful model describing the evolution of the analyzed phenomena must consider as many aspects as possible. An advantage of a correct representation of the studied phenomenon is the highlighting of the aspects hard to notice initially. To support these affirmations, two examples are brought into attention, from the dynamics of a point and from spatial kinematics respectively. Starting from the idea of improving a vertical falling sphere viscometer, a model of rotational viscometer is proposed. The viscosity is determined when the resultant torque is zero. From the equation of dynamic equilibrium for rotational motion two values for viscosity are obtained, both probable in equal manner. The concern of the paper is to find a criterion for the selection of the correct solution and also to explain the meaning of the other root.
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