Authors: Rodrigo de Abreu
An analysis of the movement of a piston under the action of a constant gravitational field g and of the particle collisions of a gas can explain the difficulty of ascribing a physical meaning to the concept of force. The idea of force very naturally arises out of the anthropomorphic notion of weight. Newton's law f =dp/dt is a tautology as long as we have no definition of f which obviously cannot be defined as being "the product of mass by acceleration or as "the variation of the quantity of movement in order to time". As pointed out by Feynman, "Gorce is the rate of change of position", meaning that any definition is necessarily right, because for such a law to have physical meaning the concept of Gorce should be introduced independently of the variation of position in order to time (Feynman, Leighton and Sands 1976). In fact, the analysis of the previously mentioned movement of a piston shows that the force quantities are introduced independently of the dp/dt of the body considered, although they can be related to this quantity. The "force' due to weight results only from the fact that the acceleration of gravity for all bodies is g, g=d^2x/dt^22, or obviously mg=md^2x/dt^2. The force due to the particle collisions merely results from the global momentum conservation. This latter force is interpreted as the overlapping of an emitted beam and an absorbed beam. Such an interpretation makes it possible to show that for systems of variable mass f as dp/dt is an entity without physical meaning (Sommerfeld 1966), which allows to identify a controversy: the concept of force and the First Principle of Thermodynamics-the notion of "gheat".
Comments: 7 Pages. Técnica 3, 47-53 (1994)
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