Abstract
In this work, the mechanism behind motion in closed systems is investigated by studying two simple mechanical systems. Firstly, a simple “ballistic” car system introduces the subject of propulsion as a function of a parameter that modulates the amount of ejected mass, thus allowing a transition between a closed to an open system. The second example provides a physically realizable (and optimizable) system, in which a mass sliding on the internal surface of a mechanical wheeled cavity is shown to produce a net movement, provided that dissipation is present. Several additional ways are discussed by which to ‘open’ a system whilst keeping the mass constant. As a function of strict momentum conservation, it is concluded that both internal mass motion and dissipation are key factors in the only way a mechanical system can move itself in a weak sense, in contrast to a strong meaning of ‘self-propulsion’.
Highlights
The subject of propulsion is traditionally introduced to students as a corollary of Newton’s third law of motion and momentum conservation of a system with variable mass [1]
It is clear that the subject of propulsion and space travel provides a strong motivation for students to delve into the meaning of many fundamental laws of mechanics, such as momentum and energy conservation
The purpose of this paper is to present students a treatment of some simple mechanical systems that inspire the application of physics to the meaning of reactionless motion, i. e.: the possibility of motion without minor mass-ejection or system-splitting
Summary
The subject of propulsion is traditionally introduced to students as a corollary of Newton’s third law of motion and momentum conservation of a system with variable mass [1]. New mechanisms have been proposed to circumvent the inherent inefficiency of this principle (as applied to rockets), in the context of interstellar trips [2,3] and to explore ‘reactionless motion’ (the idea of a self-propelled engine or the so-called ‘space drive’ [4, 5]). As a concept, this kind of device embodies part of the hope for interstellar voyages because, using the rocket principle, it is impossible currently to carry all of the fuel necessary to reach the star (and return). It is clear that the subject of propulsion and space travel provides a strong motivation for students to delve into the meaning of many fundamental laws of mechanics, such as momentum and energy conservation
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