Magnetic Levitation and Newton's Third Law

Abstract

Newton's third law is often misunderstood by students and even their professors, as has already been pointed out in the literature.1,2 Application of the law in the context of electromagnetism can be especially problematic, because the idea that the forces of “action” and “reaction” are equal and opposite independent of the medium through which they act can be muddied by the concept of “action at a distance.” While some experiments have been described3,4 illustrating Newton's third law in magnetic situations, these do not offer the student a clear way of evaluating his/her own preconceptions. The experiment we present shows how easily the student, and even the graduate student, can fail to apply the third law correctly in an electromagnetic situation. The experiment described here employs a magnetic levitator and shows the difficulty in recognizing action and reaction forces.

Dramatic (and Simple!) Demonstration of Newton's Third Law

“…5,7,8 One case used magnetic forces in a static situation to measure the reaction force using a scale. 4 In fact, the notion of explicitly measuring the reaction force due to a falling magnet has been reported earlier, 6,11 although the present setup is significantly simpler and the method outlined below provides a clearer indication of the reaction force. This method was inspired by a footnote in Ref.…”

“…Magnetic forces have been utilized before in pedagogical illustrations of Newton's third law. [3][4][5][6][7][8][9][10][11][12] Some of these demonstrations have been primarily qualitative in nature, 3,9,12 while others have used force probes to show experimentally that the forces involved are equal and opposite. 5,7,8 One case used magnetic forces in a static situation to measure the reaction force using a scale.…”

Modelación de sistemas dinámicos de la mecánica clásica

“…Si v 0 > 0, la fuerza magnética está en la misma dirección de F g hasta que el móvil alcanza una altura máxima y velocidad cero; en el descenso del móvil, la fuerza magnética está en la dirección opuesta a F g con una magnitud que aumenta de cero hasta un valor que equilibra el componente de la gravedad, esto es, cuando F g + F m = 0, de forma que el móvil alcanza una velocidad terminal v T , que permite determinar el valor de la constante de resistencia r = −mg sen θ/v T y así, la dependencia de la fuerza magnética con la velocidad. La ecuación de movimiento [22,23] queda determinada por:…”

A simple and effective magnetic dynamometer to teach Newton’s third law

“…Other important applica tions of the third law involve collisions where momentum and energy conservation concepts are P a P e r A simple and effective magnetic dynamometer to teach Newton's third law explored [11]. A very feasible way to exchange forces between objects is by means of magnetic fields, since they avoid the mechanical contact of the objects and their magnitudes can be con trolled by electric current in solenoids or varying distances between permanent magnets [12][13][14]. A remarkable experiment involves the fall of magnets that reach terminal speed inside elec tric conductive tubes, exchanging forces between them via magnetic fields generated by eddy cur rents [15,16].…”

“…A remarkable experiment involves the fall of magnets that reach terminal speed inside elec tric conductive tubes, exchanging forces between them via magnetic fields generated by eddy cur rents [15,16]. Some approaches use familiar objects, like weights and scales, while others use more involved apparatus like frictionless bead tables [11], currentcontrolled solenoids [13,14], digital force probes [12] and Arduino boards [10], just to cite a few. Despite their ability to explore different aspects of the third law, we feel the lack of a proposal that uses really cheap materials and that shows in a very direct way the qualitative and quantitative aspects.…”