ISSN:0002-9505    

DOI:10.1119/1.16635

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.16636

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.16587

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.16588

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

Strictly, Coulomb’s law and the Biot–Savart law determine the electric and magnetic fields for static sources only. Time‐dependent generalizations of these two laws, introduced by Jefimenko, are used here to explore the applicability of Coulomb and Biot–Savart outside the static domain. The fact that Biot–Savart holds for some classes of time‐dependent currents has led to much confusion in the literature, which this paper endeavors to dispel. To illustrate Jefimenko’s equations, the standard Liénard–Wiechert fields of a point charge are derived.

ISSN:0002-9505    

DOI:10.1119/1.16589

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

The dramatic Tacoma Narrows bridge disaster of 1940 is still very much in the public eye today. Notably, in many undergraduate physics texts the disaster is presented as an example of elementaryforcedresonanceof a mechanical oscillator, with the wind providing an external periodic frequency that matched the natural structural frequency. This oversimplified explanation has existed in numerous texts for a long time and continues to this day, with even more detailed presentation in some new and updated texts. Engineers, on the other hand, have studied the phenomenon over the past half‐century, and their current understanding differs fundamentally from the viewpoint expressed in most physics texts. In the present article the engineers’ viewpoint is presented to the physics community to make it clear where substantial disagreement exists. First it is pointed out that one misleading identification of forced resonance arises from the notion that the periodic natural vortex shedding of the wind over the structure was the source of the damaging external excitation. It is then demonstrated that the ultimate failure of the bridge was in fact related to an aerodynamically induced condition ofself‐excitationor ‘‘negative damping’’ in a torsional degree of freedom. The aeroelastic phenomenon involved was aninteractiveone in which developed wind forces were strongly linked to structural motion. This paper emphasizes the fact that, physically as well as mathematically,forcedresonanceandself‐excitationare fundamentally different phenomena. The paper closes with a quantitative assessment of the Tacoma Narrows phenomenon that is in full agreement with the documented action of both the bridge itself in its final moments and a full, dynamically scaled model of it studied in the 1950s.

ISSN:0002-9505    

DOI:10.1119/1.16590

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

The magnetic field dependence of the isotropic nonrelativistic one‐electron atom with nuclear spin‐1/2, in its electronic ground state, is reviewed. Attention is called to the little‐known fact that a level crossing exists (at fieldB∼17 T for1H) between the two members of the upper spin (MS=1/2) doublet. Anisotropy of such a hydrogenic atom, due to the presence of a suitable external electric field (for instance,1H trapped in crystalline SiO2) causes anticrossing of these levels and causes previously forbidden magnetic‐dipole transitions to attain appreciable intensity in thatBregion.

ISSN:0002-9505    

DOI:10.1119/1.16591

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

The equations of motion for a golf ball interacting with a hole on a putting surface are found. A computer is used to predict the outcome of the ball’s encounter with the hole as a function of the impact parameter and the initial velocity of the ball at the rim. If the ball does not skid or bounce when it collides with the opposite rim, it is predicted that a uniform ball must travel at 1.626 m/s or less to be captured; that the British ball is easier to sink than the slightly larger American ball; that a ball with a larger moment of inertia is more difficult to sink; and that bouncing and skidding (factors that vary from green to green) result in capture at greater speeds. Experimental studies support these predictions.

ISSN:0002-9505    

DOI:10.1119/1.16592

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

An oscillatory flow of water is generated when a cup, with a small orifice (ca. 1 mm diam) in its base, and filled with saline water, is placed within an outer vessel containing pure water. This simple system exhibits various nonlinear characteristics, such as a limit cycle, bifurcation of the oscillatory flow, and entrainment among the oscillators. These characteristics can be reproduced by a numerical simulation based on simple nonlinear differential equations.

ISSN:0002-9505    

DOI:10.1119/1.16593

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

The appropriateness of the standard equation of state of a rubber band is studied in an undergraduate laboratory experiment. There are two parts to the experiment: In the first part, a band is stretched at constant temperature; in the second, the band is kept at constant length while heated. The two coefficients in the equation of state of the rubber band are determined in four different ways: numerical differentiation of the data, numerical integration of the data, regression analysis, and linear regression on a ‘‘straight‐line’’ form of the equation of state. The advantages and disadvantages of each technique are discussed. The characteristic constants describing the rubber band appear to vary by up to 15% depending on the technique of analysis employed. The origins of these variations are discussed.

ISSN:0002-9505    

DOI:10.1119/1.16594

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP