“摩擦角”知识的应用

物体以某一初速度沿倾角为θ的斜面向下匀速运动，则μ=tanθ，同学们都懂，但却不能灵活应用，原因是没有深刻理解它的实质，本文将探讨“摩擦角”知识。     

自锁现象在螺旋千斤顶中的应用

当放置在斜面上的物体不会自行下滑时,斜面对于水平面的最大倾角就是该物体与斜面之间的摩擦角,用ψ表示.     

覆杯实验的科学解释

有句格言说"首先发现水的绝不是鱼."是的,同样道理,人类认识大气压强的存在经历了漫长的时间.能直立行走使用工具的人在地球上出现距今已有二三百万年,可是直到360年以前的1643年才由托里拆利(E.Torricelli,1608～1647)第一次揭示了大气压强的真实存在,并测定了其值与76 cm高水银柱的压强相当.     

容器底压强如何变化

问题 :容器中盛有部分液体 ,当其温度升高发生膨胀时 ,容器所受压强将如何变化 ?下面分几种情况分析 :一、当容器为圆柱形时 ,如图 1所示 .设容器中盛有质量为ｍ的液体 ,深为ｈ ,底面积为Ｓ .则液体密度为 :ρ1=ｍＳ·ｈ容器底所受压强为ｐ1=ρ1·ｇ·ｈ =ｍ·ｇＳ液体受热膨胀     

动力学中的“独立”与“叠加”

在动力学中，有两个独立性原理：“力的独立作用原理”和“运动的独立性原理”．将这两条原理结合起来，认识力的分解和运动的分解之间的相关性，对于我们解决那些因物体受外力作用而做较为复杂的曲线运动的问题或因短暂冲击而改变运动方向等方面的难题，是十分必要的．     

螺旋测微器演示仪

介绍自制螺旋测微器演示仪的原理、制作及使用方法。     

Frictional shear cracks

We discuss crack propagation along the interface between two dissimilar materials. The crack edge separates two states of the interface, “stick” and “slip.” In the slip region, we assume that the shear stress is proportional to the sliding velocity; i.e., the linear viscous friction law is valid. In this picture, the static friction appears as the tile Griffith threshold for crack propagation. We calculate the crack velocity as a function of the applied shear stress and find that the main dissipation comes from the macroscopic region and is mainly due to the friction at the interface. The relevance of our results to recent experiments, Baumberger et al., Phys. Rev. Lett. 88, 075509 (2002), is discussed.     

Jack Polynomials in Superspace

This work initiates the study of orthogonal symmetric polynomials in superspace. Here we present two approaches leading to a family of orthogonal polynomials in superspace that generalize the Jack polynomials. The first approach relies on previous work by the authors in which eigenfunctions of the supersymmetric extension of the trigonometric Calogero-Moser-Sutherland Hamiltonian were constructed. Orthogonal eigenfunctions are now obtained by diagonalizing the first nontrivial element of a bosonic tower of commuting conserved charges not containing this Hamiltonian. Quite remarkably, the expansion coefficients of these orthogonal eigenfunctions in the supermonomial basis are stable with respect to the number of variables. The second and more direct approach amounts to symmetrize products of non-symmetric Jack polynomials with monomials in the fermionic variables. This time, the orthogonality is inherited from the orthogonality of the non-symmetric Jack polynomials, and the value of the norm is given explicitly.     

理解磨擦起电应注意的三个问题

物理课本一般在介绍摩擦起电时,总引入这样的话:用丝绸摩擦过的玻璃棒带正电,用毛皮摩擦过的橡胶棒带负电.学生学习过程中,容易对此产生片面的理解,因此教师应引导学生澄清三个概念.     

Frictional dissipation in stick-slip sliding

The time variation of the frictional force between two surfaces, undergoing stick-slip sliding across a molecularly thin film of a confined model liquid, was examined at high time and force resolution, showing clearly that dissipation of energy occurs both during the slip, and at the instant of stick (via transfer of residual momentum). Detailed analysis indicates that, in marked contrast to earlier suggestions, of order 90% or more of the dissipation occurs by viscous heating of the confined shear-melted film during the slip, and only a small fraction of the energy is dissipated at the instant of stick.     

Frictional coupling between sliding and spinning motion

The tangential motion at the contact of two solid objects is studied. It consists of a sliding and a spinning degree of freedom (no rolling). We show that the friction force and torque are inherently coupled. As a simple test system, a sliding and spinning disk on a horizontal flat surface is considered. We calculate, and also measure, how the disk slows down and find that it always stops its sliding and spinning motion at the same moment. We discuss the impact of this coupling between friction force and torque on the physics of granular materials.     

Jack Superpolynomials: Physical and Combinatorial Definitions

Jack superpolynomials are eigenfunctions of the supersymmetric extension of the quantum trigonometric Calogero-Moser-Sutherland Hamiltonian. They are orthogonal with respect to the scalar product, dubbed physical, that is naturally induced by this quantum-mechanical problem. But Jack superpolynomials can also be defined more combinatorially, starting from the multiplicative bases of symmetric superpolynomials, enforcing orthogonality with respect to a one-parameter deformation of the combinatorial scalar product. Both constructions turn out to be equivalent.     

Frictional properties of confined polymers

We present molecular dynamics friction calculations for confined hydrocarbon solids with molecular lengths from 20 to 1400 carbon atoms. Two cases are considered: a) polymer sliding against a hard substrate, and b) polymer sliding on polymer. In the first setup the shear stresses are relatively independent of molecular length. For polymer sliding on polymer the friction is significantly larger, and dependent on the molecular chain length. In both cases, the shear stresses are proportional to the squeezing pressure and finite at zero load, indicating an adhesional contribution to the friction force. The friction decreases when the sliding distance is of the order of the molecular length indicating a strong influence of molecular alignment during run-in. The results of our calculations show good correlation with experimental work.     

Frictional cooling: Experimental results

The classical methods used in beam cooling are hard to be adapted for a beam of short-lived elementary particles. A novel method, the so-called frictional cooling – that is cooling a beam of low-energy charged particles by moderation in matter and acceleration in an electrostatic field – has been shown to be feasible. In our experiments performed in 1994/1995 a beam of short-lived particles was cooled for the first time ever. Utilizing frictional cooling on a beam of slow negative muons we observed increase in phase space density by about one order of magnitude. This revised version was published online in August 2006 with corrections to the Cover Date.     

Jack Superpolynomials,Superpartition Ordering and Determinantal Formulas

 We call superpartitions the indices of the eigenfunctions of the supersymmetric extension of the trigonometric Calogero-Moser-Sutherland model. We obtain an ordering on superpartitions from the explicit action of the model's Hamiltonian on monomial superfunctions. This allows to define Jack superpolynomials as the unique eigenfunctions of the model that decompose triangularly, with respect to this ordering, on the basis of monomial superfunctions. This further leads to a simple and explicit determinantal expression for the Jack superpolynomials. Received: 11 May 2001 / Accepted: 16 August 2002 Published online: 7 November 2002 Communicated by R. H. Dijkgraaf