动量守恒定律和角动量守恒定律辨析

辨析了系统动量守恒和角动量守恒的适用条件, 旨在加深学生对动量守恒定律和角动量守恒定律的 理解. 以3个例题为例, 根据实际条件选择参数, 计算了系统的内力和外力的数值, 分析了系统所受合外力( 外力力 矩的矢量和) 远小于系统内力( 内力矩) 的条件. 结果表明, 只定性地根据物体的质量很小就得出物体的重力远小于 系统内力的结论是不充分的, 质量很小的物体的重力是否远小于系统的内力还取决于碰撞前物体的速度; 而外力 力矩的矢量和是否远小于内力矩不仅取决于碰撞前物体的速度, 还取决于碰撞的位置     

车轮上的物理——静摩擦力扮演的角色

从牛顿运动定律和力矩做功等知识,分析汽车在启动时,发动机产生的内力转换成牵引力时静摩擦力起的作用,静摩擦力的大小与汽车扭矩的关系,以及静摩擦力在转换汽车内力做功产生的能量过程中扮演的角色.     

绳与有质量的定滑轮的作用

定滑轮模型是物理中常见的模型之一,中学物理中一般不要求考虑滑轮的质量,而普通物理部分,在学生学习过刚体转动的有关知识后,可以逐步考虑滑轮质量对运动的影响.如文献[1～3]就例举到"绳跨过有质量的定滑轮"问题.但在计及定滑轮的质量时,上述文献认为,滑轮两侧的绳张力大小不同,差值提供滑轮转动的力矩.笔者认为下此结论似乎过于匆忙,张力应是物体内部各部分由于形变而产生的内力,绳与滑轮是两个物体,它们间并不能形成张力,两者之间只有因挤压形变产生的在滑轮法向的弹力和沿切向的静摩擦力.显然,法向的弹力通过滑轮的转动点,是不能够产生力矩的,那么,只有静摩擦力才是滑轮的力矩的真正产生者.     

磁针偏转佯谬与Trouton-Noble零实验

在均匀直线电荷附近本来平行静置的磁针，对于在其中运动的参考系却 “因受外力矩而偏转”。用等效磁荷观点不仅圆满地解决了这一表观佯谬，而且还给出了普适的Lorentz力矩张量表达式，后者可对Trouton-Noble零实验的“偏转”力矩作出直接描述。     

水平滚筒内二元颗粒体系径向分离模式的数值模拟研究

发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序，对薄滚筒内二元S型颗粒体系进行了数值模拟，发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素，讨论了滚筒转速、颗粒装载率等参数对分层的影响，当转速较高时，滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式，当转速较低时形成具有波浪形界面的花瓣模式，并且随着滚筒转速的逐渐降低，花瓣的数量逐渐增加，数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形     

磁致伸缩棒力矩激励器动态特性的研究

介绍了一种能通过直接记录激振力来获得激振力矩的磁致伸缩棒力矩激励器的原理及其动态特性的理论分析和实验测试．首先建立了这种磁致伸缩棒力矩激励器动态特性的力学模型和相应的微分方程，推导了输出激励力矩的数学表达式，然后分析了影响磁致伸缩律力矩激励器输出力矩的各种因素，从而为改进这种磁致伸缩棒力矩激励器的结构设计提供了理论依据．之后，对一个磁致伸缩棒力矩激励器原型的输出激振力、输出激振力矩的纯度和频率特性及其互易关系进行了实际测试，其测试结果与理论分析非常吻合。最后还对影响测量背景噪声的因素进行了讨论．     

水平滚筒内二元颗粒体系径向分离模式的数值模拟研究

发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序,对薄滚筒内二元S型颗粒体系进行了数值模拟,发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素,讨论了滚筒转速、颗粒装载率等参数对分层的影响,当转速较高时,滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式,当转速较低时形成具有波浪形界面的花瓣模式,并且随着滚筒转速的逐渐降低,花瓣的数量逐渐增加,数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形 关键词： 分层 滚筒 模式形成 离散单元方法     

气动负载虚拟加载系统设计

飞行器在飞行过程中,舵机将承受到空气动力的力矩载荷,气动负载虚拟负载模拟器是用于模拟空气力矩载荷变化规律的地面半实验设备。针对在地面试验过程中,传统负载模拟器与承载系统因机械结构耦合而产生的“多余力矩”问题,以磁粉离合器和力矩传感器为核心,构建虚拟负载模拟器,利用系统输出转矩的大小和方向分别只与励磁电流的大小和速度滑差的方向有关的特性,较彻底地解决“多余力矩”的问题,提高力矩加载试验设备的性能,从而进一步提高飞行器虚拟试验的水平。     

"整体法"理念在力学解题中的运用

物理解题方法中有隔离法和整体法,而整体法思维方式在物理解题中若能正确运用,则解题会得心应手．整体法是在一个物理过程中把两个或者多个物体看成一个整体;这样整体内部的物体之间的所有作用力都是内力,整体以外的物体对它的作用力都叫外力．应用整体法时应注意“系统”的选取、“内力”和“外力”的确定和分析．     

“十”字形自驱单元形成寡聚体的动力学行为

探索自驱粒子形状对自组装结构和动力学的影响是软物质研究的前沿课题.组装基元形成的寡聚体及其动力学是大量粒子形成组装结构的基础.本文设计了一种“十”字形自驱粒子,发现其可以形成数种不同构型的寡聚体,计算了寡聚体(二、三、四聚体)的均方位移、角速度、角速度分布概率、轨迹曲率分布概率等.寡聚体的运动行为可分为两类:一类是合力为零但力矩不为零,寡聚体进行小半径的偏心旋转;另一类是合力不为零力矩也不为零,寡聚体呈现大半径的偏心旋转.寡聚体的平动动力学在短时间尺度(大致约为1—2 s,与其角速度有关)都呈现超扩散现象,但转动速度受寡聚体结构影响;力矩越大,转动惯量越小,角速度越大.对于三聚体,轨迹曲率与角速度有关,角速度越大,曲率也越大.     

磁流体中Helmholtz和Kelvin力的界定

磁流体磁彻体力的两种简化形式Helmholtz力和Kelvin力具有一定的适用范围.在推导磁流体中的磁彻体力表达式基础上,分析Helmholtz力和Kelvin力在磁流体中的起源,得出两种形式的成立条件.计算结果表明:当磁流体磁导率与外磁场强度无关时,磁流体磁彻体力可由Helmholtz力表示;当磁流体中磁性颗粒的平均磁矩与磁流体比体积无关时,Kelvin力为磁彻体力的简化形式;在磁流体磁化系数与其密度成正比情况下,Helmholtz力可转换为Kelvin力. 关键词： 磁流体 磁彻体力 Helmholtz力 Kelvin力     

Moment and forces exerted on the inner cylinder in eccentric annular flow

This paper presents results of numerical modeling for analysis of the moment and forces exerted on an eccentrically positioned rotating inner cylinder due to the annular flow between two cylinders with parallel axes. Laminar stationary fully developed flows of Newtonian and power law fluid flows are considered. An impact of annulus geometry, flow regime, and fluid characteristics are studied. The study indicates that the moment exerted on the inner cylinder increases monotonically with the eccentricity. Forces acting on the inner cylinder include pressure and viscous friction. The pressure forces provide a predominant contribution. When eccentricity does not exceed a certain critical value, the radial force pushes the inner cylinder to the channel wall. When eccentricity is large enough, the radial force reverses its sign, and the inner cylinder is pushed away from the outer wall. Circumferential component of the force has always the same direction and induces precession of the inner cylinder.     

Equilibrium properties of classical systems with long-range forces. BBGKY equation,neutrality, screening,and sum rules

We introduce a generalization of the BBGKY equation to define the equilibrium states for systems with long-range forces and study the properties of such states. We show that there are properties typical of short-range forces (shape independence, normal fluctuations, asymptotic behavior of correlation functions) and others which are typical of long-range forces (possible shape dependence, neutrality, sum rules and screening, abnormal fluctuations, boundedness of the internal electric field). If the force decreases at infinity faster than the Coulomb force, the properties will be those typical of short-range forces; on the other hand, if the force decreases at infinity as the Coulomb force or slower, the properties will be those typical of long-range forces.     

外电场作用下三元环状水分子团簇的特性研究

采用了HartreeFork自洽场方法在321G水平上研究了0～0.01a.u.的外电场对三元环状水分子团(H2O)3基态能量和电偶极距的影响规律,分析了由外电场产生的HellmanFeynman(HF)力(内力和外力)和在这个力的平衡作用下的团簇的稳定结构,结果表明,外电场的大小对三元环状水分子团簇(H2O)3这些性质有明显的影响.通过高斯计算可以得出,在所加的电场范围内,随着外电场的增大总能量下降,电偶极距增大,结构相对更稳定;外电场引起团簇中各个原子的电荷密度改变,导致了各个原子所受的静电力随之变化.     

磁场对载流线圈的作用

根据力系简化原理,载流线圈在磁场中所受之作用可归结为作用在简化中心的合磁力与对简化中心的合磁力矩,本文推证了合磁力和合磁力矩的计算公式。     

A new approach to the evaluation of Casimir and van der Waals forces in the transition region

This paper studies the incorporation of Casimir and van der Waals forces applied to a nanostructure with parallel configuration. The focus of this study is in a transition region in which Casimir force gradually transforms into van der Waals force. It is proposed that in the transition region, a proportion of both Casimir and van der Waals forces, as the interacting nanoscale forces, can be considered based on the separation distance between upper structure and substrate during deflection. Moreover, as the separation distance descends during deflection, the nanoscale forces could transform from Casimir to a proportion of both Casimir and van der Waals forces and so as to van der Waals. This is also extended to the entire surface of the nanostructure in such a way that any point of the structure may be subjected to Casimir, van der Waals or a proportion of both of them about its separation distance from the substrate. Therefore, a mathematical model is presented which calculate the incorporation of Casimir and van der Waals forces considering transition region and their own domination area. The mechanical behavior of a circular nano-plate has been investigated as a case study to illustrate how different approaches to nanoscale forces lead to different results. For this purpose, the pull-in phenomena and frequency response in terms of magnitude have been studied based on Eringen nonlocal elasticity theory. The results are presented using different values of the nonlocal parameter and indicated in comparison with those of the classical theory. These results also amplify the idea of studying the mechanical behavior of nanostructures using the nonlocal elasticity theory.     

Levitation and lateral forces between a point magnetic dipole and a superconducting sphere

The dipole–dipole interaction model is employed to investigate the angular dependence of the levitation and lateral forces acting on a small magnet in an anti-symmetric magnet/superconducting sphere system. Breaking the symmetry of the system enables us to study the lateral force which is important in the stability of the magnet above a superconducting sphere in the Meissner state. Under the assumption that the lateral displacement of the magnet is small compared to the physical dimensions of our proposed system, analytical expressions are obtained for the levitation and lateral forces as a function of the geometrical parameters of the superconductor as well as the height, the lateral displacement, and the orientation of the magnetic moment of the magnet. The dependence of the levitation force on the height of the levitating magnet is similar to that in the symmetric magnet/superconducting sphere system within the range of proposed lateral displacements. It is found that the levitation force is linearly dependent on the lateral displacement whereas the lateral force is independent of this displacement. A sinusoidal variation of both forces as a function of the polar and azimuthal angles specifying the orientation of the magnetic moment is observed. The relationship between the stability and the orientation of the magnetic moment is discussed for different orientations.     

Quantitative measurement of tip-sample forces by dynamic force spectroscopy in ambient conditions

We introduce a dynamic force spectroscopy technique enabling the quantitative measurement of conservative and dissipative tip-sample forces in ambient conditions. In difference to the commonly detected force-vs-distance curves dynamic force microscopy allows to measure the full range of tip-sample forces without hysteresis effects caused by a jump-to-contact. The approach is based on the specific behavior of a self-driven cantilever (frequency-modulation technique). Experimental applications on different samples (Fischer-sample, silicon wafer) are presented.     

Relativity,thermodynamics and entropic forces

A recent assertion that inertial and gravitational forces are entropic forces is discussed. A more conventional approach is stressed herein, whereby entropy is treated as a result of relative motion between observers in different frames of reference. It is demonstrated that the entropy associated with inertial and gravitational forces is dependent upon the well known lapse function of general relativity. An interpretation of the temperature and entropy of an accelerating body is then developed, and used to relate the entropic force to Newton's second law of motion. The entropic force is also derived in general coordinates. An expression of the gravitational entropy of in‐falling matter is then derived by way of Schwarzschild coordinates. As a final consideration, the entropy of a weakly gravitating matter distribution is shown to be proportional to the self‐energy and the stress‐energy‐momentum content of the matter distribution.     

Analysis of the forces acting on the saltating particles in the coupled wind-sand-electricity fields

Based on the theoretical model describing the saltation of sand particles in the coupled wind-sand-electricity fields, the numerical simulations of the forces acting on saltating particles, such as the aerodynamic drag force, Magnus effect, Saffman force and electrostatic force, are analyzed in comparison to the gravity force of the particles in the steady windblown sand movement. Furthermore, the laws of the above forces vary with the friction velocity, the diameter of the sand particle, the initial angular velocity and the lift-off velocity are discussed. Supported by the National Natural Science Foundation of China (Grant Nos. 10772075 and 10772074), the Key Project of the National Natural Science Foundation of China (Grant No.10532040), and the Program for New Century Excellent Talents in University (Grant No. NCET-04-0979)