Viscosity denotes opposition to flow. It may also be thought of as internal friction between the molecules. Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipelines. It also determines the liquid flow in spraying, injection molding, and surface coating.

黏滞与流动正好相反。又可把黏度看成是分子间的内摩擦。当流体用作滑润剂或在管道中输送时，都必须克服阻力，而黏度则是构成阻力的一个重要因素。它控制着喷涂、注射成型和表面涂覆工艺过程中的流体流动。

属类：自然科学-物理-

In transverse waves, wavelength is measured from crest to crest or from trough to trough. In longitudinal waves, it is measured from compression to compression or from rarefaction to rarefaction. Wavelength, l, is equal to the speed v of a wave in a medium divided by its frequency f, or l = v/f.

通常，在横波中波长是以波峰与波峰或波谷与波谷之间的距离来量度的;在纵波中是以密波与密波或疏波与疏波之间的距离来量度的。波长通常以希腊字母λ表示;波长等于以波的频率f除媒质中波列的速率ν来表示，即λ=ν/f。

属类：自然科学-物理-

The weight is a consequence of Isaac Newton’s universal law of gravitation, which states that the force of attraction between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.

重量是万有引力定律的后果。显然，在同一地点，物体越大，重量就越大;而距离地球越远的物体，重量则越小。尽管物体质量保持不变，重量还是随所在地的不同而异。

属类：自然科学-物理-

For a constant force, work W is equal to the magnitude of the force F times the displacement d of the object, or W = Fd.

如果力是恒量，功就可由沿物体路径方向的分力乘以路径的长度得出。即W=Fd

属类：自然科学-物理-

Work is also done by compressing a gas, by rotating a shaft, and by causing invisible motions of particles within a body by an external magnetic force. No work is accomplished by simply holding a heavy stationary object, because there is no transfer of energy and no displacement.

对物体做的功，不仅由整个物体的移动来实现，还可以通过压缩气体、转动轮轴、甚至物体内部质点在外部磁力作用下的微小运动来实现。拿一个重物静止不动时并没有把能量转移给它，因为没有位移。

属类：自然科学-物理-

Work done on a body is equal to the increase in energy of the body. Work is expressed in units called joules (J). One joule is equivalent to the energy transferred when a force of one newton is applied over a distance of one metre.

对物体做的功等于物体能量的增大，功的单位与能量的单位相同，是焦耳和尔格。

属类：自然科学-物理-

particle accelerator : Device that accelerates a beam of fast-moving, electrically charged atoms (ions)or subatomic particles. Accelerators are used to study the structure of atomic nuclei and the nature of subatomic particles and their fundamental interactions.

粒子加速器:加速高速移动带电原子(离子)束或次亚原子粒子束的装置。加速器用来研究原子核的结构，亚原子粒子的性质及其基本相互作用。

属类：自然科学-物理-

At speeds close to that of light, particles collide with and disrupt atomic nuclei and subatomic particles, allowing physicists to study nuclear components and to make new kinds of subatomic particles. The cyclotron accelerates positively charged particles, while the betatron accelerates negatively charged electrons.

在接近光速时，粒子撞击并分列出原子核及亚原子粒子，让物理学家研究原子核的组成，推算出新类型的亚原子粒子。回旋加速器加速正电荷粒子，贝他加速器加速负电荷电子。

属类：自然科学-物理-

Synchrotrons and linear accelerators are used either with positively charged particles or electrons. Accelerators are also used for radioisotope production, cancer therapy, biological sterilization, and one form of radiocarbon dating.

同步加速器与直线加速器可以用于正电荷粒子或电子。加速器也用于制造放射性同位素、治疗癌症、生物消毒及放射性碳定年。

属类：自然科学-物理-

dark matter : Nonluminous matter not directly detectable by astronomers, hypothesized to exist because the mass of the visible matter in the universe cannot account for observed gravitational effects.

暗物质:不发光的物质，天文学家无法直接发现，由于宇宙可见物质的质量无法解释观测到的重力效应而假设其存在。

属类：自然科学-物理-

Long believed to exist in large quantities, it enters into many theories of the origin of the universe and its present large-scale structure and into models of gravitation and other fundamental forces between particles. Numerous candidates for dark matter have been proposed over the years, but none has yet been confirmed.

长久以来相信有大量的暗物质存在，纳入许多宇宙起源与大尺度结构的理论，也进到重力及其它粒子基本力的模式之中。多年来，暗物质的候选名单不计其数，却从未被证实。

属类：自然科学-物理-

Nuclear power: Energy produced by nuclear fission of heavy atomic nuclei. About one-third of all electric power worldwide now comes from nuclear power plants. The navies of many countries include nuclear-powered warships; almost half of U.S. combat warships are nuclear-powered.

核能由大质量的原子核的核分裂产生的能量。现在全世界约有三分之一的电力是来自核能发电厂。许多国家的海军有核子动力舰艇，美国几乎有一半的战舰使用核子动力。

属类：自然科学-物理-

Most commercial nuclear reactors are thermal reactors. Two types of light-water reactors in use throughout the world are the boiling-water reactor and the pressurized-water reactor. In the liquid-metal fast-breeder reactor, fuel is utilized 60 times more effectively than in light-water reactors. See also nuclear energy.

大多数的商用核子反应器是热反应器。世界各地采用的轻水反应器有两种:沸水反应器与压水反应器。在液态金属高速滋生反应器，燃料利用率是轻水反应器的六十倍。

属类：自然科学-物理-

Nucleus: Central, positively charged core of an atom.It consists of positively charged protons and neutral neutrons, known collectively as nucleons, held together by the strong force.

原子核:原子中央带正电的核心。由带正电的质子和不带电的中子组成，这两者总称为核子，由强核力使其结合。

属类：自然科学-物理-

The number of nucleons can range from 1 to about 270, depending on the element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Some nuclei, especially heavier ones, are unstable, or radioactive, emitting energy in the form of alpha rays, beta rays, or gamma rays.

核子的数目从1到270，依元素而定。同位素是相同元素的原子有相同的质子数，但是中子数目不同。有些原子核并不稳定，特别是较重的核，或称具有放射性，以α射线、β射线或γ射线的形式发出能量。

属类：自然科学-物理-

The nucleus makes up nearly all the mass but only a minute fraction of the volume of the atom.

原子核几乎构成了原子全部质量，体积却是微不足道。

属类：自然科学-物理-

Sound: Mechanical disturbance that propagates as a longitudinal wave through a solid, liquid, or gas. A sound wave is generated by a vibrating object.

声音:以纵波形式藉由固体、液体或气体传播的物理扰动。声波是由振动的物体产生。

属类：自然科学-物理-

Conservation laws make it possible to predict the macroscopic behaviour of a system without having to consider the microscopic details of a physical process or chemical reaction.

。守恒定律使得人们可能预言系统的宏观行为而无须考虑物理变化和化学反应过程的微观细节

属类：自然科学-物理-

The concept, first proposed in 1850 by the German physicist Rudolf Clausius (1822-1888), is sometimes presented as the second law of thermodynamics,

熵的概念是德国物理学家克劳修斯(Rudolf Clausius,1822～1888)于1850年提出的，孤立系统的熵上能增加不能减少，有时也被说成是热力学第二定律。

属类：自然科学-物理-

The vibrations cause alternating compressions (regions of crowding)and rarefactions (regions of scarcity)in the particles of the medium. The particles move back and forth in the direction of propagation of the wave. The speed of sound through a medium depends on the medium’s elasticity, density, and temperature.

振动造成介质的粒子压缩(聚集的区域)和稀疏(稀少的区域)交替。粒子在波的传播方向前后运动。声音通过介质的速度取决于介质的弹性、密度与温度。

属类：自然科学-物理-