Statement that any particle of matter in the universe attracts any other with a force (F)that is proportional to the product of their masses (m1 and m2)and inversely proportional to the square of the distance (R)between them. In symbols: F = G(m1m2)/R2, where G is the gravitational constant.

宇宙中任何物质粒子吸引任何其它物质粒子的力(F)的大小与它们质量(m1和m2)的乘积成正比，与它们之间的距离(R)成反比的一种叙述。用符号来表示为F=G(m1m2)/R2，其中G是引力常数。

属类：自然科学-物理-

Isaac Newton put forth the law in 1687 and used it to explain the observed motions of the planets and their moons, which had been reduced to mathematical form by Johannes Kepler early in the 17th century.

1687年牛顿提出了这条定律，并用它来解释所观察到的行星及它们卫星的运动。17世纪初，克卜勒已将这些运动概括成数学的形式。

属类：自然科学-物理-

Relations between the forces acting on a body and the motion of the body, formulated by Isaac Newton. The laws describe only the motion of a body as a whole and are valid only for motions relative to a reference frame. Usually, the reference frame is the Earth.

作用在物体上的力与物体运动的关系，由牛顿系统整理而成。这些定律只适用于物体的整体运动，只对相对于一个参照系的运动有效。通常这个参照系就是地球。

属类：自然科学-物理-

The first law, also called the law of inertia, states that if a body is at rest or moving at constant speed in a straight line, it will continue to do so unless it is acted upon by a force.

第一定律(也称作惯性定律)说，如果物体处在静止状态或者沿直线做等速运动，那它将继续保持这种状态，除非受到力的作用才会改变。

属类：自然科学-物理-

The second law states that the force F acting on a body is equal to the mass m of the body times its acceleration a, or F = ma. The third law, also called the action-reaction law, states that the actions of two bodies on each other are always equal in magnitude and opposite in direction.

第二定律说，作用在物体上的力F等于物体的质量m乘以它的加速度a，或者说F=ma。第三定律(也称作用与反作用定律)说，两个物体之间的彼此作用永远大小相等而方向相反。

属类：自然科学-物理-

Division of a heavy atomic nucleus into two fragments of roughly equal mass, accompanied by the release of a large amount of energy, the binding energy of the subatomic particles.

铀-235与中子结合形成不稳定的中间物，在核裂变的过程中快速分裂为钡-144与氪-89，另有三个中子。

属类：自然科学-物理-

The energy released in the fission of one uranium nucleus is about 50 million times greater than that released when a carbon atom combines with oxygen atoms in the burning of coal.

一个重原子核分裂为质量相近的两个碎片，同时释放大量能量，这些能量来自亚原子粒子的结合能。一个铀原子核裂变时所释放的能量大约是煤炭燃烧时一个碳原子与一个氧原子结合所释放能量的5,000万倍。

属类：自然科学-物理-

The energy released from nuclear fission is used to generate electricity, to propel ships and submarines, and is a source of the vast destructive power of nuclear weapons.

核裂变所释放的能量可用来发电，推动船只或潜水艇，也是核子武器巨大杀伤力的来源。

属类：自然科学-物理-

As the temperature rises, strong paramagnetism decreases because of the greater random motion of the atoms. Weak paramagnetism, found in many solid metallic elements, is independent of temperature.

强顺磁性随温度的升高而减弱，这是因为原子磁体的较强的无规运动产生反排列作用。在许多固态金属元素中可发现与温度无关的弱顺磁性。

属类：自然科学-物理-

In general, it takes place at the boundary between two transparent media when a ray of light in a medium of higher index of refraction approaches another medium of lower index of refraction at more than the critical angle.

一般来说，当光线从折射率较高的媒质射向折射率较低的媒质，而且入射角大于临界角时，在这两种透明媒质的边界上就会发生全内反射。

属类：自然科学-物理-

The energy appears as kinetic energy of the fragments, which converts to thermal energy as the fragments collide in matter and slow down. Fission also releases two or three free neutrons. The free neutrons can bombard other nuclei, leading to a series of fissions called a chain reaction.

这些能量表现为碎片的动能，当碎片与物质碰撞时，碎片的动能转化为热能，碎片的运动就慢了下来。裂变还释放出两到三个自由中子。自由中子可以轰击其它的核，导致一系列的裂变，称为链锁反应。

属类：自然科学-物理-

Process by which nuclear reactions between light elements form heavier ones, releasing huge amounts of energy. In 1939 Hans Bethe suggested that the energy output of the sun and other stars is a result of fusion reactions among hydrogen nuclei.

氘和氚结合，形成氦，另有一个中子。轻的元素之间的核反应形成较重元素的过程，同时释放巨大的能量。1939年贝特提出，太阳以及其它恒星输出的能量是氢原子核之间发生聚变反应的结果。

属类：自然科学-物理-

In the early 1950s American scientists produced the hydrogen bomb by inducing fusion reactions in a mixture of the hydrogen isotopes deuterium and tritium, forming a heavier helium nucleus. Though fusion is common in the sun and other stars, it is difficult to produce artificially and is very difficult to control.

1950年代，美国科学家在氢的同位素氘和氚的混合物中引导聚变反应生成较重的氦原子核，从而制造出了氢弹。虽然在太阳和其它恒星中聚变是很普遍发生的，但人工核聚变却非常难以控制。

属类：自然科学-物理-

If controlled nuclear fusion is achieved, it might provide an inexpensive energy source because the primary fuel, deuterium, can be extracted from ordinary water, and eight gallons of water could provide the energy equivalent to 2,500 gallons of gasoline.

如果受控核聚变能实现，那它就能提供廉价的能源，因为主要的燃料氘可以从普通的水里提取。8加仑的水所提供的能量可相当于2,500加仑汽油所能提供的能量。

属类：自然科学-物理-

Relationship between the potential difference (voltage), electric current, and resistance in an electric circuit. In 1827 Georg Simon Ohm discovered that at constant temperature, the current I in a circuit is directly proportional to the potential difference V, and inversely proportional to the resistance R, or I = V/R. Resistance is generally measured in ohms (W).

通过实验所发现的电学基本关系，即通过各种材料的稳恒电流I的大小与材料两端的电位差V(即电压)成正比。电阻量度为欧姆(Ω)。欧姆于1827年确立了这一结论，与电阻R或I=V/R成反比。

属类：自然科学-物理-

Ohm’s law may also be expressed in terms of the electromotive force E of an electric energy source, such as a battery, or E = IR. In an alternating-current circuit, when the combination of resistance and reactance, called impedance Z, is constant, Ohm’s law is applicable and V/I = Z.

欧姆定律也可以用电源(如电池组)的电动势E来表示电压，例如，I=E/R;如交流电。电阻和电抗的组合作用称为电阻抗Z，欧姆定律是适用的。例如，V/I=Z。

属类：自然科学-物理-

" Ability of a substance to rotate the plane of polarization of a beam of light passed through it, either as crystals or in solution. Clockwise rotation as one faces the light source is "positive," or dextrorotary; counterclockwise rotation "negative," or levorotary."

物质使一束穿过它的光线的偏振平面发生旋转的能力，一如在晶体或溶液中。面对光源观测时，偏振光平面如果按顺时针方向旋转，旋光率定为正值;若反时针旋转，则旋光率为负值。

属类：自然科学-物理-

Louis Pasteur was the first to recognize that molecules with optical activity are stereoisomers (see isomerism). Optical isomers occur in pairs that are nonsuperimposable mirror images of one another. They have the same physical properties except for their effect on polarized light; in chemical properties they differ only in their interactions with other stereoisomers (see asymmetric synthesis).

巴斯德是第一个认识到具有光学活性的分子是立体异构体。光学同分异构体发生于镜像排列的成对分子，除了平面偏振光旋转角度相反和遇到其它的立体异构体时反应不同外，它们的物理和化学特性相同。

属类：自然科学-物理-

Kind of magnetism that occurs in materials weakly attracted by a strong magnet. Compounds containing iron, palladium, platinum, and the rare-earth elements exhibit strong paramagnetism because they have atoms with some incomplete inner electron shells. Their unpaired electrons make the atoms behave like tiny permanent magnets that align with and strengthen an applied magnetic field.

材料磁性的一种，这种材料可受到强磁铁的弱吸引。含有铁、钯、铂和稀土元素的化合物呈现强顺磁性，因为这些元素的原子有不完全的内电子壳层。它们的未成对电子使原子成为小型的永磁体，它们沿外磁场排列，并因此增强外磁场。

属类：自然科学-物理-

Complete reflection of a ray of light in a medium such as water or glass, from the surrounding surfaces back into the medium. It occurs when the angle of incidence is greater than a certain limiting angle, called the critical angle.

在一种媒质(如水或玻璃)中的光线从媒质的边界完全反射回媒质内部的现象。当入射角大于某个称作临界角的极限角时，便发生全内反射。

属类：自然科学-物理-