Weight and mass

In common language, “weight” and “mass” are often spoken of as though they were the same things, and a body may be spoken of as “heavy” or “massive” interchangeably: even physicist sometimes falls into a trap. However, consider what weight is. This weight of a body is the force with which it is attracted to the earth.

What will happen to the spring to Hook's law? A simple way to measure the weight of an object is to suspend it from a coiled spring. In accordance with Hook’s law the force by which the body is attracted to the earth will extend the spring, and the amount of the extension is proportional to the force. A weight-measuring device of this sort is a spring ba­lance.

The mass of a body, on the other hand, is the quantity of inertia it possesses. By Newton’s second law m=f/a: it is a force divided by an acceleration. Weight, which is a force, must by the same law be a mass multiplied by an acceleration. In the case of weight, which is the force of earth's gravitational field upon a body, the acceleration is, naturally, that which is produced by the earth’s gravitational field.

Words to be remembered:

interchangeable

trap

consider

attract

suspend

extension

device

spring balance

multiply

взаимозаменяемый

ловушка

рассматривать

притягивать

подвешивать

удлинение

прибор, устройство

пружинные весы

умножать

№8

The role of gravity

Among the fundamental forces of nature, gravity is of spe­cial interest for several reasons. It is, first of all, the only truly universal force. It acts on every material thing from electron to galaxy, and, as we have learned in this century, it even acts on particles like photons and neutrinos or energy in any form. Second, gravity played a uniquely important role in the development and growth of mechanics. Newton, in his definitive formulation of mechanics, drew upon the studies of motion near the earth, influenced by local gravity, and of planetary motion far from the earth, influenced by the sun’s gravity. Since Newton’s time, motion governed by gravitational force has provided the strictest tests of mechanics, has served as stimulus for much of the mathematical elaboration of the theory of mechanics, has led to the discovery of distant new planets, and in own era of artificial satellites, has revealed new details of the shape and structure of the earth. Through the study of the orbit of Mercury came the first hint of an imperfection of Newtonian mechanics. Mercury’s refusal to follow precisely the laws of classical mechanics stands now as one of the experimental supports of new mechanics of Einstein’s general relativity.

Words to be remembered:

gravity

reason

galaxy

growth

provide

strict

elaboration

discovery

distant

artificial satellite

imperfection

precisely

support

сила тяжести

причина

галактика

рост

обеспечивать

строгий

уточнение

открытие

далекий, отдаленный

искусственный спутник

несовершенство

точно

поддержка

№9