Gravity

The force of gravity is known to play an important part in many common phenomena of mechanics, as well as in everyday life.

We know the weight of a body to be nothing but the pull of gravity toward the earth. A body is said to weigh one kilogram if the mass of the earth exerts upon it a pull equal to one kilogram.

According to the universal Law of Gravitation, any two objects in the Universe are attracted to each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

The Law of Gravitation is universal, of course, but it is impossible to notice the force between two ordinary objects, as the attraction between them is too small to be perceptible at all.

But with a body as large as the Earth or the Moon it becomes a different thing. It is this force that holds the planets in their circular orbits and due to this very force the first man-made earth satellite created by Soviet scientists began to circle around our Globe.

We know gravity to pull on every particle of a body so that its weight is actually distributed throughout the body. But a solid body seems to have one point at which it can be supported by a single upward force; this point is called the centre of gravity. In technique the centre of gravity is considered to be the location of the resultant of all gravitational pulls exerted on the body.

Now we are to consider the question of stability, interrelated with gravity. A book lying on a table is expected to keep its position without any difficulty, as one knows it to be in a state of equilibrium. However, to make it stand on end is not so' easy at all. In the same way, a pencil will lie on the table without any tendency to tip over. With some difficulty one can even make it stand on its unsharpened end, but it will not stand at all upon its sharp end.

All common objects differ in their tendency to keep a given position, their stability depending on their shape, weight, and position, and everyday experience shows us that heavy objects are harder to tip over than light ones.

The stability of an object is measured by the amount of work to be required to make it take a new position. The lower the centre of gravity, the greater the stability of an object; that is why, to increase the stability of a motorcar, its designer tries to keep the centre of gravity as low as possible.

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BORAX

This salt, when crystallized from solution, combines with water forming a hydrate. It is made by adding calcium borate to sodium carbonate solution. It is a white crystalline salt. It is added to the glass, used for enamelling and glazing, to make it more fusible and easier to spread in a thin layer. Since it contains but a small proportion of the metallic oxide, it combines with other metallic oxides if fused with them. For this reason, the powdered salt is sometimes sprinkled on tarnished metallic surfaces being soldered or brazed. The heat of the blowpipe melts the borax and the latter removes the oxide and permits perfect running of the solder over the surface.

The borates thus formed are often coloured and the colours afford a means of recognizing the metallic compound which produced them. In chemical analyses a bead of borax, produced by fusion on a platinum wire, is heated with a particle of the unknown compound and its colour then examined.

Дата добавления: 2015-07-25