Some Properties of Matter. Грекова О.А. - 6 стр.

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   II Reading:

                                         What is mass?

         Matter is any real substance that can be weighed and measured. Clearly, then, stone or
wood or paper or any other solid is matter. So is water or milk or any other liquid: we can
weigh it and measure its volume, that is, the space it takes up, in a glass vessel suitable
marked. It is, perhaps, not quite so obvious that air is also matter. However, it can be weighed,
for on a sensitive balance a sealed glass vessel from which all the air has been pumped out
weighs less than the same full of air. It is interesting to guess the weight of air in the room in
which you are sitting. The weight depends upon the temperature. All other gases, such as
hydrogen and carbon dioxide, are, of course, also forms of matter. Matter, then, may exist as
solid, liquid, or gas.
        The quantity named mass is fundamental for the study of physics. A very important
point is that mass mast be carefully distinguished from weight, which is the gravitational pull
of the earth on the body. The mass of a piece of matter, say an iron ball, always remains the
same; it’s a property of the body that doesn’t change under any conditions that can be
imagined. The weight, however, can change. For instance, it is slightly less if the ball is 10
miles up in the air, or in a very deep mine. This change of weight could be measured with a
very sensitive spring balance of some kind.
        Actually, very sensitive methods of measurement show that a body has slightly different
weights at different parts of the earth’s surface, owing partly to the fact that the earth is not a
perfect sphere. The effect is much too small to have practical importance.
        Perhaps a clearer notion will be gained by supposing our iron ball taken to the moon. Its
mass will, of course, remain the same. But the pull of gravity at the surface of the moon is only
about a sixth of what it is at the surface of the earth, owing to the fact that the moon is much
smaller than the earth. Our spring balance, which gave the weight of the ball as 6 pounds at the
surface of the earth, would show us the ball as weighing only about 1 pound. Supposing that a
man could live healthily on the surface of the moon, his mass would remain unchanged, but his
weight would be so much lessened that he could jump easily something like 30 feet high, if his
muscles worked as usual.
        Mass, then, is a fixed property of any piece of matter, but the weight depends on
circumstances.

                                       Comprehension check
   a)   What is matter?
   b)   Is there any difference between mass and weight?
   c)   Why does a body have different weights at different parts of the earth ’s surface?
   d)   What is the pull of gravity at the surface of the moon?
   e)   Which property of a body depends on circumstances?

III Writing:

   1. Write a short summery of the text.

                                              Unit 3