Some Properties of Matter. Грекова О.А. - 8 стр.

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I Pre-reading task:

   1. Check up the meaning of the words:
   Verbs: put forward, push, strike, attempt, bring into play, deal with, be proportional to,
   multiply by, depend upon, double, treble, neglect, drop, increase, govern, take into account.
   Nouns: consideration, rock, velocity, impulse, resistance, height [     ], calculations, gun,
   bullet, grain, dust.
   Adj: celebrated, heavy, smooth, level, rough [      ], visible.
   Adv: strictly, freely, namely, nearly, gradually.

   II Reading:

   1. Read the text, find out the examples of F. C.

                                   Three laws of motion (I. Newton )

        The great Sir Isaac Newton, who may be considered to be the founder of modern
physical science, put forward three celebrated laws of motion, which are at the basis of all
scientific considerations of movement.
        Newton’s first law of motion can be stated as follows: if a body is at rest it will remain
at rest unless acted upon by an outside force, when it will at once move, and if it is moving in a
straight line at a constant speed it will continue to do so unless acted upon by an outside force.
        This may at first sight seem to be contrary to what happens every day before our eyes.
We can push against a heavy body, a rock resting on the earth, without moving it, and if we set
a body in motion, for instance by striking a ball lying on a smooth and level piece of ground, it
will not continue to move, but will come to rest.
      The fact is that when we move, or attempt to move, any body in contact with another
body there is an outside force brought into play, the force of friction. The size of this frictional
force depends upon the nature of surfaces, whether rough or smooth, and upon the force which
presses the bodies together. In the case of a body resting on a surface, this force pressing the
bodies together is the weight of the body. In the case of the heavy rock resting on the earth, the
frictional force which has to be equalled if it is to move is so large that, for all practical
purposes, the rock can be considered as a part of the earth. In the case of the rolling ball, the
friction will be a small force, acting all the time that the ball is moving, which will gradually
bring the ball to rest. The first law of motion is strictly true if we take all the forces, including
that of friction, into account.
            Newton’s second law of motion deals with bodies changing their speed, and was
expressed by him somewhat as follows: the change of motion of a body is proportional to the
force acting on the body and takes place in the direction of the force. What Newton called
‘motion’ is now called momentum and takes into account both mass and velocity: in fact it is
mass multiplied by velocity. Thus if a force is acting steadily on a body in a given direction,
the velocity in that direction will increase steadily. With a force of the same size acting on a
smaller mass the velocity will increase more rapidly. The total change of velocity will depend
upon how long the force acts. The force multiplied by this time of action is called the impulse.
So the second law is expressed thus: the change of momentum of a body is equal to the
impulse which produces it, is in the same direction.
     To get a clear notion of what this means, let us consider the case of falling bodies. First of
all, we observe that the force of gravity pulling the body down is proportional to the mass of