Учебно-методическое пособие по чтению специальной литературы для студентов 1 курса физического факультета. Дроздова И.В - 10 стр.


IV. Read the text and find definitions for friction, momentum, impulse and
acceleration. Write down four concise sentences about them.
V. Sum up the text in 10 sentences.
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.
Newtons 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 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 a 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 a 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 while 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 friction
into account.
Newtons 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 the time of action is called 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.