Английский язык. Ильичева Н.А - 15 стр.

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balanced, that is, equal and opposite, motion doesn’t occur. It occurs when the
applied forces are unbalanced, that is, when they are not equal and opposite. An
external force is one that is applied on or to the object or system. There are also
internal forces but they don’t affect velocity.


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       Galileo also introduced another concept. It appeared that objects were
resistant to changes in motion. Similarly, if an object was at rest, it seemed to
“want” to remain so. Galileo called this property inertia and we say that inertia
is the property of matter that resists changes in motion.

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       Newton went on one step further and related the concept of inertia to
something that could be measured – mass. That is, mass is the measure of
inertia. The greater the mass of an object, the greater its inertia is and vice
versa.

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       As an example of the relationship of mass and inertia, suppose you
horizontally push two different people on swings initially at rest, one a very
large man and the other a small child. You’d quickly find that it was more
difficult to get the adult moving. That is, there would be noticeable difference in
the resistance to motion between the man and the child. Also, once you got them
swinging and then tried to stop the motions, you’d notice a difference in the
resistance to a change in motion again. Being more massive the man has greater
inertia.


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       In our study of motion, acceleration was defined as the time rate of the
change of velocity. What causes acceleration? Newton’s first law answers the
question. If an external unbalanced force (net force) is required to produce a
change in velocity, then we see that an unbalanced force causes an acceleration.
Newton also related acceleration to inertia or mass. Because inertia is the
resistance to a change in motion, a reasonable assumption is that the greater
inertia or mass of an object, the smaller the change in motion or velocity is when
a force is applied. Hence we have this summary:
1. The acceleration produced by an unbalanced force acting on an object
    (or mass) is directly proportional to the magnitude of the force (a ∝ F)
    and in the direction of the force.
2. The acceleration of an object being acted on by an unbalanced force is
    inversely proportional to the mass of the object (a ∝ 1/m).