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magnitude and act in different directions. Forces (interactions) cause
alteration in motion of an object.
Force is a vector quantity that is a measure of mechanical in-
teraction of the given object with the others.

Two forces are equal and have opposite directions if their simultaneous
action on the material point (object) does not cause its acceleration.
A body is called free if forces do not act on it (in this case a = 0,

v = const, a trajectory is straight line).
The force F is defined
completely if its modulus, its
direction in the space and the point of

its application are indicated.
The resultant of several forces is
determined by the rule of

parallelogram (Fig.3.1).
  
F  F  F (3.4)
1
2
Figure 3.1
Or according to the law of cosines
in scalar form

F 2  F 2  F 2  F 2  F 2  cos 
1 2 1 2 (3.5)
Newton's second law.
Newton's first law implies that if the resultant force is other than

zero, the object will undergo acceleration. Newton's second law outlines
the factors upon which this acceleration depends and the quantitative
relationship between these factors and the acceleration.

When an unbalanced force acts on an object, the acceleration
produced is directly proportional to the unbalanced force and inversely
proportional to the mass of the object; moreover, the acceleration vector

has the same direction as the force vector:

 F
a 
m (3.6)
therefore
 
F  m a  .
(3.7)
Оn the basis of Newton's second, law unit of forse 1 N (one
newton) is introduced
One newton is that force which gives a one-kilogram mass an
2
acceleration of one meter per second: 1 N = kg • m/ s .
Each force acting on an object causes the acceleration, which does

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