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Active vocabulary
| inertial frame inertness mass momentum | — — — — | інерціальна система інертність маса імпульс | force impulse of force transformation noninertial | — — — — | сила імпульс сили перетворення неінерціальний |
If you see the velocity of a particle-like body change in either magnitude or direction, you know that something must have caused that change (that acceleration). Indeed, out of common experience, you know that the change in velocity must be due to an interaction between the body and something in its surroundings.
An interaction that causes an acceleration of a body is called a force, which is, loosely speaking, a push or pull.
The relationship between a force and the acceleration it causes was first understood by Isaac Newton (1642-1727) and is the subject of this chapter.
The study of that relationship, as Newton presented it, is called Newtonian mechanics. We will focus on its three primary laws of motion paying attention to some features.
The first
Action of one body on other is always mutual
It means that interaction of two bodies causes change of their motion. In general case all quantities which characterize motion change. But it is the most convenient to study the change of momentum of a body.
The product of the mass of an object and its velocity, 
,
is a physical quantity called momentum, which is a vector
Unit of measurement of momentum is kg · m · s–1.
The second
By observing the motions of two bodies interacting with each other, we find time after time that the resulting changes in momentum are equal and opposite,
.
It does not matter what produces the forces of interaction. They may arise from our muscles, from a distorted spring, or from a chemical explosion.
We can divide both sides of equation by 
:
or 
A sign «minus» stands in order to underline that the momentums of interacted bodies change in different directions.
These formulas mean that:
The rate of change of momentum of the first body
equals the rate of change of momentum of the second body
It is the NEWTON'S THIRD LAW
We usually used other formulation of this law:
The forces of interaction between material objects are equal in value and opposite in direction
They are acting along the line uniting the centres of mass of the material objects.
Or
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| AFTER STUDYING THE TOPIC A STUDENT IS TO | | | The rate of change of momentum of material point is proportional to the net force of all the forces acting on it and in the direction of that force |