# Principle 1

Any body keeps its resting position or uniform rectilinear motion as long as no force acts on them.

Force is the physical vector size by which a body acts on another body, transmitting the mechanical movement

## Principle 2

Newton found that a force acting on a body imparts to it an acceleration proportional to force and inversely proportional to the mass of the body:

$\vec F = m \vec a.$

(principle of force or second law of dynamics)

### Principle 3

When a body acts on another body with a force (called force of action), the second body also acts on the first one with a force (called reaction force) of the same size and direction, but of the opposite direction. This principle is also known as the principle of action and reaction.

#### Principle 4 (Overlapping Principle)

If several forces act at the same time on a body, each force produces its own acceleration independently of the presence of the other forces, the resulting acceleration being the vector sum of the individual accelerations.

$\sum_{i=1}^n \vec F_i=m \vec a$

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