The study of classical mechanics starts with **Newton’s laws (principles) of motion**. These laws are considered the foundation of classical mechanics. Newton’s laws are verified by experiments but they can not be demonstrated theoretically (mathematically).

### Newton’s first law of motion

**An object keeps its state as long as there is no force which acts upon it. The state of the body can be either stationary or moving with constant speed.**

This law can be understood easily by analyzing the image above, where the letters mean:

A: a stationary object will remain stationary…

B: until an unbalanced force will act on it.

C: a moving object, will continue to move with constant speed…

D: until an unbalanced force will act on it.

Newton’s first law can be summarized as: an object with a mass will maintain its status quo, unless there is a force which will act on it.

Newton’s first law is also know as **the principle of inertia**.

### Newton’s second law of motion

**For a given body with mass, the resultant force F [N] acting on the body is equal with the product between the mass m [kg] and the body’s acceleration a [m/s^{2}].**

**Force** is defined as **the change in time of the momentum** *p [kgm/s]*:

From Newton’s second law of motion we can deduce that the acceleration of a body in motion is directly proportional with the sum of forces acting on it and inversely proportional with its mass.

\[a = \frac{F}{m}\]Newton’s second law is also know as t**he principle of force’s action**.

### Newton’s third law of motion

**If a given body exerts a force on a second body, the second body will exert in the same time a force equal in magnitude and opposite in direction on the first body.**

Newton’s second law can be summarized as: **for each action there is a reaction**.

Newton’s second law is also known as the **principle of action and reaction**.

All Newton’s laws of motion are defining the force from three different perspectives: the first law defines the impact of a force on a body (qualitative definition), the second law defines the value of the force (quantitative definition), and the third law states that a single isolated force can not exist.

### Parallelogram Law of Force Addition

This principle is not part of Newton’s laws of motion but it’s also a foundation law for classical mechanics. The parallelogram law of force addition states that: **if upon a point P act simultaneously two forces F_{1} and F_{2}, their effect is the same as if upon the point was acting a single force F equal in magnitude and direction with the diagonal of the parallelogram formed by the two forces**.

These four principles will serve as a basis for upcoming mechanics tutorials so understanding them will help building solid knowledge of classical (Newtonian) mechanics.

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