Fundamental Laws: Newton's laws of motion [Course Physics1]

Fundamental Laws: Newton's laws of motion

Newton's laws of motion^[1], three statements describing the relations between the forces acting on a body and the motion of the body.

Newton's laws of motion

Principle of inertia - Newton's 1st law

In a Galilean frame of reference, if a body (material system) is isolated or pseudo-isolated its center of inertia (center of mass):

- It remains fixed if it is not moving.

- It remains in uniform rectilinear motion if it is in motion.

Then the sum of the external forces applied to the material point is zero

$\sum \vec{F_{ext}} = \vec{0} \to \vec{V} = cst$

Fundamental relationship of dynamics - Newton's 2nd law

Newton's second law represents the fundamental principle of dynamics. In a Galilean frame of reference, the sum of the external forces applied to a system is equal to the mass times the acceleration

$\sum \vec{F_{ext}} = m \vec{a}$

Principle of action and reaction - Newton's 3rd law

When two bodies interact, the force exerted by the first on the second is equal and opposite to the force exerted by the second on the first

Example :

Let two material points (1) and (2) interact with each other, the action exerted by (1) on (2) $\vec{F_{12}}$ is equal and opposite to that exerted by (2) on (1) $\vec{F_{21}}$

$\vec{F_{12}} = - \vec{F_{12}} \to ‖ \vec{F_{12}} ‖ = ‖ \vec{F_{21}} ‖$

These two forces are of the same nature.