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Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

An object at rest will remain at rest unless acted upon by an external and unbalanced force . An object in motion will remain in motion unless acted upon by an external and unbalanced force.

This is essentially Galileo's concept of inertia, and this is often referred to as the "Law of Inertia".


If you are holding a glass of water steady the liquid will not move. Once you apply a force and accelerate the glass, the water wants to remain in the same position and can therefore spill out of the glass. The same would happen if you were walking steady with a glass of water and stopped short. The water would continue in motion and spill out of the glass.

Lex II: Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.

The rate of change of momentum of a body is equal to the resultant force acting on the body and is in the same direction.

In an exact original 1792 translation (from Latin) Newton's Second Law of Motion reads:

LAW II: The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. — If a force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.

The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma.

Iin this law the direction of the force vector is the same as the direction of the acceleration vector.


Falling objects accelerate because of the force of gravity pulls on the object.

Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.

All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.

For every action there is an equal and opposite reaction.


When a hammer strikes a nail, the nail exerts and equal and opposite force back on the hammer.

A rocket taking off from earth which pushes fuel in one direction and the rocket in the other.


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