Newton’s first law, also called the law of inertia, was in fact discovered by Galileo. It is stated as follows:
“A free particle moves with constant speed (without acceleration)“.
A particle is called “free” if it does not undergo any interaction with the objects that surround it (the net force on it is zero).
Speed is a vector; therefore, a free particle will always move in a straight line and the magnitude of its velocity will not change.
We introduce the linear momentum (or simply momentum): 
Newton’s first law states that the linear momentum of a free particle is constant.
