# Newton's first law

Apollo 11 launch in 1969. Source: NASA

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).

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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.

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