Newton's First Law or Law of Inertia states that an object will remain at rest or move in a straight line at a constant speed unless acted upon by a net external force. In symbols, Newton's First Law can be expressed as:
\begin{align} \textrm{if}\: \vec{F}&=0, \textrm{then}\: \vec{a}=0 \: \textrm{or}\: \vec{v}=const \end{align}
FAQ
What does Newton's first law say?
An object stays at rest or moves at constant velocity unless a net external force acts on it.
Why is it called the law of inertia?
It describes inertia, the tendency of an object to resist changes in its motion.
What is the formula form?
In compact notation, if the net force is zero, acceleration is zero and velocity remains constant.
