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}