In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force. Mathmatically defined as $F_{net} = ma$ where $F_{net} = 0$.

In an inertial frame of reference, the vector sum of the forces $F_{net}$ on an object is equal to the mass $m$ of that object multiplied by the acceleration a of the object. Mathmatically the same as the first law: $F_{net} = ma$¹⁾, but where $F_{net} \ne 0$.