Newton's First Law of Motion, also known as the Law of Inertia, states that every object continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.

• Inertia: The inherent property of an object to resist changes in its state of motion.
• Force: Defined qualitatively as the agent that causes a change in the state of motion. If $\sum {\vec{F}}_{ext}=0$, then $\vec{v}=\text{constant}$.