Newton's First Law
Newton's first law states that an object stays at rest or keeps moving at constant velocity unless an unbalanced force acts on it.
Definition
Newton's first law says that things keep doing what they are already doing: an object at rest stays at rest, and a moving object keeps moving in a straight line, unless a force pushes or pulls it. Also called the law of inertia, it states that an object continues at constant velocity, meaning constant speed in a straight line, unless acted on by an unbalanced force, so balanced forces with zero net force cannot change an object's motion. In symbols, if $\sum \vec{F} = 0$ then $\vec{a} = 0$ and $\vec{v}$ is constant. Beyond being a special case of the second law, the first law defines inertial reference frames: frames in which a force-free object moves in a straight line at constant speed. In an accelerating, non-inertial frame, objects appear to accelerate with no identifiable force, and fictitious forces must be added to make Newton's laws work.
Example
When a car stops suddenly, your body keeps moving forward until the seat belt stops you; your body was obeying Newton's first law. A hockey puck sliding across smooth ice at $8\ \text{m/s}$ travels the length of the rink at nearly $8\ \text{m/s}$ because there is almost no friction, while on a rough floor friction is an unbalanced force that quickly stops it. A pendulum hanging in a bus swings backward when the bus accelerates forward at $2\ \text{m/s}^2$. From the road, an inertial frame, the bob is simply lagging behind, obeying the first law until the string pulls it along; inside the bus, it looks like a mysterious backward force of $m \times 2\ \text{m/s}^2$.
Key Insight
Objects do not slow down on their own. A rolling ball stops because friction pushes against it, not because it "runs out" of motion, and a tablecloth can be yanked from under dishes because the dishes' inertia keeps them in place while the brief, small friction force is not enough to drag them along. This idea overturned Aristotle's claim that motion requires a continuous push. Galileo's inclined-plane experiments showed that without friction a ball would roll forever, and Newton made that observation the foundation of mechanics.