Newton's first law says unchanged motion continues in what kind of line?

Newton’s first law says unchanged motion continues in a straight line. More fully, an object remains at rest or moves at constant speed in a straight line unless a net external force acts on it.

The law is called the law of inertia. It describes an object’s tendency to preserve its state of motion, not a tendency to follow a circular or curved path. A curve requires the direction of velocity to change, which means acceleration is present and therefore a net force is acting.

The statement applies in an inertial reference frame. On Earth, friction, gravity, air resistance, and other forces often hide the ideal behavior, so everyday objects rarely glide indefinitely. A hockey puck on smooth ice is a useful approximation: with little friction, it travels nearly straight at nearly constant speed.

Newton published the three laws in Philosophiæ Naturalis Principia Mathematica in 1687. Earlier thinkers, including Galileo and Descartes, helped develop the idea that force is needed to change motion rather than to maintain it.