Which statement best describes the law of inertia?

The law of inertia, a fundamental principle of classical mechanics formulated by Sir Isaac Newton, asserts that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. This principle highlights the natural tendency of objects to maintain their current state of motion.

In practical terms, this means that if no force—such as friction, gravity, or an applied push or pull—acting on an object will cause it to either start moving from a stationary position or alter its existing motion. This concept is crucial for understanding how forces influence the motion of objects and is foundational in physics and engineering.

The other options do not accurately capture this core principle. One suggests that an object will change motion only when a force is applied but ignores the possibility of maintaining motion. Another implies that an object in motion will gain speed indefinitely without any external forces acting, which contradicts the law of inertia. Lastly, the idea that an object will speed up based on its surroundings does not reflect the need for an external force to change its state of motion. Thus, the statement that best aligns with the true definition of the law of inertia is that an object will stay at rest or in motion unless acted upon by an external force.