Which law states that a body at rest will remain at rest unless acted upon by an external force?

The law that states that a body at rest will remain at rest unless acted upon by an external force is known as Newton's First Law of Motion. This principle, also referred to as the law of inertia, highlights the tendency of objects to maintain their current state of motion—whether that be at rest or in uniform motion—until an unbalanced force causes a change.

This law is foundational in understanding how forces interact with objects and emphasizes the concept that motion is not spontaneously initiated or ceased without an external influence. For instance, if a book rests on a table, it will stay put until a person pushes it, demonstrating how the presence of an external force can alter its state of rest.

In contrast, the other laws mentioned focus on different aspects of motion. The second law relates to the relationship between force, mass, and acceleration, stating that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The third law states that for every action, there is an equal and opposite reaction, which addresses the interactions between two bodies rather than the state of a single object. Galileo's Law, while influential in the study of motion, does not directly define this specific principle of inertia as clearly as