What is the principle that an object at rest will stay at rest?

The principle that an object at rest will stay at rest is encapsulated by the Law of Inertia. This law, which is derived from Newton's First Law of Motion, states that an object will remain in its current state of motion—whether at rest or moving at a constant velocity—unless acted upon by an external force. Essentially, the Law of Inertia is foundational in understanding the behavior of objects in both static and dynamic contexts.

In the absence of forces, such as friction or applied force, an object will not change its state of motion, illustrating that rest is a stable state. This concept is crucial in fields like physics and engineering, as it helps predict how objects will react under various conditions.

The other principles mentioned, such as Newton's Second Law, deal with the relationship between force, mass, and acceleration, while Newton's Third Law describes action and reaction forces. The Law of Conservation of Motion relates more broadly to the conservation of momentum in systems rather than specifically addressing the state of objects at rest. The unique focus of the Law of Inertia makes it the correct answer for this question.