What concept explains why an object remains at rest until acted upon by an external force?

The concept that explains why an object remains at rest until acted upon by an external force is known as the Law of Inertia. This principle, formulated by Sir Isaac Newton, states that an object will not change its state of motion (whether at rest or moving uniformly in a straight line) unless acted upon by a net external force. In simpler terms, inertia is the property of matter that resists changes in motion. This fundamental law underpins many aspects of physics and is crucial for understanding how forces interact with objects in various exercises and movements in group fitness settings.

In contrast, momentum pertains to the quantity of motion an object has and is dependent on both its mass and velocity. Equilibrium refers to a state where all forces acting on an object are balanced, resulting in no changes to its motion. Energy conservation involves the principle that energy cannot be created or destroyed but only transformed from one form to another. While these concepts are essential in physics, they do not directly explain the state of an object in terms of remaining at rest until an external force is applied.