The scientific principle explaining how forces act on objects in motion is known as Newton's Laws of Motion. Specifically, the first law states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless acted upon by an external force. This principle is foundational in mechanics and helps us understand how objects respond to different forces. The second and third laws further elaborate on the relationship between force, mass, and acceleration, establishing key concepts in physics.