Final answer:
Newton's first law of motion, or law of inertia, states that an object will remain at rest or in motion at a constant velocity unless acted upon by a net external force. This resistance to changes in motion is a result of the object's inertia, which is directly related to its mass.
Step-by-step explanation:
Newton's first law of motion, which is also known as the law of inertia, states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. Inertia is the property of an object that quantifies its tendency to maintain its current state, whether that is at rest or in uniform motion. The greater the mass of an object, the more inertia it has, and therefore, the more force is required to change its motion or bring it to rest. This principle is foundational in understanding the behavior of objects in response to applied forces.