Final answer:
Newton's first law, or the law of inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force. This means that an outside force is necessary for an object to change its state of motion. The law also introduced the concept of an inertial reference frame.
Step-by-step explanation:
Newton's first law, also known as the law of inertia, states that an object will remain at rest or in motion in a straight line at a constant speed, unless acted upon by an outside force. This law underscores the concept of inertia, which is the tendency for an object at rest to stay at rest, or, if moving, to keep moving at a constant velocity. It implies that a force is required to change the velocity of an object. Therefore, the velocity of an object only changes when a net external force acts on it.
In effect, the law is a statement about reference frames, identifying a special kind of frame referred to as the inertial reference frame. Inertia is directly related to an object's mass. The heavier the object, the greater its inertia, and thus, the more force it will take to alter its velocity or direction of motion.
This law is different from Newton's second law, which states that the change in motion of an object is proportional to the force applied on it, and Newton's third law, which states that for every action there is an equal and opposite reaction.
Learn more about Newton's First Law