How do friction and air resistance affect Newton’s first law of motion?

Newton’s first law of motion, also known as the law of inertia, states that an object at rest or moving at a constant speed in a straight line will maintain that state unless acted upon by an unbalanced force.

In everyday experience, objects that are not being pushed tend to come to a stop, which can seem to contradict this law. However, this occurs because friction and air resistance act as external, unbalanced forces on the object. These forces oppose the object's motion, causing it to decelerate and eventually come to rest.

In the context of Newtonian mechanics, these forces are not exceptions to the law but rather the specific external influences that change an object's state of motion. When these resistive forces are present, they prevent an object from continuing its uniform motion indefinitely. Therefore, the common observation of objects slowing down is simply evidence of these forces acting upon them, consistent with the fundamental principles of Newton’s laws of motion.1