Final answer:
A moving object slows down due to a net external force of friction, not on its own. Newton's first law states that without such a force, an object in motion would continue moving at a constant velocity. So, objects in our environment slow down due to the force of friction acting upon them.
Step-by-step explanation:
If the first law of motion is a true description of nature, why do moving things in our environment eventually slow down? The answer lies in understanding Newton's first law of motion, which on the surface, might seem to conflict with our day-to-day observations that a moving object will usually slow down and stop unless an effort is made to keep it moving. In the case of a sliding box, for instance, it will slow down not on its own, but due to a net external force. This force is typically friction, which acts opposite to the direction of motion and slows the object down.
Note the law implies that an object's motion will remain constant unless acted upon by this net external force. Without such a force, Newton's first law indicates that an object in motion would not slow down. Therefore, when we see an object slowing down, we can infer the presence of a net external force, usually friction in everyday scenarios. If this frictional force disappeared, the object would not slow down and would continue to move at a constant velocity, preserving its status quo of motion.