Final answer:
Newton's first law of motion, also known as the law of inertia, states that objects will remain at rest or continue moving in a straight line at a constant velocity without any external force acting on them. It explains why objects don't accelerate on their own.
Step-by-step explanation:
Newton's first law, also known as the law of inertia, states that objects will remain at rest or continue moving in a straight line at a constant velocity unless acted upon by an outside force. This means that an object will not accelerate unless there is a net external force acting on it. For example, if you push a book across a table and then stop pushing, the book will eventually come to a stop because the frictional force between the book and the table is acting as the external force to slow it down.