Final answer:
Newton's first law of motion states that objects at rest stay at rest and objects in motion stay in motion unless acted on by an external force.
Step-by-step explanation:
Newton's first law of motion, 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. In other words, an object will continue to do whatever it is doing until a force is exerted on it.
For example, if you slide a book on a table, it eventually stops because of the force of friction. If you push the book, it will continue to move until an external force (such as friction or hitting another object) stops it.
So, the correct answers for the blanks in Newton's first law are: Things at rest stay at rest. Things in motion stay in motion. (Unless acted on by a net external force).