Newton’s first law states that every object will remain at rest or in motion in a straight line at a constant velocity unless acted upon by an outside force. Constant velocity is when an object's speed netiher increases nor decreases. The first law basically means that everything and everybody stays as they are unless something or someone tries to change that. This law can also be the definition of inertia. In fact the law of inertia and Newton’s 1st law are exactly the same. Inertia is the reluctance of any body to change ists state of motion. When an object is at rest it is not moving. When it is at equilibrium all the forces acting on it are equal and balanced so that it is not in motion.

A model rocket being launched into the air or a ball falling through the atmosphere are examples of inertia. A ball could fly in the air forever if there was no air resistance or gravity pulling it backwards. A model rocket would not launch if there was no fuse and gunpowder pushing it up from the bottom. The rocket would also fly up in the atmosphere forever unless gravity pulls it back down.