Newtons first law of motion, also known as the law of inertia, states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity in a straight line, unless acted upon by an unbalanced force. This means that an object will not change its motion unless a force acts upon it. The law of inertia was first formulated by Galileo Galilei for horizontal motion on Earth and was later generalized by René Descartes. The law of inertia is fundamental to understanding motion and forces.