Newton’s laws describe the fundamental principles that govern the motion of all objects in the universe. Newton’s first law is often called the law of inertia, and it describes how objects will continue to move in a straight line at a constant speed unless acted upon by an external force. The second law explains how forces relate to changes in an object’s motion. The third law tells us that for every action there is an equal and opposite reaction, meaning that when two objects interact, they always exert equal and opposite forces on each other. These laws are used in all sorts of applications, such as building bridges and designing spacecraft.