In classical mechanics, there are three main postulates about the objects’ motion and the forces that influence them. They are called Newton’s laws of motion and should be memorized by everyone who studies physics.
The first one states that if an object (or a body) is still, then it remains at rest. If it is in motion, it continues to be in this state. Under the conditions that the object or body moves at a constant speed in a straight line, it keeps moving at a constant velocity unless an external force acts on it. This rule is also known as the law of inertia.