Galileo first introduced the concept of free fall. His classic experiments led to the finding that all objects free fall at the same rate, regardless of their mass. According to legend, Galileo dropped balls of different mass from the Leaning Tower of Pisa to help support his ideas. A freely falling body is an object that is moving under the influence of gravity only. These objects have a downward acceleration toward the center of the earth. Newton later took Galileo's ideas about mechanics and formalized them into his laws of motion.
Newton's first law states that "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." This law is also called "Law of Inertia" or "Galileo's Principle. Inertia is the tendency of any state of affairs to persist in the absence of external influences. Specifically, in physics, it is the tendency of a body to maintain its state of uniform motion unless acted on by an external force. This law actually has two parts to it and they are: 1. An object at rest will remain at rest unless an unbalanced force causes it to do otherwise and 2. An object in motion will continue in motion in a straight line with constant speed unless an unbalanced force causes it to do otherwise.
A force is a push or pull upon an object resulting from the object's interaction with another object. When this happens and there is a force acting on each of the two objects, there is an force acting on these objects. When there isn't any interaction the objects will not have a force on them it would only exist as a result of a interaction. There are two categories that forces between objects can be placed into. 1. Action-at-a-distance forces and 2. contact forces.
Contact forces are types of forces in which the two interacting objects are physically in contact with each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance...