Force is a push or pull on an object in simple terms. There are forces all around us. Some forces we can see and some we can only see act on other objects. For example, if I throw a ball, I apply a force to the ball with my hand. There are unseen forces such as magnetic and gravitational. If I let go of the ball, it will drop to the floor. I cannot see the gravitational force, but I can see the ball fall to the floor. The Earth pulls the ball to the floor. If you hang an object on a spring, there are two forces involved. The Earth pulls down on the object and the spring pulls up on the object. Robert Hooke found a relationship between the force on the spring and how far it stretches. We are going to study this relationship.