Expressive vocabulary refers to the combination of all the words that a person has acquire throughout his/her life and can be used in various type of situations.
This would include all words in the child vocabulary, starting from the child's written language, spoken language or even the child's manually signed words.
A soccer ball would keep moving forever without physics, because without force to act upon the soccer ball, it could, or will not be able to stop the acceleration. And force is a factor in physics.
