Hey people, I searched by this question in the forum but don´t found the answer. I have a simple doubt about this theory in mechanic. The question is: There is negative force? I know that when I am working in math with a vector that represents a force I can use the negative signal to represents it is in the opposite direction, like a convention. In the college people always say that the negative force don´t exists, but to me this can exist just like the positive force (that is a convention too) and negative direction. Can someone give me an explanation about this concept? Thanks. Carlos Dutra Sorry the english, I am a brazilian.