This can be a confusing topic, so you've got a lot of help. There is this SCREENCAST, this INTERACTIVE FIGURE, and three videos. I don't want this to be overwhelming, but I do want you to have a few ways to understand this topic.
Source: Hewitt, P. G. (2015). Conceptual Physics (12th ed.). Boston, MA: Pearson.
Source: HEWITT, P. G. (2015). CONCEPTUAL PHYSICS (12TH ED.). BOSTON, MA: PEARSON.
Source: HEWITT, P. G. (2015). CONCEPTUAL PHYSICS (12TH ED.). BOSTON, MA: PEARSON.