There are two videos in this lesson, so watch both and ask any questions below. Also helpful is this INTERACTIVE FIGURE. Here, too is a SCREENCAST on balanced torques with a seesaw, and here's another SCREENCAST showing a plank.
Source: Hewitt, P. G. (2015). Conceptual Physics (12th ed.). Boston, MA: Pearson.
Source: HEWITT, P. G. (2015). CONCEPTUAL PHYSICS (12TH ED.). BOSTON, MA: PEARSON.
