One of several Newton’s 3rd Law activities that students explored today was this setup of 2, 1-kg masses and 3 spring scales connected by string:
Students were instructed to draw free body diagrams for each mass and each scale. They then were asked to predict the reading of each scale. Students that trusted their FBDs made the correct prediction. Students who didn’t most frequently predicted that the scales would each read 20 N. Some predicted that the scales would each read 7 N (20 N split three ways). A great activity to reinforce how powerful a tool free-body diagrams are if they are trusted!