Tag Archive | force diagram

Day 32: Interaction Diagrams and Force Diagrams


College-Prep Physics: Now that we have gravitational forces, spring forces, and normal forces under our belts, we can analyse more complex situations. Today was a direct instruction lesson* on drawing interaction diagrams and forces diagrams. You might notice some changes from how I drew them from last year.

I’m using agent-object notation on the force diagrams, rather than last year’s force type + agent, in order to combat the misconception that the force diagram represents what the object is doing, rather than what is being done to the object. This also helps with getting the students to focus on the objects that are exerting the forces, because “every force has a source.” To make the force diagrams easier to read and label, we’re not including the force types on the force diagram vectors. Force types are labeled on the interaction diagram only, to help reinforce that a force is a single interaction between objects.

I’m also starting with complex scenarios early, and also asking students to draw more than one force diagram for a given situation. Last year, some students had the misconception that there must always be one force up, down, left, and right. The didn’t realize you could have 2 forces in one direction or no forces at all.

Drawing multiple force diagrams also allows for identifying 3rd Law pairs (the two vectors with circles in #4 above, though we haven’t formally called them 3rd Law pairs).

We also started with numerical values early. Although the scenarios don’t ask a specific question, we determined the values for as many forces as we could based on what was given.

In hopes of avoiding another common misconception, you’ll see that in both scenarios the normal forces aren’t equal to the weights of the objects.

We are only looking at static cases right now. Up next is tension, then friction. After friction, we’ll consider the dynamic cases.

The two scenarios pictured are taken from Preconception in Mechanics, though PiM doesn’t have the students draw interaction diagrams or force diagrams — a fault I found out too late last year. You can get the entire handout here: ForcesSchemaFBDDevelopmentStatic2015

PS: I haven’t been using the HW sheets from PiM at all. Rather, I’ve been using the occasional PiM HW problem as a bell ringer/do now/warm up.

*If you have a more engaging way of introducing interaction diagrams and force diagrams, please share!


NGSS Science and Engineering Practice #2: Developing and Using Models

Day 29: Interaction Diagrams as a Bridge to Free-Body Diagrams

College-Prep Physics: Students attempted the above problem for homework (taken from Preconceptions in Mechanics). Not surprisingly, answers were all over the place. Although the question didn’t ask students to draw anything, I asked students to draw an interaction diagram (aka system schema) for the situation. Then, based on their interaction diagram, I asked them to draw separate force diagrams (aka free-body diagrams) for the box, the table, and the spring scale.


I like interaction diagrams (left side of whiteboard) because I think they help off-load a lot of the complexity of scenarios like this one.  Once the interaction diagram is drawn, the force diagram (right side of whiteboard) is easily transposed from it. The interaction diagram also helps identify 3rd law pairs, since there is just one line (one interaction) connecting each pair of objects.

Two points to bring up:

1. This year I’ve decided to go with a circle instead of a dot for force (free-body) diagrams because (a) it allows for several forces in the same direction to be more easily drawn side-by-side; (b) it clearly labels what the dot represents.

2. I still have a number of students think that the force (free-body) diagram represents what the object/dot is doing rather than what is being done to the object/dot.