Interaction Diagrams

How to construct an interaction diagram

By Marc Reif

We draw these interaction diagrams to organize our thinking at the beginning of a force problem. They also help to visualize Newton's Third Law. 

1. Below is a typical drawing from a physics problem. Look carefully at the drawing and read the question carefully.

I see that the object, a block, is pulled by a force at an angle to the horizontal. Also, that the object is on a horizontal surface.

2. Decide what objects are interacting in the picture.

It rubs on the surface, so the surface is interacting with it. It is pulled, let's say by a string. I know that it's near the surface of the earth, so I know that the entire earth's gravitational pull is acting on it. Let's include the air, although in most problems air resistance is ignored.

So, for objects, I'll say there's the block, the string, the surface, air, and the entire earth. Draw a circle/ellipse for every object.  Draw them in the general physical orientation of the actual system. 

3. Draw the arrows that represent the interactions. Each arrow is double-headed to show that the interaction is between two objects. For interactions that are contact forces, we draw solid arrows. For interactions that are gravitational, we draw dashed arrows to symbolize that no contact is necessary. For clarity, I named each interaction. "Drag" is the short name for air resistance. "Tension" is the name for a pulling force exerted by a string. "Gravity" is our name for the attraction between two masses.

4. In the last step, we draw a dashed square around the object the problem is asking about, in this case the block. The arrowheads that cross the dashed square and end on the object show you what to draw for your Free-Body (Force) diagram. In this case you know that the entire earth exerts a gravitational force on the block, the string a tension force, air a drag force (which is usually negligible, and the surface a contact force.