Blog Post – Unit 4 on Balanced Forces Model (aka Inertia and Interactions – Free Particle Model)

Force – Operational Definition:  We started this unit by attempting to develop an operational definition for the term FORCE. In contrast to other topics, we started with the vocabulary term and then attempted to define what it meant.  This seemed to go against the suggested method of concept before term, but because of the ubiquitous use of the term force in both everyday language and in the classroom, we chose to violate the rule.  I liked the way Don introduced the topic…”since this is not your first day as a human, you’ve heard of this term before…”   A long list of terms (or phrases) associated with the term “force” was written on the board and then Don attempted to narrow the list down by “combining ideas”, eliminating if the word did not correspond to a “process used to determine its existence, duration, quantity” (from Wikipedia definition of “operational definition”).  The list of words eventually was reduced to “push or pull, or rubbing”.  I’m still a bit confused about how to eliminate the words that don’t fit…that is, how to determine if the word belongs in or can be used for an operational definition.

Force – What is it exerted BY?  We then attempted to refine our concept of force by looking at ONLY THE OBJECT that exerts a force.   So the question posed “what forces are acting on a block” (a person holding a block stationary) was reduced to only the objects that exert a force, not the force itself.  Our list was reduced to “Hand” and “Earth”, and temporary included the term “air”.  Words eliminated were “light”, “mass”, “inertia”, “moon”, “sun”, “magnetism”, etc. 

Don was careful to list all words thrown out by the class, but then narrowed it down to just the two objects.   This is to ensure that the list is generated by the class so it’s not just the list we use in physics class and it is something else outside of class…it is THEIR list.    Our first general rule was established:

1:   “When naming forces, you must name the physical object doing the “pulling/pushing/rubbing” – objects must be made of MATTER”

Force – System Schema:  A representational strategy (new to me) was shown called a system schema.  In this representation, the object under study and the object(s) that exert force on it are shown in circles and connecting lines are shown representing the interaction (force).  This was very effective in showing that objects exert force, and that if you can’t identify the object, there is no force.  I felt that this will go a long way in helping students realize there is no impetus force.  The forces of interaction represented by connecting lines are drawn either as solid lines or dashed lines, which help illustrate the difference between “contact” forces and “non contact” forces.   A list was made of “non-contact forces” and “contact forces” and under the “non-contact forces” was listed three of the four fundamental forces (gravitation, magnetism, electrostatic, and strong nuclear) and under the “contact forces” was listed the phrase – “everything else”.

 

(Personal notes:  This appears to be the opportune time to address the fact that there are only 4 fundamental forces and although for convenience we can talk about “contact forces”, in reality, nothing ever actually touches anything else.   Granted, this is not the time to delve into details or mathematical descriptions of these forces.  The premise of “modeling” is that nature behaves in patterns that can be understood through some basic models… these models serve as the foundation upon which one can understand more complex processes as they can be described through simultaneous application of the basic models.   Narrowing the list of forces to the 4 fundamentals may help strengthen the foundations of these models as they are being built and reinforce the notion that nature can be described by only a handful of basic laws applied in a myriad of ways.   

Our second rule was established:  2:  If vel = 0 then forces are balanced (will be replaced later with if change in velocity = 0 then forces are balanced, and then ultimately if acceleration = 0 then forces are balanced)

A demonstration was shown, but not explored, in which boards bend or change shape to provide the supportive force necessary to maintain equilibrium.  A lot of discussion took place about how this could be used to answer the question…”how does the table know to push up with 4 N on the board and by 20 N on the book?”  In our reading, there was discussion about how a spring behaves as an “automatic force adjuster”, but I’m honestly not sure where this is going or how it’s helping students build a model just yet.  A suggestion was made that a series of magnets around a dowel rod can be used to show the compressibility of matter.

 

A demonstration using a “Kick Disc” was used to illustrate the fact that there is no “impetus” force.  This helped provide the visualization for students that there is “no contact force” that remains acting on the disc after it is set in motion.  A list of objects was generated that could be exerting forces to keep it moving, but then subsequently they were removed from the list as they weren’t making contact.  It was concluded that there is no force acting in the direction it is moving, and after describing the motion of the disc after set in motion, we changed our #2 rule to the “no change in velocity means balanced forces”. 

Activity:  Relationship between mass of an object and the force that earth exerts on it:  Using spring scales we measured the “weight” of a variety of masses and plotted on graph paper in our usual method.  From this activity, we determined the relationship that “W = (0.01 N/g) * M”  or “W = (10 N/kg) * M which experimentally confirms our fundamental equation relating mass to weight (W = mg).  This relationship was summarized in words as “for every 1 kg of mass the earth pulls on the object with a force of 10 N”.  Although the mathematical representation of this relationship was not utilized in our classroom setting, I see this as a good opportunity to show how the equation W = mg describes this relationship and discuss with my students the meaning/interpretation of the value of “g”. 

 

Atwood’s machine demo – balanced and unbalanced:  A demonstration was done to see how well we understand balanced forces.  An atwood’s machine with equal weights was shown, Don then lifted one higher than the other and asked what would happen when released (system stayed put), then he said he would throw one downward giving it a slow but steady speed.  What would the system do?  (It continued at a constant velocity)

Worksheets were done to practice drawing force diagrams and schemas for a variety of different scenarios.  During discussion period, Don nicely led us to see that direction of an unbalanced force and the direction of acceleration are always the same.  Following one of the more challenging worksheets (dealing with 3^rd Law) we used force sensors attached to dynamics cars to study forces of interaction between two objects that are in contact while one is pushed, and then also studied forces of interaction between objects as they collide.

 

This was all done PRIOR to any discussion of the 3^rd law.  Prior to 3^rd law worksheets, Don shared a story about “confrontational Stu” to help introduce the idea that forces always come in pairs.  We were all asked to think of a situation in which an object can be propelled and discuss how the 3rd law can explain the propulsion.  Drawings were created as well....

 

How I feel about all of this?  I really like how the activities allow students to learn these conceptually difficult concepts through kinesthetic experience.   However, it is very apparent that true learning will take place only if the instructor does a thorough job of questioning and guiding student thinking through the common misconceptions (or preconceptions).  That is to say, the teacher cannot sit idly by and hope that students make sense of the material or that they will perform the necessary reflections on their own.  

How I intend to implement:  At this point, I can see myself implementing most or all of these activities in my classroom.  I am inclined, at this point, to introduce more mathematical representations as we go along, however.    This is not to emphasize any particular form of representation, but rather to reinforce that all representations have their own merit, and that best practice may necessitate using more than one representation in order to convey a particular idea or provide additional insight.      

Difficulties I see coming:    What I will struggle with is the pacing and depth of the material to keep the students (especially at the higher end) from getting bored and “zoning out”.  Unfortunately, many introductory physics students believe they already understand this material and that they don’t really need to pay attention as it may appear (at least to them) that this material is “below them”.  In reality, this material is rife with student mis and preconceptions that, if not fixed, will continue to plague their problem solving abilities as the rest of the course unfolds.  I can see myself performing a balancing act for a lot of this unit, attempting to balance the necessary focus on “basics” with the students’ desire to be challenged with meaningful learning activities that stretch them a bit.