Monday, July 8, 2013

The First Rule of Forces

We're on day 10 of our workshop and there is some discussion about how to name forces.  This is necessary because there are a couple of different naming conventions that are popular.  Regardless of the convention I feel like we should all follow the "First Rule of Forces" which states that:

"When naming forces you must name the physical object doing the pushing or the pulling."

I can't remember where I got this rule - probably from my physics teaching mentor, Mark Davids.  He grounded every concept in a physical experience and this has helped me get the students to really examine their own understandings of forces.  By sticking to the FROF kids are forced to think more critically about the forces involved in any situation.

If a student has to name the physical object supplying the force then they can't throw around physics words like, inertia, momentum or gravity as supplying forces because none of them are physical objects.  Too often kids imbue these physics words with special properties.  Popular culture doesn't do us any favors here.  I was watching American Ninja Warrior the other day - which is a game show in which contestants have to complete an elaborate obstacle course - and the commentators couldn't stop themselves from saying, "His momentum kept him swinging..."  I wanted to reach into the TV and throttle someone!

Anyway, below are some of the conventions you may see for naming forces.  Consider the gravitational force supplied by the earth on a block.
1. Agent Object - in this the force is named with an F and the subscripts list what is doing the pushing or the pulling, the word "on" and then the object on which the force is acting.  For example, Fearth on block.
2. Object Agent - in this the force is name with an F and the subscripts list the object on which the force is acting, the word "by" and then what is doing the pushing or the pulling.  For example, Fblock by earth.
3. Physical Object - in this the force is named by the physical object doing the pushing or the pulling.  For example, "earth".
4. Nickname - in this the force is named by its "nickname".  For example, "weight".
5. F notation -  in this the force is named with an F and the subscript is the nickname. For example Fg.
6. Formula - in this the force is named as a formula.  For example, "mg".

There may even be more!  My opinion?

Not all of these are created equal!   

Soapbox time; there are a couple of these that really are better than some others.  Agent Object and Object Agent are both good.  They allow us to follow the FROF.  I also like #3 in that it is the most basic.  But other than those three - I really have a hard time getting on board.  

Let's talk about nicknames: weight, friction, tension, normal...these are all force nicknames well known to physics teachers - but not necessarily to physics students.  When do we introduce these to our students?  Earlier this year I had a student come up to me and ask, "when are we going to start calling it the normal force?"  First of all, I have no idea how he knows we call it the normal force.  Secondly, what is his hurry?  So I told him this story.  

Last weekend Mrs. Pata and I went out with some of her friends and their significant others.  One of her friends has a new boyfriend and this was our first time meeting him.  I have a nickname; my close friends and family call me DP.  When I introduced myself to this guy I said, "Hi. my name is Don its nice to meet you."  He responded with, "DP great to see you!"  I'm like, um...what?  In my head I'm thinking, dude - you don't even know me!  

It is not appropriate to use people's nicknames if you don't know them.  It is just an inappropriate in a physics class to use the nicknames of forces if we don't know them.  Textbooks throw around these nicknames like "tension" all willy-nilly.  No students can gain the right context from a textbook definition.  This idea is manifested by numbers 4 and 5 that remove the context of the force and require that students have an understanding of these ideas already.  #6 removes the context of the force and asks students to relate it to an even more abstract idea and equation!  
Regardless of the convention that you choose classroom vocabulary can/should only be used when the class has been involved in the making of the definition or if 100% of the class in on board with its adoption.  I urge you to be aware of these problems ahead of time and make the choices that are best for the conceptual development of your students.

I love the first rule of forces and the kids always want to know what the other rules of forces are.  I haven't come up with any yet; but its early!  

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