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13 September 2010

Email discussion group and an excellent conversation

I set up an internal email conference accessible only to my classes.  Each class has a separate folder, to which I post assignments and commentary.  Students are also encouraged to ask specific questions of each other and of me on the folder.

Getting the students to use this folder properly and usefully is difficult.  Most are too timid to post anything.  Then when someone does post, they say "I can't do #4.  Help."  I have to tell such a student to try again with a SPECIFIC question:  say, "On #4, I drew the free body diagram, but I can't figure out how to get the components written."  Specific questions can provoke excellent e-discussions.

I also use this conference myself to hammer home points that I don't want to make in class; or, points that I have made in class, but need reinforcement.  For example, yesterday when I graded homework I saw some ridiculous free body diagrams; posted the picture above with the subject line "What are the two things wrong with this free body for the boat?"  The body of the message included the statement, "First to post the answer gets candy!"

A few minutes later, I found a student's homework contradicting Newton's second law.  Instead of screaming, I merely posted to the folder, offering another piece of candy:  "What's wrong with the statement 'Since the resistive force exerted by the water is less than the forces exerted by the ropes, the boat can move in the direction of the ropes.' "

I got excellent answers.  To the latter question, student Peter Chen replied: 

"You don't necessarily need to exert greater forces than the resistive force to make it move. What you really need is the same quantity of forces on the opposite direction to the resistive force. If you don't fulfill this requirement, no matter how great the forces are, you cannot move the boat.  eg: Even if you exert 1 billion Newton of forces to both sides of the boat, which are perpendicular to the direction of the resistive force, you will never move the boat. Because the quantity of forces on the opposite direction to the resistive force is 0.

To the question about the free body diagram, I reposted James Moyler's answer with commentary:

1.You shouldn't write 600N on the free body diagram. It is a rope so it should say T1 and T2. You don't put quantitative values on a FBD.
Correct.

2. F isn't enough. F of what? I believe it should be Fr for Force of Resistance.
In this particular problem, the resistive force was DEFINED as "F." So the plain old label "F" is okay here. But generally, Mr. Moyler is correct -- put a subscript or words on the label on a free body.

Another student, Frederic Lamontagne, followed up:

Firstly, there is no normal force when the object is on water. The force on water is Buoyant Force.  Secondly, [this force] would not be acting to the left (as it is indicated in the free body diagram)...it would act perpendicular to the water.
Absolutely correct.

Thing is, everyone in the class read that exchange.  Everyone in the class had done those problems just the day before, so the information was current and relevant.  The five minutes of my time it took to post and reply were worth 20 minutes of lecture time.


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