Re: [Phys-l] momentum dissipation?

["John Clement" <clement@hal-pc.org>]

> John C states, "Conservation
> reasoning should have clicked in by then, but it usually does not.  Even
> in
> the senior year of HS there are students who have still not grasped
> conservation reasoning."
>
> I ran into this very issue 3 weeks ago.  I am using McDermott's Physics By
> Inquiry with some juniors and seniors.  They were working on water
> displacement.  A group of 3 students had difficulty deciding if a
> graduated cylinder must be emptied in between each trial.  They had a 50
> mL cylinder with some water.  Measure the initial volume, placed an object
> in the cylinder, measured the 2nd volume.  Then they could not decide if
> they could simply add another object for the next test, or if they had to
> completely empty the cylinder and start over.  It took a full 4 minutes of
> discussion for them to reach the conclusion that they probably /could/
> just add the water.  One student stated that while they probably could, it
> would just be easier to dump the water and start over...
> Normal kids.  Typically categorized as smart and hard working.
What is really eye opening is the research done by Piaget on this topic.
Generally below below age 8 as I recall they will tell you that the weight
of an object will determine how high the water rises because it pushes up
the water more.  They think that if the object is dropped into the cylinder
that this will raise the water more, and if lowered slowly less.  You can
then demonstrate that this does not happen, but they will come up with
reasons why you did it wrong.  The paradigm is so strong that students will
not believe what they see.  But he found that usually by age 9 the
understanding of water displacement clicks in.  He did know that this type
of understanding was culturally dependent, but he looked at students who
were from an exclusive private school.  We now know that his markers of
thinking do vary widely in age, and that some indicators of low level
thinking persist into adulthood.

So momentum conservation, or indeed any conservation principle will be
difficult to understand until students have learned conservation reasoning.
There are programs that teach such reasoning, but they rely on inquiry
lessons and not on explaining or lecturing.  Unfortunately PER materials do
not generally build in enough reasoning tasks aimed at developing the
important types of reasoning.  But they are probably still better than
conventional lectures.

John M. Clement
Houston, TX