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Re: [Phys-l] definitions ... purely operational, or not



Agreed. I work with people (primarily elementary and middle school teachers) who are often confused about science concepts. The concept of weight is extremely confusing for them, especially when they get conflicting messages not just from laypeople but from physics teachers. An example: I was doing a workshop with HS teachers who taught a basic physical science course to freshmen. One teacher said she always had trouble with "the elevator problem." Her confusion was all about the reading on the scale, and she was carrying around several definitions of weight. Once I provided her with a clear definition of weight--the gravitational force exerted by the Earth on an object--all became clear to her. She lamented that things weren't this clear in the courses she took.

So again, although it might be okay for physics educators talking to one another to alter their definition of weight to suit their purposes, we do students a disservice when we don't stick to one clear definition. Understanding basic physics is difficult enough for most students without changeable basic definitions. And I don't think one can use "lay" uses of a term as justification for altering the physics definition. We don't succumb to using "deceleration" in teaching physics (at least we shouldn't--too many elementary and middle school texts do just that), so why succumb to common uses of the term "weightless?" There are times when physics concepts are at odds with common language. Better to just clear up the differences than try to accommodate the common language and thus confuse students. When we teach students that astronauts are not using the proper term when they say they're weightless, won't that simply highlight the difference rather than confuse? Wouldn't it be great if your students stopped and thought a bit every time they heard the term weightless? When we help students generate a bit of cognitive conflict, we're helping them. With acceleration and deceleration, I show audiences a diagram of the inside of a car, with the gas pedal, the brake, and the steering wheel all labeled "accelerator." It generates laughter, but also thought.


Bill


William C. Robertson, Ph.D.


On Nov 8, 2010, at 10:30 AM, Anthony Lapinski wrote:

These concepts can be confusing to kids.

I believe that Einstein's happiest thought was that if a person jumps off
a cliff (no air), then she has no sensation of weight. This led to his
general theory of relativity.

True weight is mg, while apparent weight is given by a bathroom scale,
which shows the upward support force. Jump off a cliff, and that scale
reads zero. You have no apparent weight, just like those floating
astronauts in the orbiting shuttle. Plenty of true weight. You can also
hang a wood block with a spring scale. Release the block and it falls due
to its true weight. But the scale reads zero while the block falls.

Scale can also be used with fluids. Suspend a 1-kg mass from a spring
scale. It weighs about 10 N. When completely submerged, the scale reads
about 9 N. The difference is the buoyant force. So the buoyant force is
abut 1 N -- the weight of water displaced (which can be measured).

Kids understand these true weight/apparent weight ideas using these scale
activities and their applications.

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
On Nov 8, 2010, at 8:40 AM, Scott Orshan wrote:

I'm fine with that definition. Now can we get the astronauts to stop
telling everybody that they are weightless, and in Zero G?

But, with that definition they *should* be saying that they are
weightless.

"... the weight of a mass M in a specified frame of
reference is M times the free-fall acceleration in that specified
frame of reference."

John Mallinckrodt
Cal Poly Pomona
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l