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



My specific take on it is that it works just fine. My particular interest now is how teachers view the definition and how it affects students. Like it or not, most elementary and middle school students learn the difference between mass and weight as the fact that mass does not change when we go to the moon, but weight does. That only works if weight is defined as the force of gravity acting on an object. If weight changes all the time depending on the viewer's frame of reference, then all we have is confusion for the students. When we say that objects "weigh less in water" or other conventions, then it's confusing for students. Even young students can understand relatively abstract ideas, but not if we all use different definitions.

And I do think it's fine to discuss changes of frame of reference for students as early as middle school. I just think we need to be clear about what we're doing, and not use various "notions" of something like weight when trying to help them understand how scientists view the world.

Just this last week I was discussing with a group of teachers the fact that scientists consider all changes in velocity as accelerations, and that using deceleration might be lay talk, but not scientifically appropriate. One teacher said that she accepted that, but that all reductions in speed were to be considered negative accelerations and all increases positive accelerations. In the short time we had, I tried to help her understand that the world does not come with pluses and minuses attached. She was the victim of previous educators who deemed it "simpler" to assign increases in speed as positive and decreases in speed as negative. It might have suited the purposes of those educators at the time, but it blocked this teacher from seeing a bigger picture of how scientists analyze the physical world. That's a case where educators used a technique that actually constrained this teacher's understanding. I think similar things happen when different educators use different definitions for something that really shouldn't be all that confusing. One definition of weight would be a really good thing, at least for the "beginners" in studying physics.

Bill


William C. Robertson, Ph.D.

On Nov 8, 2010, at 7:10 PM, John Mallinckrodt wrote:

William Robertson wrote:

If you agree that we should use a consistent definition, then why not the one we all learned by reading Halliday and Resnick, Sears and Zemansky, Hudson and Nelson, or even Feynman?

I do agree that we *should* settle on a common definition of weight although I suspect it's not possible. In any event, what is *your* specific take on the definition that we all learned from the old textbooks?

John Mallinckrodt
Cal Poly Pomona
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