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Re: [Phys-l] g...



I don't see the benefit of starting with acceleration as opposed to, for example, mass. Once you have persuaded your students to differentiate mass from weight, then weight per unit mass (as a proxy for F/m) can follow directly. It is inntuitively and experienbtially obvious
that it is harder to push a truck, than a wheelbarrow - the essential difference is mass, not weight. The notion of field is not necessary at this point, so I don't see that F/m is all that difficult.
Regards,
Jack


On Sat, 18 Nov 2006, Paul Lulai wrote:

It depends on the academic level. Most of those that post here(and I assume this is true for those that read these posts) appear to be at the college level. For those, my point is not significant. If you are truly talking about a person's first exposure, the stuff below might be appropriate.

If you are talking to high school kids (and perhaps college freshman). Average high school kids... refering to a_g as a measure of field strength or the F/m ratio is a death-blow.
They have not been *exposed* to fields. They are discussing acceleration quantitatively for, perhaps, the first time EVER.
They have not really discussed what a force is.
They don't *know* the difference between an object's weight and its mass.
Telling them it is the ratio of F/m, or a measure of field strength on a first run through is telling them that "a_g is something that they can not understand right now. Not really."

These points are all excellent. But for the average intro high schooler, it's bringing a gun to a knife-fight. Bring up the F/m and field strength discussion once the kids have a footing in physics. Otherwise we may as well skip all of classical physics in the high school and start at the standard model, quantum mechanics and general relativity.

Have them get an understanding in what acceleration is. Then *after they know what acceleration for 1-d horizontal motion ismeasure a_g for an object with an insignificant drag force.

Later, when investigating forces, measure m, calc F_g, use a lab to find a--> wow, similar to a_g. Then do the same lab with coffee-filters or you favorite object with significant drag in relation to its F_g. Wow, a-->0, it definitely isn't a_g.

Later still, F_G=Gm1m2/r^2. For a cat dropped from a 1m high tree, use F_G and F_net to find the cat's a_net. For a cat on a 100m tree, find a_net. For a cat on a 1000m tree... for a cat on a 10,000 m tree...
why does a_net change? what does a_net seem to reflect or measure? Now a_g as a measure of field strength really works. For the average high schooler this is at least 1 month after their first a_g talk.

Later still still, looking at Electrical potential energies, now we can really hit the a_g is a measure of field strength. Look how similar it is to *E*. Neato.

I would maintain that much of the above is true for college freshman. For the average person, not the physics gods, they have to see or use this stuff multiple times before they 'get it'. The college freshman that has already taken high school physics is not going to really get the field strength arguement. Fields, to them, appear completely abstract. Perhaps F_g/m will hit more of them.

Again, I like the mention of a_g=F_g/m and a_g as a measure of field strength. I use them in class myself. But I maintain that using this as an introduction is overkill (as is both much of this email and my tendancy to use parenthesis).


Paul Lulai (where I will have someone else start my car for me)
Physics Teacher & Online Learning Coordinator
St. Anthony Village Senior High
Saint Anthony Village, MN
55418
(w) 612-706-1144
(fax) 612-706-1020
plulai@stanthony.k12.mn.us


--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley