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Re: [Phys-l] unbiased experiments +- index of refraction



At 13:14 -0400 05/12/2009, Philip Keller wrote:

Are we sure that there is NO educational value in "verifying" labs? That's a strong claim, and if it is true, I am troubled to hear it because I sure do a lot of them in my first-year high school physics classes.

For example:

We use motion sensors to get velocity data to see if momentum is conserved in a series of collisions. We also check if energy is conserved, and if not, we calculate the fraction that is "lost" in the collision. Is this a pointless exercise for a first year class?

Or here's another: we just used resonance with tuning forks to find speed of sound. Oversimplified? Yes. Handed correction factors without explanations...guilty. But this is a first-year, non-honors class. I think they are more likely to believe and remember what resonance is after raising the glass and hearing the tuning fork magically amplified by the resonating column of air.

Almost any lab that is usually designed as a "verifying" experience can be recast in a "discovery mode" with a little thought. For example, the conservation of momentum can be done by looking at a whole bunch of collisions using motion sensors, calculating the before and after momentum of each collision and graphing the befores vs. the afters. Fitting a straight line to the data should yield a line with a slope of 1 and passing through the origin. This can be done fairly early in the momentum sequence--before one talks about conservation of momentum, so they have in fact "discovered" the principle, and in the process seen that there is a good bit of "experimental error" since the slope will never be exactly 1, nor will the intercept be exactly at zero. But in zero net momentum collisions, you don't have to worry about percentage discrepancies around zero.

Modern lab software has various statistical routines built into it, including uncertainties of slope and intercept in linear regression, that will let them gain insight into how the "error bars" can be used to support a conclusion or refute it, and it is also a good time to introduce the students to the ideas around systematic errors in lab design (e.g., non-level slope of the collision surface, friction, etc.). They can also see that the idea of momentum conservation applies to *all* collisions, not just the perfectly elastic variety that are mostly dealt with in introductory classes, but inelastic, partially elastic and even "super-elastic" ones where energy is introduced to the process at the time of collision (or "explosion").

This isn't "rocket science," gang. Labs operating on the discovery principle have been around for a long time, and have been used in elementary grade science, too. We need to expand that principle, not abandon it, when we get to the older kids.

Hugh

PS. The idea for the version of the momentum lab I have described above comes from Bob Morse a long-time physics teacher in Washington, DC, whom I am sure many of you know.

--
Hugh Haskell
mailto:hugh@ieer.org
mailto:hhaskell@mindspring,.com

So-called "global warming" is just a secret ploy by wacko tree-huggers to make America energy independent, clean our air and water, improve the fuel efficiency of our vehicles, kick-start 21st-century industries, and make our cities safer. Don't let them get away with it!!

Chip Giller, Founder, Grist.org