### Notes and Thoughts about the Core Knowledge Presentation:

I have added one slide to my presentation - a slide that I did not actually show at the conference. It is the slide pointing out the "core knowledge" aspect of what I try to teach in How Things Work. The point of that slide is to recognize that much of what is taught as "physics" is not physics at all. Struggling with the concepts of physics is hard work for both instructor and student, so it is easier to fiddle with formulas, numbers, words, dates, names, and the like and pretend that all those are somehow equivalent to real understanding. I don't think so.

The slides do not show what demonstrations I did in the presentation. Here is a brief discussion of those demonstrations, most of which can be found at:

http://howeverythingworks.org/demonstrations.pdf

Roller Coasters:

• Pulling tablecloth out from under dishes (demonstrates inertia--inertia keeps the dishes in place)
• Slicing a motionless banana with a moving knife (inertia keeps the banana stationary while the knife cuts it)
• Slicing moving banana with a motionless knife (inertia carries the coasting banana through the knife)
• A ball on a string traveling in a circle (inertia tries to make the ball go straight and steady, while the string bends its path into a circle)
• The ball traveling in a vertical circle (at high speeds, gravity is a bit player)
• A book traveling in a vertical circle in my palm (inertia trying to make the book go straight while my palm bends its path into a circle)
• A wineglass on a swinging pizza platter (the same physics, but a more challenging version)

Bicycle:

• Balancing on a chair (static stability relates to a stable equilibrium: an inertial situation to which you automatically return when tipped)
• Balancing a stick on end (static instability relates to an unstable equilibrium: an inertial situation to which you do not return when tipped)
• Most of the rest of the discussion was words, since I didn't bring a bicycle with me. The key issues are:
• that a tricycle is statically stable, but dynamically unstable--it tips over easily during high-speed turns
• that a bicycle is statically unstable, but that it can be dynamically stable--leaning during turns prevents the tipping over
• that a bicycle steers automatically so as to return you to the unstable equilibrium situation
• that automatic steering is based on (1) gyroscopic effects and (2) flexure of the steering and front wheel
• The concept that underlies all of these movements and stabilities is that objects tend to move (they accelerate) in the direction that reduces their total potential energies as quickly as possible. Thus a tipped tricycle returns to equilibrium (to lower its gravitational potential energy), a stationary tipped bicycle falls over (to lower its gravitational potential energy), and a forward-moving tipped bicycle steers under its center of gravity by flexing its steering (to lower its gravitational potential energy).

Clocks:

• Clocks are all based on Harmonic Oscillators
• Harmonic oscillators have a stable equilibrium, about which they vibrate or oscillate
• The restoring force associated with that stable equilibrium is proportional to how far the system is from the equilibrium
• Pendulum (a swinging ball on a string)
• Bouncing mass on spring (the classic harmonic oscillator)
• A flexing plastic ruler (a harmonic oscillator that makes sound--relates to musical instruments, which are all harmonic oscillators)
• A tuning fork
• A tuning fork quartz oscillator from a wristwatch

Microwave Ovens:

• Putting metal in a microwave oven is OK, as long as that metal is not thin or sharp
• Many common metal or metal-containing objects behave surprisingly in a microwave oven. All but the last of my demonstrations are pretty safe and do not hurt the microwave, as long as you limit the on-time to a few seconds.
• A CD in a microwave (thin aluminum heats and tears, then the sharp edges spark beautifully) (when the sparking stops, turn off the microwave immediately or it will stink).
• A neon bulb in the microwave (currents flow through the metal contacts of the bulb and spark into the neon, which glows)(don't cook the bulb for more than about 5 seconds to avoid overheating and overpressuring the bulb--which can explode).
• An ordinary incandescent bulb in the microwave (same as neon bulb)(again, 5 seconds maximum to avoid overheating and overpressuring. Don't burn your fingers on the bulb!)
• A burning match in the microwave creates plasma balls which rise upward (this can scortch the top of the microwave oven. It works best in a microwave without a fan or bulb. You can put the burning match under a plastic dome, cut from a 2-liter soda bottle. That will protect the match from wind from the microwave's fan and will catch the plasma balls for a couple of second. Stop the microwave as soon as the plastic melts.
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