GOALS OF BOWLING BALL ENERGY DEMO The bowling ball on a rope (demonstrated here by graduate student Jeff Peischl) graphically demonstrates energy conservation. The larger the lecture hall and the longer the rope, the better the demo. Whereas carts on tracks tend not to have low enough friction to conserve energy well, the bowling ball is massive (and slow) enough to lose only a few inches of stroke on each swing. For greatest effect, I pull the bowling ball up to touch my nose or forehead. The trick is not to flinch as the ball comes back. Closing ones eyes is one solution. The only hazard of this demo is that one gently leans back when "loading" the bowling ball. When it is released, there is a natural tendency (physics!) to lean forward a couple of inches. This could be a problem, so I always brace myself on a wall to prevent this from occurring. GOALS OF INERTIA DEMO Mass -- The most fundamental of mechanical properties, is generally costly. In the transport intensive, off-shore-manufacturing-driven economy of the US, there are many economic incentives to make products out of lightweight and thin materials and to use plastics rather than wood, metals or stone in their manufacture. As a result, entering college students are "mass deprived" compared to those 30 years ago, who were themselves mass-deprived compared to those iron age students of the early 20th century. Certainly our students have run into massive objects, but they do not necessarily have a very good feel for inertia. If they played with toy cars as children, they almost certainly were of plastic rather than iron. When teaching Newton's Laws, it is common to teach about cable tension in an accelerating railroad train, and to note that it is maximum behind the engine and decreases at each additional coupling down the line. One can almost think of the pulling force being "used up" by the successive train-car masses. I personally think of a line of train cars as being equivalent to a resistor chain in an electrical circuit. Voltage is maximum at the "front" of the circuit and drops successively across each resistor. This analogy is actually mathematically exact if we replace force with voltage, acceleration with current and mass with electrical resistance. The same concepts apply to pushing forces. If one pushes on the first brick in a line, the force exerted on the second brick is lower, the third brick lower still, etc. In the demo shown above, a hardware-store cinderblock is placed atop a jelly jar. (I use small canning jars, but I doubt it's necessary). The cinderblock is struck with a sledge hammer, with the expected consequences. The UNEXPECTED result is that the jelly-jar always survives intact. This demo can be treated as illustrating the concept of "inertia", which is really just the effect of a large mass. It also shows the "delay" that mass puts into application of force by being a second-derivative of position. Alternately, it demonstrates that the large mass "uses up" some of the force. Finally, the demo could be used later to have a discussion about impulse, maximum forces applied, yield strength and materials failure caused by collisions. In the second short segment, we must do the "control" experiment of striking the jelly jar without the brick ... with the expected results. This demo was done at the end of lecture, in the parking lot, near a dumpster, on a carpet. The carpet and debris were readily disposed of after class. R.Sonnenfeld New Mexico Tech Dept of Physics 2/20/2005