1997-2006 Academic Years
21 November 2000 Don Kanner (Lane Tech HS)
Then he began a discussion of centripetal [inward toward center] versus centrifugal [inertial forces that appear to act away from a center of rotation] forces. When a mass is attached to a string held in the hand and swung in a circle, the mass experiences a centripetal [not centrifugal] force, being continually accelerated inward toward the center of the circle. Conversely, the hand cannot swing the mass while being held perfectly still; in fact the hand moves (roughly) in a circle, so that the net force acting on it is centripetal, as well.
Comment by Porter Johnson [IIT]: the earth and moon rotate about their center of mass, which lies inside the earth. An astronomical investigation of this "earth wobble" was made in the first determination of the mass of the moon.
01 May 2001 Ann Brandon (Joliet West HS, Physics)
28 January 2003: Karlene Joseph [Lane Tech HS,
Physics] Spinning Eggs
In connection with the concept of impulse, Karlene had been having her students throw an egg into a loosely held cloth sheet in her physics class, to demonstrate that the eggs don't break, unless of course, you happen to miss the sheet! Karlene brought in one raw egg and one boiled egg, and asked how we could tell which was which, simply by spinning them on the table. She showed that it was quite easy to rotate the boiled egg about a vertical axis, with the long axis of the egg parallel to the table. And when she quickly stopped-and-released the egg, it simply remained in place. By contrast, under similar circumstances the raw egg was harder to get started in rotation, and it had the curious property that, if you momentarily stopped it, it would start rotating again. How come?
Ben Butler [L Ward Elementary School] --- a veteran egg-spinner of some repute! --- showed us that it is easy to rotate the boiled egg about the long axis of the egg when it is vertical --- whether the boiled egg is right-side-up or upside down. However, it is essentially impossible to get the raw egg to rotate about the (vertical) long axis. Now, why is that true?
No egg on our faces today! Fascinating, Karlene!
12 April 2005: Betty Roombos [Gordon Tech HS, physics] and
Marilynn Stone [Morgan Park HS,
Penny and twirling coat hanger
The twins took coat hangers that had been bent into a roughly rhomboid shape, supporting them on one finger at the top, with the hook at the bottom. They each placed a penny on the flat end of the hooks of their hangers, and began to cause the hangers to rotate in a vertical plane about the supporting finger. The pennies stayed on the hangers! How come? They were able to make the pennies stay there as the hangers rotated, and on several occasions pennies remained there even after the rotation was stopped. Most remarkable -- but did they use "twin power" to do this? Actually, it is largely a question of balance and steady motion, but there are a few tricks. Be sure to file the end of the coat hanger hook flat with a file, and then make sure that the flat surface lies perpendicular to the pivot point. An excellent demonstration of centripetal force! Thanks, Betty and Marilymn.