Roy Coleman (Morgan Park HS,
Using Spools to Teach Physics
Roy followed up on Karlene's presentation on cycloids last week by placing a piece of chalk into a hole near the edge of a circular face (on one side or the other) of a large spool, about 50 cm in diameter. When that face of the spool was held against the board and rolled without slipping along the chalk tray at the bottom of the blackboard, the chalk traced its own cycloidal path on the board.
Roy then wrapped a string around the axle of the spool, placed the spool on a horizontal table, and slowly pulled on the string. In which direction does the spool begin rolling? Well, it depends upon the direction of the torque produced about the axis of rotation. This actually is the horizontal axis through the line of contact of the spool with the table. There is a critical angle at which there is zero torque about the contact point. This happens when the string points directly at the axis of rotation. The spool simply slides along without rotation when the string is pulled with a force sufficiently large to overcome static friction. For details see the Rolling Spool entry on the Oberlin College Physics Demonstration website: http://www.oberlin.edu/physics/catalog/demonstrations/mech/spool.html. Good show! Thanks, Roy.
Ann Brandon (Joliet Central HS physics,
Testing the Royal Inch
Ann recently visited American Science and Surplus, and called our attention to their 20% off Teachers' Appreciation Day sale, Saturday, September 24. She earlier had bought a Vernier caliper with an electronic digital display there. She had each of us use it to measure the distance from the first to the second knuckle in our left index fingers to see how close our class was to this classical definition of an inch. We obtained the following set of 24 measurements, in millimeters, arranged in decreasing magnitude:
Ann gave away some GE Magic Cubes, which she had found in a recent housecleaning episode. These old fashioned photo flash cubes were known as Magic Cubes because they could be flashed without a battery, by touching both terminals with a wire. They are very useful for studies of chemical reactions. [Flash cubes can still be obtained from Wizard Devices Inc: http://www.wizard-devices.com/flash.shtml.] Splendid! Thanks, Ann!
Bill Shanks (Joliet Central HS and Joliet JC physics,
Balloons and aircraft.
Bill brought in a great toy that he had obtained at Walgreen's --Rocket Balloons -- which are large, long balloons with a hand pump. When fully inflated with the pump, a balloon was about 1.2 meters long and about 8 cm in diameter. When the balloon was released, it flew around the room and remained in the air for about 7 seconds, while making a squawking noise. Bill discussed this rocket flight in terms of conservation of momentum. As the air inside the balloon is expelled, it gains momentum (to the left). This change in momentum of the expelled air is equal and opposite to the change in momentum of rocket balloon, which moves to the right.
Bill then asked whether we thought a rocket would work better in the atmosphere (ignoring friction and gravity) or in empty space. Bill thought that it would work better in the atmosphere, although most of us did not agree. Bill also described lift, and questioned whether Bernoulli's principle played a crucial role in explaining flight. Bill discussed the work of Physicist David Anderson of Fermilab and Aeronautical Engineer Scott Eberhardt of the University of Washington. For details see The Newtonian Description of Lift on a Wing: http://home.comcast.net/~clipper-108/Lift_AAPT.pdf. They conclude that the Bernoulli effect does not account for enough lift to hold the plane in the air. Instead, lift occurs because the wing pushes the air down, and by Newton's Laws the air pushes up on the wing.
Bill then described the "Ground Effect" in flight of a plane, When a plane flies just above a smooth surface (such as a body of water), the air is pushed downward and actually compressed. This layer of compressed air under the plane provides additional lift. This effect is especially important for flight of Helicopters. For details see the Wikipedia article Ground Effect in Aircraft: http://en.wikipedia.org/wiki/Ground_effect.
Fred Schaal mentioned that tips on propellers of some aircraft actually move near or above the speed of sound. For details on the Thunderscreech airplane, see the website http://hsfeatures.com/features04/xf84hbd_1.htm.
Bill also discussed kite flying in the wind. The kite catches the breeze and pushes it downward, thereby providing lift to the kite. The purpose of the tail is to keep the kite properly oriented toward the wind, and to provide stability in flight. For details see the NASA article Kite Launch and Flight: http://www.grc.nasa.gov/WWW/K-12/airplane/kitefly.html. Great stuff! Thanks, Bill!
Carl Martikean (Proviso Math and Science Academy,
Module: Learning to see Behind
Carl has developed some Project-Based Learning Modules in his new school. He passed out one such module, which presented the following problem:
"Your task is to devise a scheme that uses the three car mirrors so that there are no blind spots --- or at least minimal ones."The analysis proceeds through the following steps:
Here were some items on our KTN table:
|Know||Think we know||Need to know|
|There are 3 mirrors.
The left mirror is closer.
Head of driver can move.
The mirrors are adjustable.
Different people of different sizes have different problems with the blind spot.
|Views from the 3 mirrors must overlap.
Mirrors show us things that are behind us.
Different height cars may result in complications.
|What is a ray and how do we draw it?
How does light react with a mirror?
How big are the the mirrors?
Do passengers obstruct the view from the rear view mirror?
Do other obstructions, such as small windows in the rear, complicate the problem?
Pat Riley (Lincoln Park HS,
Rate-Determining Steps in Chemical Reactions
Pat had a beaker containing about 800 ml of a green liquid (water with green dye) for visualizing how sequential chemical reactions occur, and the concept of a "rate-determining step". Pat attached three funnels to a ring stand, in a vertical stack. These three funnels had differing sizes, spout lengths, and spout diameters. She poured the liquid into the top funnel. As it passed through the top funnel, it went into and through the middle funnel and then through the bottom funnel, and was collected in a beaker underneath. Pat set up the apparatus so that the funnel with the most narrow spout was at the middle position. We saw that liquid accumulated in that funnel and slowly passed through it -- this seemed to be the rate-determining step. We measured the time elapsed from when we starting filling the top funnel until the liquid all landed in the collecting beaker. Why does the spout diameter make so much difference? Does it matter where we place the funnel with the narrow spout? Here are the data (averaged over several observers) taken for various locations of the narrow spout:
|Narrow Spout Location||Time|
Notes prepared by Ben Stark and Porter Johnson.