Class Notes, March 2015

Pulley, Pendulum, Pressure, Potential Energy

Pulleys – We’ve drawn diagrams of various styles of rigging, and then built them in the classroom and recorded their properties.
Here are some of the setups we looked at:
Fixed Pulley
Movable Pulley W/2 advantage
Gun Tackle Show W/2 advantage
Gun Tackle, “ Rove to Advantage “, W/3 advantage
Luff Tackle, adds fixed pulley to rove advantage, very smooth, W/3 advantage
Double Tackle Show, 2 fixed pulleys, 2 movable pulleys, W/4 advantage
The children enjoyed the smooth, easy lifting power of the more advanced pulleys, and would try them out during breaks.

Pendulums – We did an extensive study of pendulums, starting with tying washers, nuts and bolts of various sizes and weights to one yard of string and thread. Then we also tied string to a large ball of clay, creating a pendulum obviously much larger than the other various-sized pendulums we used. We set the pendulums swinging from fixed points, working in groups of three students. We developed and filled out a chart. From a 45degree angle we dropped the pendulum and counted the movements in one minute (using a stopwatch). We repeated this from 90 degrees and 25 degrees. We used heavier pendulums and lighter pendulums, and finally the enormous clay pendulums. Then we calculated the average of all the findings of each group for the many different trials. No matter what the angle was, or the weight, all the pendulums swung about 70 times per minute. The children were surprised. Then we talked about Galileo discovering pendulum motion sitting in church, and the astounding regularity they seem to swing with, and the physics of it. The first pendulum clocks replaced mechanical spring clocks and greatly increased their time-telling accuracy and regularity, from 15 minutes lost in a day in 1500 to 5 seconds lost in a day in 1600; a good quality in a clock. I also mentioned Poe’s  “Pit in the Pendulum“, much to their excitement, and Mrs. Finn’s chagrin.

Is Air Heavy? – We balanced two balloons (that the students inflated and tied) on a suspended dowel. The kids worked in pairs. One child popped their balloon with a pin. The dowel dipped decidedly. Then the other student popped their balloon with a pin, and the dowel was level again. As you can imagine, they hated popping balloons with pins.

The Power of Air Pressure – We placed a large sheet of paper on a table, then placed a wooden ruler or piece of wooden lath under the paper with about two inches of wood exposed and hanging over the table edge. I asked the children to hit the ruler sharply with their fists to try and lift the paper. They couldn’t ; there seemed to be an enormous weight on the thin paper sheet making it immovable. The next day I brought a hammer and more expendable wooden straight edges. The students were asked to strike the rulers sharply with the hammer. Instead of lifting the flimsy paper, the ruler snapped cleanly off at the table edge! They realized that there was a column of air pressing down on that sheet. Everyone had a turn to hit with the hammer and snap a ruler. Believe it or not it wasn’t that hard to get them to try it, some more than once.

Down – Using our pendulums tied to the center of  dowels, the class, in pairs, holds both ends of the dowel  in such a way that the dowel is level and the pendulum hangs straight down. Then one student lifts and the other dips their end of the dowel. Then they reverse that motion and the other lifts and dips. In each case the pendulum remains straight up and down, plumb to the center of the earth.

Equal – Using scales we measured out exactly 50 grams of clay and then, in pairs, dropped the clay balls into the hand of  partners, from 1 ft., 2 ft., 3 ft., and 4 ft. We observed the difference in force at each height. We learned about  Energy (E), Work (W), Potential Energy (PE), Gravitational Potential Energy (GPE) , Kinetic Energy (KE), and Joules, a measurement of work.

Earth Pressure – We fastened balloons to a bicycle air pump and then, using charts, counted the pumps and monitored the appearance of the balloons as each child pumped them up. We continued until the balloons got so big and transparent, and the tension in the room got so excited, fearful, and insanely expectant — until – SUDDENLY – POP!!!. Though we did it twelve times, and the  number of pumps varied from 85-115, the students went crazy with tension each time we got to around 70 pumps. It was a palpable, measurable tension and insanity. Nothing prepared anyone for the sudden, loud, upsetting explosion, even after the first eleven times. It was remarkable, scientifically but also psychologically. This segued easily into our study,   which we will be continuing with, of Volcanoes, and Earth Forces,  and  Earth Science.
Needless to say, this was another arduous experiment, wherein I had to convince children to explode things.

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