Last Wednesday was “Decades Day”. It was a part of homecoming week. There were lots of 80’s hair and socks, but one student chose to a bit further back. Far enough back to be expressed in black and white.
Students in Black and White
Hedge Clippers as a Combination of Two Simple Machines
Now that we’ve studied simple machines, we’re practicing by calculating the mechanical advantage of some of the complex machines students have around the school. The clippers my outdoor group uses every Friday, for example, are a combination of a lever (the arms) and a wedge (the blade).
For levers the mechanical advantage (M.A.) is:
which in this case is measured as the distance from the bolt that the arms pivot about to the handholds (for the input force) and the stick (for the output force):
The wedge is an inclined plane, and its mechanical advantage is:
which we calculated to be about 3.
Combined, the two mechanical advantages multiply each other to give a total mechanical advantage of 36, which means that any force you apply to the handles of the clippers gets multiplied 36 times to the thing you’re trying to cut.
Chasing Water
Ms. Mertz’s biology class chased water droplets around a piece of wax paper to study the properties of water. It was pretty neat how she had them name the droplets and then use a toothpick to drag them around, join them up, and split them apart.
The water sticking together to form droplets is due to the hydrogen bonding between the water molecules: each water molecule has a slightly positively charged end and a slightly negatively charged end that causes molecules to stick together.
The ability to drag the water droplets around using a toothpick is because the cellulose fibers in the wood have their own slight charges that make them hydrophilic.
The students then tried dragging the water droplets around using a small piece of plastic straw, which was not supposed to be hydrophilic. However, it was a little hard to tell the difference between the straw and the wood. We’re not sure why, so we’ll have to revisit that part of the experiment again.
Ms. Mertz followed up with another nice little demonstration of the effect of soaps on water. She sprinkled some black pepper onto the surface of some water in a bowl, and then took a toothpick, dipped it into a bottle of liquid dish-washing soap, and then dipped the tip into the center of the bowl. The result was quite immediate, and quite dramatic.
The soap molecules are forced to form a thin layer on top of the water as their charged end is pulled down toward the water and their uncharged, hydrophobic end is pushed away.
Principles of Generators
Please don’t take this as an endorsement of energy drinks, especially not for adolescents; they don’t need the extra sugar, caffeine, and who-knows-what. However, it does take a little knowledge of how generators and motors work to get the joke in this ad. I think I’ll ask my middle schoolers to explain what’s going on as a short quiz after we talk about electricity and magnetism.
Life on the Hill
Last week, on one of our daily hiking trips up the slope for P.E., we came across two turtles. It was odd enough to find the first one on the way up the hill since they’re so well camouflaged against the brown leaves littering the floor of the forest.
The students wanted to take it with them, but since we’ve had a turtle in the lab this semester already I told them they should leave it there.
They left it on the ground and we continued on. It was only about 15 meters off the top of the ridge, so they wanted to stop by and see it again on our way back down. I bet them they couldn’t find it again, even though it had only been five minutes and turtles are known to be slow. They still couldn’t find it, but less than a minute later they found the second turtle on a different place.
It was quite a bit of fun looking for turtles in the forest. It occurred to me that it would be nice to have another objective on our hikes. So now, every time we go up the hill, we’re bringing a bunch of sample jars. Since I’ve been thinking about arthropoda lately our first few outings will be to collect insects and spiders on different parts of the slope to see if there’s an ecological difference due to the microclimatic differences.
Copper Plating
As an introduction to ionic compounds, my chemistry students hooked up a dime to an electrode in a copper chloride solution. It’s not exactly copper plating, but the color is quite interesting.
It was also interesting to see how the color of the copper chloride solution changed as well: from a dark to pale blueish green as the copper was extracted by the electrolysis.
The Math of Planting Garlic
One of the jobs my class helped with at the Heifer Ranch was planting garlic in the Heifer CSA garden. The gardeners had laid rows and rows of this black plastic mulch to keep down the weeds, protect the soil, and help keep the ground warm over the winter.
We then used an improvised puncher to put holes in the plastic through which we could plant cloves of garlic pointy side up. The puncher was a simple flat piece of plywood, about one foot by three feet in dimensions, with a set of bolts drilled through. The bolts extended a few inches below the board and would be pressed through the black plastic. Two handles on each side of the board made it easier for two people to maneuver and punch row after row of holes.
As I took my turn punching holes, we did the math to figure out just how much garlic we were planting. A quick count of the last imprint of the puncher showed about 15 holes per punch. Each row was about 200 feet long, which made for approximately 3,000 heads of garlic per row.
We managed to plant one and a half rows. That meant about 4,500 garlic cloves. With ten people planting, that meant each person planted about 450 cloves. Not bad for an afternoon’s work.
Arkansan Spiders
The Heifer Ranch is home to quite the variety of large spiders, including the tarantulas we found a couple years ago. Most of them work hard at keeping the insect pests down. Here’s a collection of some of them we ran into this year.
