Wednesday, December 06, 2006

Cheap Science Labs: Friction

When I was in elementary and middle school, we did a lot of really, really boring labs. They were so dull, I don’t even remember them. But the basic principles being taught- how plants grow, how gravity works, etc.- are fascinating stuff.

I’ve been trying to come up with easy, cheap labs that teachers could use anywhere, without training, to teach in interesting, relevant ways. The point of these labs is not to have the right answer. Mild inaccuracy can surely be corrected later by memorization. I want to design labs that are easy for kids to think about in familiar terms, but that make them really consider and analyze what they're seeing.

Today: Elementary Science: Friction.

Equipment: 3 clear cups, jars, or beakers (or 3x number of groups); 6+ pennies and/or ball bearings; toy car or something with wheels; small tray; sandpaper; dirt; aluminum foil; water; corn syrup; oil. Optional: something flat that can make a ramp, another small tray.

-Set out aluminum foil, a small tray with dirt in it, and sandpaper.

-Get a kid to give the toy car a push at the left side of each of these materials. Have another kid recording about how fast they go.

-Ask them about their experiences with bikes or cars going through dirt or mud, or running across a beach, or sliding (like in sock feet). Ask them to tell you what’s different about the materials, that makes things move across them differently (whether they ‘stick’ or whether you can slide). Ask how a toy car would move across a board with a bunch of needles stuck through it.

-Now have the kids drag the car across these materials with a string attached to it. Is it harder? Easier? On which materials? Why do they think it’s that way? What’s different between rolling and sliding?

-Lesson on friction: rough materials are like a bunch of needles, or like soft dirt: they catch little rough bits on other things, and slow them down. Rolling friction is different from sliding friction because the contact is different: a large surface moving as one, or something that only contacts through a small and changing area.

-Fill one jar each with water, corn syrup, and oil.

-Drop a penny into each jar, flat side down. Ask the kids to tell you how they’re moving. Why? Drop a penny edge-on. How is it different? Why?

-Try for the connection between solids and liquids: that sticky liquids have something like friction, too. If you’re feeling ambitious, pour a little water, then corn syrup down a little ramp (into a container) to show that corn syrup moves more slowly on its own, and things move through it slower.

-Ask why does it work like this? How is it like a car going over dirt? How is it different? What would happen if you tried to swim through corn syrup? To drive over it? What if it were on top of a slick surface? (More slippery, to a point, then drag cuts in.) On top of dirt? (Enhancement of friction.) If you don’t care about the car, let the kids pour corn syrup on the dirt and on the aluminum foil and check it out.

-If you feel like it, introduce viscosity as liquid friction: how much something doesn’t want to move (down a ramp) and so how much it doesn’t want things to move through it. You can discuss rolling friction vs. dragging friction in combination with viscosity to explain the sliding-on-corn-syrup but dragging-through-syrupy-dirt effect.

Next: Solubility!

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