Johnny and Susie's Wonderful Summer
Teachers Guide
The Science
Here's a brief overview of the science behind friction. There might be more in here than you really need to know, but it's always useful to know more than you teach, and anyway it's interesting!
What is friction?
Friction is the sticky force that slows things down or keeps them from moving. There are two kinds of friction: static friction and kinetic (sliding) friction. Kinetic friction happens when two solid objects are in contact and one of them moves with respect to the other. In all cases of kinetic friction, the frictional force acts to slow down the moving object. Static friction is the interaction when two surfaces are not moving with respect to each other.
 | Is this accurate: An example of static friction in the story is when the inhabitants of sticky world have to exert force to "unstick" their feet from the ground. |
What causes friction?
Sliding friction happens because atoms stick to each other (think how hard it is to slide a piece of scotch tape along a table top - sticky side down!). Drag adds to that force the fact that the solid object has to push the atoms of the fluid out of the way in order to move through them (think of think of that final scene in "Crocodile Dundee," where Paul Hogan is elbowing his way through the rush hour subway crowd to get to the girl).
Where does the energy go?
Motion is energy. As friction slows things down it "robs" them of their energy. But energy is conserved and cannot be destroyed, so the energy has to go somewhere. It goes into heating things up. The effect is very small so most of the time we don't notice it, but sliding friction comes in handy every time you rub your hands together to warm them, and drag is what heats up the space shuttle when it re-enters the earth's atmosphere (and destroyed the shuttle Columbia when it came down with a damaged heat shield).
How can you reduce or increase friction?
There are only two things you really need in your toolbox: duct tape and oil (like WD40). If it moves when it shouldn't, use duct tape; if it doesn't move when it should, use oil. How do these magical products work? Lubricants like machine oil work by separating two surfaces (by a thin film of oil) so that the atoms don't attract each other. The molecules in adhesives like duct tape literally bond to the other surface, making it very hard for the tape to slide around.
Can you really walk up walls in sticky world?
The Pedagogy
What's important?
What isn't important?
How should I use the materials?
Questions kids ask (or might!)
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Wikipedia describes static and kinetic friction. There are three types of kinetic (dynamic) friction:
Posted by cmcintyre at Aug 14, 2007 10:52
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