Projects: UDL : Friction Teacher's Guide
This page last changed on Aug 20, 2007 by mateoaw.
Johnny and Susie's Wonderful Summer
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!
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.
The difference between static and kinetic friction is outside the scope of this unit. Also note: drag is a special form of kinetic friction: the frictional force that acts on a solid body (like a boat or an airplane) moving through a fluid (like water or air.)
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).
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).
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.
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