Take a toy pull-back car. Pull it back and let it go. Ask what made it go?
Student responses vary (usually the spring made it go) but seldom mention
the table. Pull back the car, lock the drive wheels with fingers, release
the car over the table in the air. Much spinning and noise, but no forward
motion until contact with the table.
The table provides the forward force that makes the car go.
Place a file folder on a bed of straws so that it slides freely. Place
wound-up car on file folder and release. Car pushes road backwards.
I've been using this for years as a third law/friction demo.
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S.Goelzer
Physics Teacher
Coe - Brown Northwood Academy
Northwood NH 03261
603-942-5531 ext43
sgoelzer@coebrownacademy.com
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