Created by Dale Olive and Randy Scoville
Future Flight Hawai`i instructors
Demonstration at a glance:
A chopstick or thin piece of wood is placed under a poster or paper and quickly broken in two with a quick hand chop.
Place the chopstick or wood 3/4 of the way under the paper and test with a quick downward tap. There should be some resistance. If not, slide the chopstick under the paper a little further. The chopping motion should be quick and steady, don't hit it half hearted or it may not break! Thin strips of wood can be obtained from any hardware store.
This demonstration can be explained in two ways: the law of inertia and the concept of air pressure. Since the wood and paper are at rest, they want to remain at rest (law of inertia.) If you hit the wood too slowly it begins moving but if hit fast enough it remains at rest. Air pressure exerts a force of 14.7 pounds per square inch so a poster or paper as small as 1 square foot in area can be held down with as much as 2000 pounds of pressure. Why doesn't it stick to the table then? Luckily, paper doesn't act like a suction cup where all the air is squeezed out from under it.
Besides the many other pressure demonstrations listed here and elsewhere, a good demonstration of inertia is the old pull the table cloth out from under the dishes. If you want to spare the dishes you can take a narrow mouth bottle (2 inch diameter) a sheet of oaktag or thick paper and a penny to make a neat inertia demonstration. Cut a one-inch strip of the paper along its longest side. Tape into a hoop and place on the opening to the bottle. Directly above the opening balance the penny on the hoop. Challenge a student to get the penny into the bottle without lifting the hoop. Many will want to hit the hoop on the outside but this distorts the hoop upwards throwing the penny off. If you pull the hoop from the inside it distorts downward. Due to the low friction between the hoop and penny it hangs in mid-air for a split second before falling into the bottle.
Communications: Hawaii Space Grant Office
This activity is featured in Future Flight Hawaii, a K-12 Education Project of Hawaii Space Grant Consortium.
FEB 27 2001.