STRAW THROUGH THE POTATO
Created by Dale Olive and Randy Scoville
Future Flight Hawai`i instructors
Demonstration at a glance:
A straw is punched through a raw potato or apple.
Straight straws work better than the flexible ones. Hold the straw with your thumb over the hole in the top as you punch it through the potato. Be careful not to hold the potato in such a way as to hit your hand after the straw has gone through it.
There are at least two reasons why this demonstration works. First, the straw is sharp on its edges and is strongest when the force applied is end to end parallel to its length. Second, by putting your thumb over the hole, air is trapped inside the straw making it more rigid.
There are many simple but good demonstrations of pressure. Another one that requires a straw is the drinking challenge. Challenge a student to a drinking contest (juice or soda of course) where the rules are you have to drink from a cup with a straw. The straw you give them is rigged, of course, with small holes that can be made by sticking a paper clip through the straw several times. A second version of the challenge can then be done where you let them drink with two straws - only, one of them has to remain outside the cup at all times as they "suck up." You should win each and every time due to the physics involved with using a straw. You actually don't suck liquid up a straw, air pressure pushes it up because of the higher pressure on the drink and lower pressure in your mouth as you "suck up." The holes in the straw don't allow the students to create low pressure when they "suck up." In the second case of the straw outside the cup, low pressure cannot be created again due to the fact that as they "suck up" they get air in their mouths from the outside straw which pushes down on the liquid at the same rate as the air trying to push the liquid up. A number of great questions can be asked after this demo such as "Would a straw work on the surface of the moon?" The answer is "no" because if you had your helmet off to use a straw you would be dead! Besides that, there is no air on the moon to push the liquid up the straw when you "suck up." Would a straw work inside the space shuttle while it is in space? Yes! There is pressure inside the space shuttle while in space, otherwise, how would the astronauts breathe?
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.