Materials Required:
l Narrow drinking glass
l Water
l Cork
Watering can (like the kind used for house plants)
Procedure:
- Full a narrow drinking glass with water to about 1/3 of an inch from the lip of the glass. Set a cork on the water surface and note that it drifts to the side. No matter how carefully you try to center it, the cork will always move to the edge of the glass.
- Now remove the cork. Using the watering can, or any other small pitcher with a spout, pour additional water slowly into the glass. Continue pouring until the water level is above the rim of the glass. Carefully place the cork on the surface once more. This time it will float in the center.
This Is What Happens:
In Step 1, where the water is 1/3 of an inch from the lip, the water clings to the walls of the glass. Because the water level is slightly higher at the walls than in the middle of the glass, the cork floats to this higher point. However, in Step 2, where the water is above the rim, the shape of the water is just the opposite—higher in the center. So, again, the cork floats to the highest point, but this time it’s the center. Do you know why you were able to “pile up” the water above the normal level in Step 2? If your answer was “surface tension,” you’re right! The attraction of water molecules to each other allowed you to add water to the glass slightly above the normal level.
