| MadSci Network: Physics |
Suzanne; When a wet glass is placed on a smooth, wet surface, a ring of water first makes a seal around the bottom edge of the glass. Then, as the glass continues toward the countertop, air that is under the glass bottom is compressed. Under ideal conditions, this small volume of pressurized air is able to support the weight of the glass. The water around the bottom edge of the glass functions as a seal to prevent the air from escaping. For a short time, the glass is floating on the surface water, and moves with almost no friction. A different, but related behavior is possible with a container that has a very smooth, flat bottom. Here, the water can not escape quickly, and temporarily supports the glass on a squeeze film of water alone. This is much like an ice skate, which melts the ice under the blade, so the skater glides freely on a thin film of water. The first case is like a hovercraft, where a large fan pressurizes the air under the craft so it can move freely over water or even smooth land. Technically, the glass uses what is called a 'squeeze film' bearing to temporarily support the glass. Hovercraft are supported by 'externally pressurized' bearings. Ice skates employ 'self-lubricating' bearings. These are all ways to reduce friction, using lubrication. Another way to reduce friction involves the use of magnets. Then it is called magnetic levitation. You might want to experiment with different smooth-bottomed containers to learn more about this behavior. Note especially how large a recess is on the bottom of the containers. Very flat Bottomed containers can support themselves for several seconds. Some glycerin or liquid detergent added to the water will increase the support time. Containers with a very large recess on the bottom will not glide well, as their large air volume will not pressurize enough. Remember to be careful. Glasses can easily slide off a table or countertop during these experiments.
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