Demos: 2B-05 Pressure Forces in Liquids

An aluminum disk is held tightly by a string against the bottom of an open cylinder. The cylinder is then slowly lowered into a large beaker of water and held in place. The string is released and the disk stays in place because of the pressure forces exerted upward from the water. Water containing red food coloring is slowly poured into the cylinder. When the red liquid reaches the height of the water in the beaker, the pressure is the same on both sides of the disk which then drops away from the cylinder.

Directions: Hold the cylinder in one hand, place the disk firmly against the bottom and hold the string tight with the same hand that’s holding the cylinder. Push the cylinder about half way down into the water and hold it in place and release the string. With your other hand, slowly pour the red water into the cylinder.

Suggestions for Presentation: This is a fairly straightforward presentation. First focus on the fact that there is a pressure differential on each side of the disk because air is on one side and water is on the other. Remind them of the concept that pressure forces act in all directions, so there is an upward force on the disk. To emphasize the fact that the liquid pressure depends only on depth, not surface area or volume, suggest that although the inner cylinder has a much smaller cross-section, when the red water reaches the same level as the water in the beaker, the pressures should be the same. Now there is no net upward force to hold the disk in place.

Applications: None directly.

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