Buoyancy

Find out more about
Buoyancy

The Point of the Experiment

In drilling for oil, one of the many functions of drilling fluid is to provide buoyancy for the drill pipe. This is a beneficial side effect of having a liquid in the borehole. Drilling fluids are not specifically designed to increase buoyancy, although this is desirable. The density of the drilling fluid, and hence its buoyancy, is determined by other considerations: the need to counteract fluid pressure in the hole without exerting so much pressure as to cause fracturing of the formation and loss of fluid to it.

The buoyancy experiment is a simple way of demonstrating how liquids provide support for heavy objects and how liquids differ in density. Water provides a standard by which to measure the other liquids, since its density is 1 g/cm³. We chose corn syrup because it is readily available and easy to handle and because its density is different enough from water to produce noticeable results.

Measurement Procedures

Spring scales and beam balances

The spring scale is easy to use for this experiment, but you can achieve greater accuracy with a triple beam balance. You have to position the balance so the pan end is beyond the edge of a table. The thread is attached to the underside of the pan—there is probably a hook there. Then the container with liquid is raised up from below to submerge the hanging object.

Measuring volume

In our experiment we did not actually measure the volume of the screw. Instead, we calculated the volume based on the fact that it weighed 4 g (0.14 oz) less in water than in air. Therefore it must have displaced 4 cm³ (.24 cu in) of water.

You could also measure the volume of the object directly by filling a graduated cylinder with sufficient water to submerge the object and then measuring the rise in water level when the object is placed in the cylinder. The result should be the same as that calculated from the change of weight.

Related Activities

Viscosity

Another important property of drilling fluid is viscosity. This is explored in the series of viscosity experiments. Some people may believe that viscosity and density go together—that thick, gooey liquids are also dense. Comparing water and corn syrup would support this view. But oil is less dense than water and generally quite a bit more viscous. Using water, corn syrup, and oil in both experiments offers a useful comparison.

Floating and sinking

Our "Buoyancy Experiment" uses an object that is heavier than water and most liquids. If the object is less dense than the liquid it is in, it will float, displacing a volume of water equal to its weight.

Buoyancy and density can be explored through activities involving floating and sinking. A good collection for middle school children is published by AIMS:

Floaters and Sinkers

Mass, Volume, and Density

AIMS Education Foundation

PO Box 8120

Fresno, CA 93747-8120

USA

What if?

What would happen if we put the steel screw into a jar of mercury? (The density of mercury is 13.5 g/cm³.)

Ice has a density of 0.9 g/cm³. It floats in water. People don't find that odd, but it is unusual for the solid form of a substance to be less dense than the liquid form. What if ice were denser than water? How would the world be different?