Buoyancy and Specific Gravity

EXPERIMENT #25

Buoyancy and Specific Gravity

Purpose: In this experiment, Archimedes' principle will be studied in the application of determining the densities and specific gravities of solid and liquid samples.

Apparatus:

Triple-beam pan balance with swing platform (or single-beam double-pan balance with swing platform and set of weights).

Overflow can (or graduated cylinder and eye dropper).

Two beakers.

Metal cylinder or irregularly shaped metal object or metal sinker.

Waxed block of wood.

String.

Hydrometer and cylinder.

Procedure: Set up the apparatus as in the lab manual.

Results: In lab manual.

Discussion: Buoyancy is what makes a piece of wood float in water. It is also what makes a battleship float on the high seas, or a block of steel float in a pool of liquid mercury. The first principle of buoyancy is very simple: (1) If a solid immersed in a fluid weighs less than an equal volume of the fluid, the solid will float. Another way of saying the same thing is, if a solid has a lower specific gravity than a fluid, the solid will float in that fluid. Specific gravity is defined as the weight of a substance divided by the weight of an equal volume of water (specific gravity of water = 1).

- If the first condition is met, then the level at which the solid will float is determined by the second principle: (2) A floating object will displace its own weight in a fluid. Thus, the percentage of the solid immersed in the fluid will be equal to the specific gravity of the solid divided by the specific gravity of the fluid. For a block of wood having a specific gravity of 0.7 floating in water (specific gravity 1.0), 70% of the block will be below the water's surface.

- Specific gravity is the ratio of the density of a particular solid or liquid to the density of pure water, which is given a specific gravity. Substances with a value less than 1000 will be less dense than pure water and by definition would float in pure water. Substances with a value