- Principle that states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid.
- The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases.
- It explains not only the buoyancy of ships and other vessels in water but also the rise of a balloon in the air and the apparent loss of weight of objects underwater.
- In determining whether a given body will float in a given fluid, both weight and volume must be considered; that is, the relative density , or weight per unit of volume, of the body compared to the fluid determines the buoyant force.
- If the body is less dense than the fluid, it will float or, in the case of a balloon, it will rise. If the body is denser than the fluid, it will sink.
- Relative density also determines the proportion of a floating body that will be submerged in a fluid.
- If the body is two thirds as dense as the fluid, then two thirds of its volume will be submerged, displacing in the process a volume of fluid whose weight is equal to the entire weight of the body.
- In the case of a submerged body, the apparent weight of the body is equal to its weight in air less the weight of an equal volume of fluid.
- The fluid most often encountered in applications of Archimedes' principle is water, and the specific gravity of a substance is a convenient measure of its relative density compared to water.
- In calculating the buoyant force on a body, however, one must also take into account the shape and position of the body.
- A steel rowboat placed on end into the water will sink because the density of steel is much greater than that of water.
- However, in its normal, keel-down position, the effective volume of the boat includes all the air inside it, so that its average density is then less than that of water, and as a result it will float.
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