Floating and sinking

When you put an object in a fluid, the weight of the object pushes down and the fluid pushes up.

The upward push of the fluid is equal to the weight of the fluid it displaces. It displaces its own volume of the fluid.

If the weight of the fluid a body displaces occupies a volume that is less than that of the object, the object will float. The buoyant force will equal the weight of the object in air and the two will cancel each other out.

Will it float or sink?

Whether a body will float in a particular fluid, both weight and volume must be considered. We need to look at the density of the body compared to the density of the fluid.

If the body is less dense than the fluid, it will float. If the body is denser than the fluid, it will sink.

If it floats how much will be under the surface?

Relative density (comparing the two densities) 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.

Apparent weight of a body in a fluid

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.

Shape and position when floating

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.