The upward push of water
Drop a stone into a bowl of water and it sinks. Lower a cork onto the surface and it floats. Both objects experience gravity pulling them down, but water also pushes up on them. That upward force is called buoyancy. Whether an object rises, sinks or stays suspended depends on how buoyancy compares with its weight.
Water pressure increases with depth. When an object is surrounded by water, the pressure pushing on its underside is generally greater than the pressure pushing down on its top. Together, these pressures produce an upward force. The same principle applies in other fluids, including air. A balloon can be buoyant in air just as a cork is buoyant in water.
Displaced water sets the buoyant force
An object placed in water takes up space that water would otherwise occupy. We call this displaced water. Archimedes’ principle describes the result: the buoyant force on the object equals the weight of the fluid it displaces. Displace more water, and the upward force grows.
For an object held completely underwater, the amount of displaced water depends on its volume, not its mass. Imagine two solid blocks of equal size, one made of wood and one of stone. When fully submerged, they displace the same volume of water and receive the same buoyant force. The stone weighs more, however, so that force is less likely to support it.
A floating object settles at a level where the weight of the water it displaces equals its own weight. Push a floating cork farther below the surface and it displaces more water, increasing the upward force. Release it, and it rises until the forces balance again.
Where density comes in
Density describes how much mass is packed into a given volume. For a solid object fully immersed in water, comparing its average density with the density of water provides a quick prediction. If the object is denser than water, its weight exceeds the buoyant force available at that volume, so it sinks. If it is less dense, it rises and can float with only part of its volume submerged. If the densities match closely, it can remain suspended beneath the surface.
This comparison also explains why the surrounding fluid matters. Salt water is generally denser than fresh water, so an object can receive a greater buoyant force from the same displaced volume of salt water. It will typically float a little higher there. An object whose density falls between those of two fluids might sink in one and float in the other.
Density is useful, but it is important to compare the right volumes. A dry piece of wood may float because its structure contains spaces that lower its overall density. If those spaces fill with water, the piece may sit lower or eventually sink. The material and the object’s condition both matter.
How a steel ship stays afloat
A compact steel nail is denser than water and sinks. A ship can be made largely of steel and still float because its hull encloses a large volume that includes air. Consider the ship as a whole—steel, contents and enclosed spaces—and its average density can be lower than that of water. Its broad hull also allows it to displace enough water before the deck goes below the surface.
Loading a ship makes it sit deeper. The added weight requires it to displace more water to reach a new balance. If it takes on too much water or too much cargo, it may no longer be able to displace enough water while keeping water out of the hull. The contrast between nail and ship is therefore not an exception to the density rule: it is a reminder to consider the shape and volume of the entire object.
Try the idea for yourself
A small piece of modeling clay offers a simple demonstration. Rolled into a tight ball, it may sink. Reshape the same clay into a shallow, boat-like form and place it gently on water. If the shape holds and keeps water out, it may float. Its mass has not changed, but the hollow form can displace more water before becoming submerged.
Not every object at a water surface is held up mainly by buoyancy. A carefully placed small needle, for example, may be supported by surface tension, which is a different effect. For ordinary submerged objects and boats, though, the central question remains straightforward: is the weight of the displaced fluid enough to balance the object’s weight?
