Floating is the upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In order for an object to float, the density of the object must be less than the density of the fluid.
The density of an object is defined as its mass per unit volume. The SI unit of density is kilograms per cubic meter (kg/m^3). The density of water at room temperature (25°C) is approximately 1000 kg/m^3.
If the density of an object is greater than the density of water, the object will sink. If the density of an object is less than the density of water, the object will float.
For example, a boat floats on water because the average density of the boat is less than the density of water. The boat displaces an equal amount of water that weighs more than the boat, resulting in a net upward force that keeps the boat afloat.
On the other hand, a rock sinks in water because the density of the rock is greater than the density of water. The rock does not displace an equal amount of water that weighs more than the rock, resulting in a net downward force that causes the rock to sink.
The density of water can also be affected by temperature and salinity. As water temperature increases, its density decreases. This is why ice floats on water. As water salinity increases, its density increases. This is why it is easier to float in saltwater than in freshwater.
