Floating and Sinking

Buoyancy When you pick up an object underwater it seems much lighter due to the upward force that water and other fluids exert known as the buoyant force The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter Since the pressure of a fluid increases with depth, the upward pressure on the bottom of the object is greater that the downward pressure on the top, resulting in the buoyant force The weight of a submerged object is a downward force An object sinks if its weight is greater than the buoyant force If the weight of the submerged object is equal to the buoyant force, the object floats

Archimedes’ Principle Submerged objects displace, or take the place of, a volume of fluid equal to its own volume Archimedes, a Greek mathematician discovered a connection between the weight of a fluid displaced by an object and the buoyant force acting on it Archimedes’ Principle states that the buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces

Density The density of a substance is its mass per unit volume By comparing densities, you can predict whether an object will float or sink in a fluid An object that is more dense than the fluid in which it is immersed will sink An object that is less dense than the fluid in which it is immersed will float If the density of the object is equal to the fluid, it will float at a constant depth

Changing Densities Submarines are a prime example of changing densities When a submarine dives, it takes on water increasing density and making its weight greater than the buoyant force When a submarine surfaces, it releases water decreasing the density and making it weight less than the buoyant force