Floating and Sinking

The Titanic What do you know about the Titanic? How is it possible that a huge ship can easily float in water under certain conditions and then in a few hours become a sudden wreck? Buoyancy!!

Buoyancy Water exerts a force called a buoyant force that acts on a submerged object. The buoyant force acts in the upward direction, against the force of gravity, so it makes an object feel lighter.

Buoyancy As you can see in the picture, a fluid exerts pressure on all surfaces of a submerged object. Since the pressure in a fluid increases with depth, the upward pressure on the bottom of the object is greater than the downward pressure on the top. The result is a net force in the upward direction. This is the buoyant force.

Submerged Objects Submerged objects take the place of a volume of fluid equal to it’s own volume. Objects that float on top of the water only take the place of the volume of fluid equal to the volume of the amount of the object in the fluid.

Archimedes’s Principle Archimedes’s Principle relates the amount of fluid a submerged object displaces to the buoyant force on the object. Archimedes’s Principle states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

Floating and Sinking There is always a downward force in a submerged object, the object’s weight. If the weight of the object is greater than the buoyant force, the net force on a submerged object will be downward and the object will sink.

Sinking and Floating The object will only sink deep enough to displace a volume of fluid with a weight equal to its own. At that point, it will stop sinking deeper and will float.

Floating and Sinking If the weight of the object is less than the buoyant force, the object will float. If the weight of the object is exactly equal to the buoyant force, the two forces are balanced.

Density The density of a substance is its mass per unit volume. Density = Mass Volume An object that is more dense than the fluid in which it is immersed sinks. An object that is less dense than the fluid in which it is immersed floats to the surface.

Density If the density of an object is equal to the density of the fluid in which it is immersed, the object neither rises nor sinks in the fluid.

Densities of Substances

Densities of Substances Changing the density of an object can make it float or sink in a given fluid. For example, submarines change their density by pumping water out of it’s floatation tanks. The mass of the submarine decreases but the volume remains the same.

Buoyancy and Density Another way to change density is to change the volume.

Ships The shape of a ship causes it to displace a greater volume of water than a solid piece of steel of the same mass. The greater the volume of water displaced, the greater the buoyant force. A ship stays afloat as long as the buoyant force is greater than its weight.