Buoyancy and Archimedes Principle

"Eureka !"
("I've got it !")
Archimedes

If an object is submerged (or partially submerged) in a fluid the buoyant force is the force which tends to push the object towards the surface.

Archimedes Principle

" The buoyant force on a submerged or partially submerged object is equal to the weight of the fluid displaced by the object "

The magnitude of the buoyant force determines whether an object will sink or float.

weight of object > buoyant force => object sinks and its density is greater than the fluid

weight of object < buoyant force => object floats and its density is less than the fluid

Interesting example of buoyancy. A can of diet coke floats in water, a can of classic coke does not float - demonstration.

Icebergs are frozen blocks of freshwater "calved" from glaciers or formed during the break-up of Winter pack-ice. Icebergs float in the ocean because the density of freshwater is less than that of saltwater. However, since there is only a small difference between these densities icebergs float with most (about 90%) of their mass underwater. This can lead to disasterous results as in the case of the RMS Titanic . This picture demonstrates that "what you see is not necessarily what you get...". But is this a "real" photograph ? Click here for an explanation.

Problems involving buoyancy and Archimedes principle are often most easily solved by considering the equilibrium or otherwise of the forces acting on the object in question.

"Have you heard the one about the retired general who said he had not had sex since 1956? His friend said, 'That's a long time ago.' 'I don't know,' the general replied. 'It's only 20.27 now."

Ronnie Barker

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu