Everything Floats

Today I'm gonna introduce a physical concept that is used a lot in Chemistry. Like some physical properties, this one is hard to explain; I never had a clear concept on college and only understood it correctly in university.

Let’s use this example:

What weights more, a pound of rocks or a pound of feathers?

                                  

Both are the same, a pound is a pound, the difference is that to have a pound of rocks we would only need a few smaller rocks, but to get a pound of feathers we will need a lot of them.

The difference of the volume occupied by these 2 objects is because of the difference of DENSITY between them.

The density of a material is its weight divided by the volume occupied by that mass. The density of liquid water is 1, this means I liter of water weights 1 Kilogram, but the density of ice is 0.9 because 1 liter weights 900 grams. This is the explanation of why the ice floats in your drink.

Hydrostatic principle

If we tie a rock to a string and we immerse it into water we observe that the necessary effort to hold it is less than when it is in the air; it’s like if its weight was decreased. In the same way, if we try to sink a glass pushing its bottom into the water we will notice a big resistance.

These 2 phenomenons obey the same reason: Every liquid exert over the objects immersed in them a force in the contrary direction of the weight, called Push or Bouyancy.

Archimedes discovered that the push experimented by any object immerse in a liquid or a gas is equal to the weight of the fluid being displaced but in opposite direction.

Floating

 

Therefore, on any object immerse in a liquid act 2 forces: its own weight, directed to the center of the Earth (gravity) and the push force of the fluid, directed in the opposite direction. When the weight is bigger than the Push, the object sinks. When the push is equal to the weight, the object remains suspended in equilibrium without movement. And when the push is bigger than the weight, the object goes up to the surface until the weight of the immersed section equals the weight of the displaced fluid.

 Any object can float, as long as it is immersed on a substance more dense than itself, like this bronze object that is floating in Mercury. 

 Source: Ciencia Recreativa (1992) - Planeta Agostini