THE LAW OF FLOATATION

FLOATATION

Ships, canoes and ferry boats move on the upper part of the water bodies, we say they float.it may happen that due to accident or over loading they sink. Example Titanic went to the bottom of North Atlantic Ocean after hitting larger ice- berg.

A balloon filled with lighter gas such as hydrogen will keep rising up when released from the hand and a hot air balloon will keep rising as well with the people in carriage once the air in balloon has been heated and expanded.

Therefore floatation can be defined    as   the tendency of an object to rise up to the upper levels of the fluid or to stay on the surface of the fluid.

The opposite of floatation is sinking and can be defined   as the tendency of an object to go to the lower levels of the fluid.

MV.Liemba floats on lake Tanganyika

THE LAW OF FLOATATION

The law of floatation states that, a floating body displaces its own weight of the fluid in which it floats.

This means if a log of 200kg (2000N) floats in water displaces 200kg (2000N) of water, if the same log is placed in other liquid and be able to float it will displace the same 200kg of fluid in which it floats.

CONDITIONS FOR OBJECTS TO FLOAT

        I.            The average density of the object should be less than the density of the fluid in which the object has to float.

Example, a ship is very heavy but it floats because it is hollow inside it contains air, this causes its average density to be lower than that of water.

     II.            The upthrust force of the fluid on the object must be equal  total weight of the object.(law of floatation)

Example, a coin will sink to the bottom when placed on the surface of water, this is because the upthrust of water on coin is less than its weight

   III.            The volume of object submerged must be large so as to displace large amount of fluid.

RELATIONSHIP BETWEEN UPTHRUST AND REAL WEIGHT OF FLOATING BODY

We know that,

Real weight = apparent weight + apparent loss in weight

Apparent loss in weight = weight of fluid displaced = upthrust

Therefore,

Real weight =apparent weight + upthrust

But when an object floats its apparent weight is          zero

Apparent weight   = 0

Then,

Real weight = 0 + upthrust

Real weight = upthrust.

Therefore relationship between upthrust and real weight of floating object is such that upthrust is equal to real weight, and since upthrust is equal to weight of fluid displaced then floating body displaces its own weight of the fluid in which it floats (law of floatation)

In order an object to float its real weight must be equal to upthrust.

If an object floats the volume of fluid displaced is equal to the volume of an object submerged and the percentage of the volume of an object submerged is equal to the relative density of an object with respect to the density of fluid in which it floats.

Thus, 

Example

An ice cube of density 0.9g/cm³ floats in freshwater of density 1.0g/cm³ what fraction of volume of ice is submerged? If the same ice is floating in sea water of the density 1.3g/cm³ what is the percentage volume of ice will be submerged?

Solution

Percentage volume of an object submerged in fresh water,

Percentage volume of an object submerged in sea water

APPLICATION OF LAW OF FLOATATION IN EVERY DAY LIFE

In transportation by water ways

The law of floatation is applied in all vessels which travel by waterways that include ships, submarines and ferry boats.

In transportation by air ways

It is also applied in some vessels which travel by air ways such as hot air balloon and air ship.

In decoration

Balloons of different colors and shapes are filled with lighter gas so that will float in air.

In measurement of specific gravity of liquids

Hydrometer is an instrument which is used to measure specific gravity of liquids, in its operation it employs the law of floatation.

Other uses include making of bulges and  transportation of logs down the river.

Ships

Ship is like an iron bowl floating in water, although iron is denser than water the bowl does not sink. Being hollow it contains air causing its average density to be lower than that of water.

Sand can be poured in the bowl but it will keep floating, in excess mass of sand the bowl will sink, in ship we call it overloading.

Ships are made of metals such as steel or aluminium alloy which are denser than water, so to make it be able to float it is hollow and filled with air making the average density lower than density of water even when it is loaded with her cargo.

Ship travels in different densities of water, sea water or fresh water, (hot or cold)

So by changing water ways ship can displace more or less water in which it floats that means it may gain or lose upthrust. If the upthrust becomes lower than its own weight the ship sinks.

So for safety loading of the ship under different sea conditions plimsol lines are provided.

Plimsol lines are lines which show maximum height of the ship that should be under water.

Plimsol  lines are also referred as plimsol marks.

Pic of marksxxxxxxxxxx

Submarine

Submarine is ship which can travel deep under water and also at the surface of water, it owes its ability to dive and come up to law of floatation.it contains ballast tanks at either sides which are filled with water when it has to dive and are emptied  when it has to come up. The tanks are filled and emptied by special pumps.

submarine at different levels from sea surface

submarine at the surface and underwater

Hot air balloon

To watch animals at Serengeti helicopters is noisy it would scare the animals,plane is faster it would pass without clear vision but hot air balloon will do.

Hot air balloon consist of three parts,the balloon,the burner and the basket.

The burner uses the propane gas to heat up air in the balloon as the air in  the balloon gets hot it expands,its density becomes lower than the surrounding air so it raises.

To get the balloon down the pilot can open a parachute valve at the top of the balloon causing cold air to enter in the balloon and decrease the temperature inside the balloon so the density,also the pilot can let the air inside the balloon to cool itself by burning less fuel.

hot air balloon

Air ship

Air ship uses lighter gas such as helium, hydrogen or hot air to rise up, and maneuvering is done by means of propeller attached to it,example at cabin.

The lighter gas gives it lifting and the engine is used to propel it .

                                             airship

Balloons

Balloons used for decoration are filled with lighter gas such as hydrogen or helium so that they stay in the air,when filled by blowing air to them the can not rise because the air we blew in them denser or equals to the density of air when it is air itself(if not contaminated by exhaled air).

Pizxxxxxxxxxxxxxxxxxxxxxxx

Transportation of logs by the liver

Picxxxxxxx

Bulge

A bulge is floating  platform which can be used for several activities for example recreational,transportation,research or working platform,bridge etc.

Picxxxxxx  bulge,,

The hydrometer