How Does a Boat Float in Water ?

 The Principle of Flotation


   When the weight of an object acting downwards is equal to the upward buoyant force exerted by the liquid, the buoyant  force will balance the weight of object due to which the object will float  in the liquid.  So, for an object to float in a liquid, the weight of object should be equal to buoyant force acting on it. But by Archimedes'  principle,  the buoyant force acting on an object is equal to the weight of liquid displaced by it. This condition gives us the principle of flotation which can be stated as follows. 

           According to the principle of flotation : An object will float in a liquid if the weight of object is equal to the weight of liquid displaced by it. That is, for flotation :

 Weight of object = Weight of liquid displaced

                                                       by it 

How does a Boat float in water

A boat is kept afloat by an upward force from the water. This upward force is called Buoyant  force (or upthrust) and it is caused by the pressure of water 'pushing up' on the bottom of the boat. When a Boat is gradually lowered into water ( in a river or sea ) it displaces more and more water due to which the upward 'buoyant  force' on it increases. The boat stop sinking down into water when the buoyant force acting on it is just enough to support the weight of boat. Now, Archimedes' principle says that the buoyant forcing is equal to the weight of liquid displaced by the boat. So, when a boat is floating, the weight of water displaced by the submerged part of the boat is equal to the weight of the boat.


The Density of Floating Objects

We have just learnt that for an object float in a liquid, the weight of liquid displaced by it should be equal to its own weight. This can be happen if the object has a lower density (or lower average density) than the liquid. If, however, the object has a higher density than the liquid, then the  weight of liquid displaced will be less than the weight of the object and the object will not float, it will sink.

1. An object will float in a liquid if its density (or average density) is less than that of the liquid. See the above figure for clearation : 

(I) The density of cork is less than that of water, so cork floats in water.


Why Ships Float

When we put a piece of iron in water, it sinks immediately because iron is denser than water. Why should then ship made from iron and steel float in water ? This can be explained on the basis of 'average density' as follows.

Ship is not a solid block of iron and steel. A ship is a hollow object made of iron and steel which contains a lot of air in it. Air has a very low density. Due to the presence of a lot of air in it, the average density of the ship becomes less than the density of water. And since the average density of ship is less than that of water therefore, a ship floats in water. Thus, A ship is  made of iron and steel floats in water because its average density is less than that of water (due to the presence of a lot of air space in it). In fact, all the hollow object made of dense materials (like metals) float in water because 



due to  the presence of a lost of air in them, their average density becomes less than the density of water. The floating of ships can also be explained in another way as follows. 


 
When a ship is placed in water it sinks to a certain level such that the weight of water displaced by its submerged portion is equal to the weight of the ship. Since the ship fulfills this  condition of floatation, it floats in water. Thus, a heavy ship floats because it displaces a large weight of water (which provides a great  buoyant force to keep it afloat)
      
                                 Density
Some substances appear to be heavy whereas  other are light. For example, . iron is a heavier than aluminium and water is heavier than alcohol. In physics we describe the lightness or  heaviness of different substances by using the word density. The density of a substance is defined as mass of the substance per unit volume.

           Density = Mass
                            Volume








I am Saheb Sarkar from purnea .

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