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Archimedes Principle

At any point in a liquid at rest, the liquid exerts pressure equally in all directions. So, when a body is immersed in a liquid, the liquid exerts an upward thrust at the bottom of the body. This upward thrust that the liquid exerts on any body immersed in it is called buoyant force. Due to this upward thrust exerted on the body by the liquid, the weight of the body appears to be less when the body is immersed in the liquid.
 

Principle of Archimedes

When a body is immersed wholly or partly in a fluid at rest, it suffers an apparent loss of weight, which is equal to the weight of the liquid displaced by it.
 

Verification of the Principle of Archimedes
The socket and cylinder apparatus consists of a hollow cylinder called the socket and a solid cylinder, which fits exactly into the socket. The volume of the cylinder is equal to the capacity of the socket. The socket is suspended from the hook of a physical balance and the cylinder is hung from the hook of the socket. The balance is counterpoised with weights so that the beam is horizontal.
 

A hydrostatic bench is placed across the left pan of the physical balance and a beaker of water is kept under the cylinder so that it is immersed completely in the water without touching the sides or the bottom. There should be no air bubbles sticking to it. It is found that the equilibrium of the balance is disturbed and it tilts to the other side, showing that the body now weighs less.

A body appears to lose some weight when immersed in a liquid due to a property of liquids called buoyancy. The body loses weight when immersed in a liquid because the liquid exerts an upward force on the body. The upward force experienced by a body which is partly or completely immersed in a liquid is called the buoyant force. It is due to this force that the weight of a body immersed in a liquid appears to be less than its actual weight.

The weight of a body is the force with which it is pulled downwards by the gravity of the earth. A body immersed in a gas also experiences a buoyant force. Liquids and gases are called fluids.

When a body is partly or completely immersed in a fluid, it experiences an upward force called buoyant force.


Activity 2
Take a jug made of glass or plastic, two pieces of cork (one bigger than the other) and some common salt. Fill the jug with water. Push the smaller piece of cork to the bottom of the jug and leave it. The cork rises to the water because the upward buoyant force exerted by the liquid on the cork is greater than the downward pull of gravity.
 

Try the above experiment with the bigger cork piece and carefully observe its rise to the surface. The bigger cork rises faster than the smaller one which means that the buoyant force depends on the size (or volume) of the cork.

 

Now add salt to the water and stir it. This increases the density of the solution. Try the above two experiments in the salt solution. You will find that both corks rise faster in the salt solution than in water. Thus, the buoyant force also depends on the density of the liquid in which the cork is immersed.
 

The buoyant force depends on:

1. The size or volume of the body immersed in a liquid and

2. The density of the liquid in which it is immersed.

 

To measure the buoyant force, take a solid (a piece of stone or metal). Suspend it by a thin thread from the hook of a spring balance. The reading on the scale of the spring balance gives the weight of the solid in air. Let us suppose it weighs 200 g. Gently lower the body into the water in a beaker. The new reading on the scale gives the weight of the body when it is immersed in water, which is less than the first reading. Let us suppose it weighs 140 g. Thus, the buoyant force is 200 – 140 = 60 g wt. 
 

Repeat the experiment with a bigger solid and measure the buoyant force when it is completely immersed in water. The buoyant force will be greater than 60 g wt.

Finally, perform the above experiments using a different liquid, such as kerosene. The buoyant force will be less than that with water because the density of kerosene is less than that of water. Thus, the buoyant force on a solid completely immersed in a liquid is greater if the volume of the solid is increased or if the density of the liquid is increased.
 





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