From the experience of our daily life we observe that any solid substance when immersed in liquid it appears lighter. Its cause is on an immersing body vertically upward force or buoyancy acts on it.

Before 300 B.C. the Greek philosopher Archimedes invented that if a body is partly or fully immersed in a liquid or gas in equilibrium, it seems to have lost a part of its weight. This apparent loss of weight is equal to the weight of the displaced liquid or gas.

**Experiment:** Take a body of known weight. Tie the body by a light thread and dip it into a container completely filled with water. Some water will spill out of the vessel. Take the weight of the body in completely submersed condition. Now find out the apparent loss of weight by subtracting the weight of the body immersed in water from the known weight of the body in air. Now determine the weight of the spilled out water. It will be observed that the apparent loss of weight of the body is equal to the weight of the displaced liquid. Thus we may have a proof of Archimedes law principle in a simple way.

**Calculate:** The surface area of the bottom of a rectangular block is 25 cm. It is dipped in water. Density of water is 1000kgm-3. The depth of the upper surface of the block from the open surface of the water is 5cm. If the height of the block is 2 cm, then

- Find the water pressure P
_{1}on the upper surface of the block - Calculate the pressure P
_{2}at the bottom of the block - How much force will be applied by water on the upper surface of the block?
- How much force will be applied by water at the bottom of the block?