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    Mechanical Properties of Fluids
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    Explain with an example that atmosphere exerts a huge pressure.
    Take a hollow sphere. Cut it into two hemispheres. Place the two hemispheres in contact with each other. You will find that when there is air inside the hemispheres, two get easily separated. This is because there is air on both the sides of sphere at same pressure and hence net force on each hemisphere is zero and could be easily separated.

    Take a hollow sphere. Cut it into two hemispheres. Place the two hemi

    Now remove the air by using vacuum pump from the two hemispheres by placing in contact with each other. When there is prefect vacuum inside, you will see that you are not able to separate them by applying a large force. As shown in figure, if we have a sphere of surface area 1 m2, we need the weights more than 10 quintal to separate the two hemispheres. This experiment shows that air exerts a huge pressure.
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    Why the passengers in aeroplane are advised to remove the ink from their fountain pens?
    As altitude increases, the atmospheric pressure decreases in comparison to that on the surface of the earth. Ink on the fountain pens is filled at the atmospheric pressure on the surface of the earth. Therefore, as height increases ink will come out of the pen.
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    To empty an oil tin, two holes are made diagonally. Why?
    When hole A is made in the tin, the oil coming out will be less because the volume of the air above the oil increases and hence the pressure of the air decreases, obstructing the flow. When the pressure of air above oil plus pressure due to height column of oil becomes less than atmospheric pressure, the oil stops to ooze out. 

    If another hole B is made which is diagonal to the first hole, atmospheric air from this hole keeps on entering and hence pressure at A remains greater than atmospheric pressure and hence oil will continue to flow out from the tin.
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    Straws are used to take soft drinks. Why?

    Straws make use of the capillary action as a result of the difference in pressure.

    When a person suck though the straw, pressure inside the straw decreases and due to the difference in pressure outside and inside the straw, the liquid rises in the straw. 
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    In a dropper, the liquid does not come out unless its bulb is pressed.
    The pressure due to the height column of liquid inside the dropper is less than atmospheric pressure. That is the liquid is held inside the dropper against the atmospheric pressure. On pressing the dropper, the pressure inside is increased and when the pressure inside exceeds the atmospheric pressure, liquid comes out from the dropper. 
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