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    Mechanical Properties of Fluids
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    Why it is easy to cut with a sharp knife than blunt one?
    Sharp knife-edge has smaller area than a blunt one. When the force is applied on the knife, the sharp edged knife produces more pressure on the surface and gets penetrated into the surface.
    Thus, it is easy to cut with a sharp knife than blunt one.
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    Why does a siphon not work in vacuum?
    The functioning of siphon is based on the atmospheric pressure. On sucking, the pressure of the air inside the siphon decreases and the atmospheric pressure on the liquid outside pushes the liquid up in the siphon. If there is vaccum, i.e., there is no atmospheric pressure, then there is no force on the liquid that can push the liquid into the siphon and siphon does not work. 
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    Explain why water cannot be used in place of mercury in a barometer?

    The atmospheric pressure at sea level = 76cm of Hg = 1.013x105 Pascal. 

    If we fill the water in barometer tube, the height of water column required to balance the atmospheric pressure will be,

       h =Atmospheric PressureDensity of water×acceleration due to gravity   =1.013×1051000×9.81   =10.326m 

    That is, if water is used in barometer tube instead of mercury, the length of the tube must be greater than 10.326 cm. A greater height will be difficult to hold in a vertical position and needs a high tower to install the barometer. 

    So, we cannot replace mercury by water in the barometer. 
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    What do you mean by absolute pressure and gauge pressure?

    Absolute pressure is the pressure above that of vacuum. 

    Absolute pressure = Gauge pressure + One atmospheric pressure.

    Gauge pressure is the pressure above that of one atmospheric pressure. Since the pressure of vacuum is zero, it is referenced against an ambient air pressure.

    Note: Atmospheric pressure is the pressure at any point equal to the weight of a column of air of unit cross-sectional area extending from that point to the top of the atmosphere. 

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    Why are paper pins and nails made to have pointed ends?
    Pointed paper pins or nails have small area as compared to the blunt one. Lesser the area, greater is the force applied. When force is applied on the pins, they exert large pressure on the surface and hence easily pierce the surface and get penetrated into it. 
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