Floatation

•    According to law of floatation, when a solid is floating in a fluid, then the weight of the whole solid acting vertically downward at its centre of gravity is equal to the weight of the fluid displaced by immersed part of solid acting vertically upward, at its centre of buoyancy.
•    A floating body has the weight equal to the weight of the fluid displaced by immersed part of body and its apparent weight and apparent density is zero.
•    Floatation of ships, fishes, submarines and icebergs are based on the law of floatation.

                                            
 

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  • Q1

    How icebergs are dangerous to ships?

    Marks:2
    Answer:

    As the density of ice is less than the density of sea water, it floats on water such that a large part of it remains immersed inside the sea which displaces just enough water to balance its enormous weight. As this underwater part of the icebergs is not visible to the sailors, icebergs are extremely dangerous for ships as ice can hit the ship and sink it.

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  • Q2

    How do submarines are able to float inside water?

    Marks:1
    Answer:

    A submarine is like an air tight ship. It displaces more weight of water than its own weight and hence floats on the surface of water. It can be sunk lower or completely inside the water by admitting the water to its internal tanks.

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  • Q3

    Why are ships filled with ballast before allowed to sail out?

    Marks:1
    Answer:

    For an empty or partially loaded ship, centre of gravity of the ship tends to be higher than the meta-centre and hence the ship is in unstable equilibrium. So it can overturn in a rough sea.

    To avoid such a situation, the ships are filled with ballast before they are allowed to sail out.

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  • Q4

    State the forces acting on the wooden block. Also give the direction in which they act.

    Marks:1
    Answer:

    The forces acting on the wooden block are:

    (a) weight of the block (W) in vertically downward direction.

    (b) Upthrust (buoyant force F) acting in vertically upward direction.

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  • Q5

    State whether the following objects will sink or float when dropped in water

    (a) metal ball

    (b) wooden block

    (c) stone
    (d) a paper sheet

    Marks:1
    Answer:

    Objects with the density more than the density of water will sink and vice-versa.

    (a) Sink

    (b) float

    (c) sink

    (d) float

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