Q. How is the tonnage of Ship related to water displaced?
Ans. According to the law of floatation, a body floats in a liquid, if the downward weight of the body is equal to the upward thrust produced by the liquid displaced by it. Thus, the law stipulates that the weight of the water displaced should be greater than or equal to the tonnage of the ship.
Q. Why does a ship rise as it enters the sea from a river?
Ans. The density of sea water is high as compared to river water. Consequently, the upthrust produced by the sea water on the ship is great as compared to the upthrust of river water. This explains the reason for the rising of ship, as it enters the sea from a river.
Q. Why is it easier to lift a heavy stone under water than in air?
Ans. According to Archimedes’ principle, when a body is immersed either wholly or partly in a fluid at rest, it experiences an upward thrust and loses weight equal to the weight of the fluid displaced by its immersed part. Thus, when the stone is put under water, it loses weight and hence is easier to lift.
Q. Why does an iron nail gain weight by rusting?
Ans. When iron is left exposed to air and moisture, it gets rapidly oxidised. This is known as rusting of iron. The chemical composition of rust varies somewhat, but it consists mainly of hydrated ferric oxide (2Fe203.3H30) and a small amount of ferrous carbonate (FeC03). The increase in the weight of iron nails is, therefore, equal to the amount of oxygen and water vapours taken up from the atmosphere.