Gravitation : NCERT Intext Questions

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Page 141

 Q.1      Why is it difficult to hold a school bag having a strap made of a thin and strong string ?
Sol.         It is difficult to hold a school bag having thin straps as pressure exerted by the thin straps on the shoulder will be more than broad strap.The reason being Pressure \alpha {1 \over {Area}}

Q.2      What do you mean by buoyancy ?
Sol.       Buoyancy is the upward force experienced by an object placed in a fluid.

Q.3      Why does an object float or sink when placed on the surface of water ?
Sol.        An object floats or sinks when placed on water depends on the net force. If the weight is less than buoyant force it will float, and if more than it will sink.

Page 142

Q.1      You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Sol.       On the weighing machine mass is measured by comparing weights. Since our actual weight is slightly more than that measured by machine (as buoyancy reduces the  weight)  our mass is slightly more than that measured.

Q.2      You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighting machine. In reality, one is heavier than other. Can you  say  which one is heavier and why?
Sol.        Actual weight = Measured weight + Buoyant Force .
              Cotton bag is much larger than iron bar, so its buoyant weight is more .
              Hence , actual weight of cotton is more than the iron bar.




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