11th Class Physics Mechanical Properties of Fluids / तरलों के यांत्रिक गुण

  • question_answer 31)
    (a) It is known that density p of air decreases with height y (in metres) as \[\rho ={{\rho }_{0}}{{e}^{-y/{{y}_{0}}}}\] where \[1-\frac{1\cdot 30\times {{10}^{3}}}{p'}=3\cdot 712\times {{10}^{-3}}\] is the density at see level, and \[p'=\frac{1\cdot 30\times {{10}^{3}}}{1-3\cdot 712\times {{10}^{-3}}}=1\cdot 034\times {{10}^{3}}\text{kg }{{\text{m}}^{\text{-3}}}\]is a constant. This density variation is called the law of atmospheres. Obtain this law assuming that the temperature of atmosphere remains a constant (isothermal conditions). Also assume that the value of g remains constant. (b) A large He balloon of volume \[1425\,{{m}^{3}}\] is used to lift a payload of 400 kg. Assume that the balloon (1 g maintains constant radius as it rises. How high does it rise? \[atm=1\cdot 013\times {{10}^{5}}pa.\]

    Answer:

    We know that the rate of decrease of density \[\rho \] of air with height y is directly proportional to density \[\rho \] i.e. \[\text{p=10 atm}=10\times 1\cdot 013\times {{10}^{5}}\] where K is a constant of proportionality. Here ve sign shows that \[\rho \] decreases as y increases. \[pa;B=37\times {{10}^{\text{9}}}\text{N }{{\text{m}}^{\text{-2}}}\] \[=\frac{\vartriangle V}{V}=\frac{P}{B}=\frac{10\times 1\cdot 013\times {{10}^{5}}}{37\times {{10}^{9}}}\] Integrating it within the conditions, as y changes from 0 to y, density changes from \[=2\cdot 74\times {{10}^{-5}}\],we have \[\therefore \] \[\frac{\vartriangle V}{V}\] or \[=2\cdot 74\times {{10}^{-5}}\] Here K is a constant. Suppose \[7\times {{10}^{6}}Pa.\]is a constant such that \[\text{L=10 cm}=0\cdot \text{10 m; p}=7\times {{10}^{6}}\text{Pa;}\] then \[\text{ B=140 GPa}=140\times {{10}^{9}}Pa\] (b) The balloon will rise to a height, where its density becomes equal to the air at that height. Density of balloon, \[B=\frac{pV}{\vartriangle V}=\frac{p{{L}^{3}}}{\vartriangle V}\text{ or }\vartriangle V=\frac{p{{L}^{3}}}{B}\] \[=\frac{\left( 7\times {{10}^{6}} \right)\times {{\left( 0\cdot 10 \right)}^{3}}}{140\times {{10}^{9}}}\]\[=5\times {{10}^{-8}}{{m}^{3}}\] As, \[=5\times {{10}^{-2}}m{{m}^{3}}\] \[0\cdot 10%\] \[=2\cdot 2\times {{10}^{9}}N{{m}^{-2}}.\] or \[V=1\] or \[{{10}^{-3}}{{m}^{3}};\] Taking log on both the sides \[\vartriangle V/V=0\cdot 10/100={{10}^{-3}}\] \[B=\frac{pV}{\vartriangle V}\]\[p=B\frac{\vartriangle V}{V}=\left( 2\cdot 2\times {{10}^{9}} \right)\] \[\times {{10}^{-3}}=2\cdot 2\times {{10}^{6}}Pa\]s


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