question_answer

Problem 1. Explain why : (i) A balloon filled with helium does not rise in air indefinitely but halts after a certain      height (Neglect winds). (ii) The force required by a man to raise his limbs immersed in water is smaller than the force for the same movement in air.                         

Answer:

                (i) A balloon filled with helium goes on rising in air so long as the weight of the air displaced by it (i.e., up thrust) is greater than the weight of filled balloon. We know that the density of air decreases with height. Therefore, the balloon halts after attaining a height at which density of air is such that the weight of air displaced just equals the weight of filled balloon. (ii) Water exerts much more up thrust on the limbs of man than air. So the net weight of limbs in water is much less than that in air. Hence the force required by a man to raise his limbs immersed in water is smaller than the force for the same movement in air.