question_answer 78)

                  A balloon filled with helium rises against gravity increasing its potential energy. The speed of the balloon also increases as it rises. How do you reconcile this with the law of conservation of mechanical energy? You can neglect viscous drag of air and assume that density of air is constant.

Answer:

                  When balloon goes up, the increase in the K.E. and P.E. of the balloon is equal to the change in K.E. of the air which comes down.