question_answer 45)

                  A raindrop falling from a height h above ground, attains a near terminal velocity when it has fallen through a height (3/4)h. Which of the diagrams shown in Fig. correctly shows the change in kinetic and potential energy of the drop during its fall up to the ground?                 (a)    (b)                   (c)       (d)                

Answer:

                  (b) Speed attained after falling through a height                 \[\frac{3h}{4},\,v=\sqrt{2g\,\times \,\frac{3h}{4}}=\,\sqrt{\frac{2gh}{2}}\]                 \[\therefore \] K.E. after falling through a height \[\frac{3h}{4}\]                 \[=\frac{1}{2}\,m\,\times \frac{3gh}{2}=\,\frac{2mgh}{4}\]                 At height h, P.E. = mgh (maximum) and K.E. = 0.                 As raindrop falls, its P.E. decrease and its K.E. increases.