question_answer 34)

                  At a metro station, a girl walks up a stationary escalator in time                 . If she remains stationary on the escalator, then the escalator take her up in time                 . The time taken by her to walk up on the moving escalator will be                 (a)                                         (b)                                 (c)                             (d)                                  

Answer:

  (c) Velocity of girl, \[\upsilon \,=\,\frac{L}{{{t}_{1}}}\]                 Velocity of escalator, \[{{\upsilon }_{2}}\,=\,\frac{L}{{{t}_{2}}}\]                 Relative velocity of girl w.r.t. escalator \[={{\upsilon }_{1}}\,+{{\upsilon }_{2}}\]                 \[\therefore \] \[t\,=\,\frac{1}{{{\upsilon }_{1}}\,+{{\upsilon }_{2}}}\,=\,\frac{{{t}_{1}}{{t}_{2}}}{{{t}_{1}}+{{t}_{2}}}\]