Page 57 - Physics - XI
P. 57

Q19.  Who discovered simple pendulum?
          Ans.  Galileo

           Q20.  What will happen if the bob is made to vibrate in a liquid?
          Ans.  Due to the upthrust of liquid, the eff ective weight and hence g will decrease and hence time period
                            l
                    T   2        will increase.


                            g
           Q21.  How will the amplitude of oscillation change if the pendulum oscillates in perfect vacuum?
          Ans.  The pendulum will continue oscillating forever without any decrease in amplitude.
           Q22.  Can you use rubber string in the simple pendulum?
          Ans.  Rubber string cannot be used as it is extensible and hence its length changes. The time period
                 therefore increases.

           Q23.  Why should there be no spinning motion of the bob during its oscillations?
          Ans.  Due to the spinning motion of the bob, there will be twists in the suspension thread which will eff ect
                 the length and hence the time period of the pendulum.

           Q24.  Interpret the graphs between L and T , and also between L and T that you have drawn for a
                                                          2
                 simple pendulum.                                                                           (NCERT)
          Ans.  The graph between L and T  is a straight line passing through the origin. It means that the square of
                                             2
                 the time period of a pendulum is directly proportional to the eff ective length of the pendulum. The
                 graph between L and T is curved, convex upward. It means that increasing the length increases the
                 time period only up to a certain extent.

           Q25.  How can you determine the value of 'g', acceleration due to gravity, from the T  vs L graph?
                                                                                                      2
                                                                                                            (NCERT)
          Ans.  We know that the time period of a simple pendulum is given by,

                                       L
                              T =  2π
                                       g

                                          L
                 So,          T  =  4  2                                                                       …(1)
                               2
                                          g
                 For an eff ective length of the pendulum, we fi nd the corresponding value of T  from the graph. On
                                                                                                2
                 putting these values in equation (1), we get the value of g.




                                           COMPETENCY-BASED QUESTIONS
                                           COMPETENCY-BASED QUESTIONS

          Multiple Choice Questions
           Q1.  The time period of a second's pendulum is
                  a.  1 second             b.  2 seconds            c.  3 seconds             d.  4 seconds
           Q2.  In a second's pendulum, mass of bob is 30 gram. If it is replaced by 90 gram mass, then its time

                 period will be
                  a.  2 seconds            b.  1 second             c.  3 seconds             d.  4 seconds

                                                                                                              55
   52   53   54   55   56   57   58   59   60   61   62