1)

 Two pendulums begin to swing simultaneously. The first pendulum makes nine full oscillations when the other makes seven. The ratio of the  lengths of the two pendulums is 


A) 4981

B) 6481

C) 89

D) 79

Answer:

Option A

Explanation:

 As two pendulums begin to swing  simultaneously , then

    n1T1   = n2 T2    ...........(i)

 where n1  and n2 are the numbers of oscillations of the first and second pendulum respectively and T1 and T2 be their respective time periods.

  The time period of simple  pendulum is given by

  T=2πlg

 where, l= length  of pendulum

and g= acceleration  due to gravity

      T2l  .......(ii)

 So, from  Eqs. (i) and (ii), we get

     l1l2=T21T22=n22n21

   Here, n1=9, n2=7

   l1l2=(7)2(9)2=4981

 Hence, the ratio of pendulum  lengths l1:l2= 49 :81