The force experienced by an electric charge is given as,
F=qE ma=qE a= Substitute the values in above expression.
a= a=2.1m∕s2 The expression of time period in absence of an electric field is,
T0=2π√=2π√ The expression of time period in presence of an electric field is,
T1=2π√=2π√ =2π√=2π√ Here, the effective acceleration on bob is
ae The ratio of time period is,
=√= T1=×=2s The time taken to complete 15 oscillation is,
T1=2s×15=30s