The bob of a simple pendulam has a mass of 40 g and a positive charge of 4.0 x 10^-6 C.It makes 20 oscillations in 45 s.A vertical electric field pointing upward and of magnitude 2.5 x 10⁴N/C is switched on.How much time will it now take to complete 20 oscillations?

mass of bob, m = 40 g = 0.04 kg

Bob performs 45 oscillations in 20 seconds.

so time period, T = 20/45

find out g using the formula

After electric field is switched on, effective g' = g - qE/m

and again use the above formula to find T' (new period of oscillations)