the bob of a simple pendulum has a mass of 40g and a positive charge of 4.0uc(uc=micro coulomb). it makes 20 oscillation in 45 s. A vertical electric field pointing upwards and of magnitude 2.5*10^4 N/c is switched on. how much time will it now take to complete 20 oscillation?

Asked by Rohan Kumar | 3rd Jun, 2013, 11:35: AM

Expert Answer:

charge on bob, C = 4.0 micro coulumb
electric field, E = 2.5 x 104 N/C
thus force on the bob, F = CE = 0.1 N
mass of bob, m = 40g  = 0.04 kg
thus external acceleration on bob = F/m = 2.5 m/s2
thus nt acceleration of bob in downward direction, A = g-a = 9.8 - 2.5 = 7.3 m/s2
let lenght of pendulum = L
initial time period = 45/20 = 2pisqrt(L/g)
thus we can find L
and hence the new time period
and thus time taken for 20 oscillations

Answered by  | 5th Jun, 2013, 12:22: AM

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