Here is how you get the answer to this question.
Given that the ascending is with an acceleration of g/3, now, let's look for the time period of a simple pendulum suspended from the ceiling, given that its time period in stationary lift is T.
So,
Fnet = T - mg
Fnet = ma = T - mg
m(g/3) = T-mg
T=4mg/3
This tension decides the apparent weight, and therefore the apparent
"g"
so apparent g= 4g/3
now,T=2π √ l/g(apparent)
therefore,T′=3–√/2T
Hope this answer helps. Thanks for posting your question.
