Simple Pendulum & Elevator Homework | Period Formula for Accelerating Elevator

[hansel13]

Homework Statement

A simple Pendulum is suspended from the ceiling of an elevator. The elevator is accelerating upwards with acceleration a. The period of this pendulum, in terms of its length L, g, and a is:
2\pi*(L/a)^(1/2)
OR
2\pi*(L/(g+a))^(1/2)
OR
2\pi*(L/g)^(1/2)

Homework Equations

T = 2\pi*(L/g)^(1/2)

The Attempt at a Solution

Not sure where to start. It doesn't seem like there's really math involved here. I tried drawing a free body diagram, but that made things worse. I'm pretty sure it's the middle one, because the other 2 just don't make sense, because the formula needs to both a and g into account, right?

 

[Carid]

Right.

"we [...] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system." (Einstein 1907)

 

[hansel13]

OK thanks