system
Given the following post, respond as if you're commenting on Reddit:
human
Would a grandfather clock keep time in an elevator?

The pendulum will be heavier accelerating up but lighter accelerating down. Do they cancel out so the clock will work?
gpt
Simple answer is no.  First of all, when the gravity changes during the period is very important.  If the accelerations are quick and happen near the bottom of the pendulum swing, there would be no impact on period (and large impact on amplitude), but at the top of the swing it would have the largest impact on period (and no impact on amplitude).  So right there, if the acceleration and deceleration were not on the same part of the phase of the swing, time would not be kept properly.

If we assume the period of acceleration and deceleration is long compared to the period (and reduce this phase impact) the answer is still no.  Let's say we have 1g of positive acceleration for 1 second.  Gravity goes from 1g to 2g.  Frequency goes by square root of g, and so it increases by a factor of sqrt(2), or about 1.4.  Now to come to rest, we need 1 second of 0g.  Assume this happens right as the pendulum stops (to prevent it whirling around and around).  It has now lost 1 second of time, but only gained 0.4 or so seconds.

  So on average, the clock should lose time, but this could change drastically depending on when the acceleration happens with regards to the phase.