Swaying stacked objects

I have a game which stacks objects and I'm wondering if there's a way to sway and move objects (horizontally) at the top of a stack. For example, if I'm a waiter and there are bar objects falling out of the sky (such as beer mugs and bottles) which are stacked when they land on top of each other, and the waiter is moving back and forth (on the x-axis), how would I move the bar objects at the top of the stack to slowly follow (and sway) with the direction of the waiter?

Any help would be greatly appreciated! :)

Thanks,
Kevin