Comment by mlyle

> They should, in my view, have had y as a function of t, and dropped x.

Then you could have only had a flat floor that moved up and down. If you need shape that changes, you need it to be a function of both x and t.

So e.g. (x-t)^2 / 5 is a parabola shaped "bowl" that moves right at 1 unit per second.