Corrected entry: As the first layer dream was in zero-gravity (i.e: the van falling off the bridge), the second layer was also in zero-gravity (i.e: the hotel room). But the zero-gravity effect did not continue to the third layer dream (i.e: the snowy mountains).
Knever
12th Aug 2010
Correction: They were not in zero-g in the first level. They were in normal gravity. That's why the van was falling. It was zero-g in the second level because their bodies felt zero-g as the van was falling. The third level was also not affected because the second level wasn't doing anything to make them feel like they were in zero-g. The van only looked like that because the filming was slowed down to remind people that dream time goes by much slower than real time. Basically, a dream level is only affected by the level directly above it.
Knever