Hi. I'm unsure how to know where expressions will work in Houdini. It seems very hit/miss but there is surely some logic to it?

A tutorial used this expression to determine when an object would begin to drop:

if( ($F < 50), 0, 1000)

This was in the massdensity channel under the Model tab of a FEM Solid Object node sitting inside an AutoDopNetwork. It was great. The object floats in the air with a massdensity of 0 until frame 50 and then suddenly drops with a massdensity of 1000 after that.

Today, I tried to use the same expression but instead, in the Density channel under the Physical tab of an RBD Object node sitting inside an AutoDopNetwork. Absolutely nothing happened.

So why did it work on the FEM Solid Object node in a DOP network but not on an RBD Object node in a DOP network?

Thanks.