I'm learning along with the video:
"The Secret Language of Houdini from SideFX Houdini on Vimeo"
https://player.vimeo.com/video/240187599 [player.vimeo.com]

Not doing too badly - some of the details seem to have changed...
I'm not sure if the video shows a different version of Houdini? (I'm on 18.5)

in the video at timestamp 09:26
there is the following expression, used to rotate the ball as it rolls...

ch("../Translate/tz")/(2*+3.414*ch("../Rotate/ty"))*360

I have tried loads of things and googled but cannot get this to work.

I have also searched the forum.

Any tips/advice welcome - Thank you in advance.