Inertia and gravity are the same on all the levels. I don't know why it would feel different... Yes, levels 8 and 9 are meant to be more challenging. you might encounter obstacles like that in the random levels. Each level of the tutorial is meant to give you practice tackling different kinds of obstacles. I would love to see what automatic inputs can do for some of these levels :)

I was able to manually succeed on level 8, but I have no idea how it's possible to accomplish level 9. I can't figure any way to get enough inertia or upward thrust to escape the narrow spot once I'm most of the way through it.

EDIT: and... literally on my next attempt, I made it through the narrows. It's possible.

EDIT #2: and... made it. >:D