Hello! Excuse my ignorance. The forum is full of difficult questions so I even feel a bit ashamed of posting this, But that is the only way I can learn. I do not understand some concepts stated below in the images, and I am not able to grasp what is being said. For instance, I do not know why spin momentum is turned into orbital momentum and so on... To be more specific, what does the author mean with: "the ﬁrst phase in which the ball slides/rolls and during which (with the help of sticky friction) slipping and abnormal spin angular momentum is converted to orbital energy; and then the second phase where slipping has ceased and the ball is performing a more or less regular rolling motion and where the points of contact between the ball and the rim of the wheel trace out a 3-dimensional roulette curve" As far as I know, "slipping" stands for a movement in which surfaces slide with each other, isn't it supposed to be happening the whole time during a spin of a roulette? What is exactly saying the author? I'd be really grateful if someone could explain it to me with easier words Moreover "Our physical model described by equations (1) is only valid after the ball has connected to the roulette and before it disconnects from the roulette and leaves the track" Does it mean that the model is valid once the ball is already in movement? what does it mean with "has connected to the roulette"? Thanks in advance.