Say a ball rolls down to the right with slipping with on an arc incline so that at the bottom it leaves horizontally. The instant it leaves horizontally and is now on a cart with mass with frictional surface. So the ball will roll on the cart with kinetic friction pushing to the left altering the Vcm and forward spin such that at one point, it rolls without slipping. By Newton's third law, there would be friction from the ball to the cart pushing the cart to the right. So it seem like this is an inelastic collision where both ball and cart would move forward with the same linear speed. So is the cart moving to the right with the ball just spinning while its position is stationary at one point on the cart while the cart moves forward? So they both have the same v forward as observed from someone outside the system. So from the reference point of someone on the cart, the ball is just spinning in one spot. Is this right? But the weird part about this scenario is that the angular momentum (the final spin is faster)is not conserved even though the system is just mass and cart. How to explain this?