# I Rolling Bowling Ball with backspin

1. Nov 18, 2016

### FallenApple

Ok so a ball starts moving to the left while the rotation is clockwise, hence the backspin. On this surface there is friction. So clearly as the ball moves to the left, there is Kinetic friction to the right slowing down the Vcm and the spin as well.

So assuming that the initial backspin is large enough such that that eventually the Vcm becomes momentarily 0 while the ball is still spinning clockwise.

At this point, what will happen? I've heard two things.

1: the kinetic friction pointing to the right will reverse the Vcm and increase it until it starts rolling without slipping.

2: A the moment the Vcm is 0, the ball immediately starts pure roll to the right and returns to where it started.

Which one is more plausible?

I'm thinking #1 is because the backspin will cause friction to the right even though the Vcm is 0. Hence causing a short time interval where there is still slipping.

But #2 is somewhat plausible as well. If the material of the friction is low, maybe the kinetic friction will not exist at that instant when Vcm is 0, because the backspinight be weak to cause friction.

Last edited: Nov 18, 2016
2. Nov 18, 2016

### Staff: Mentor

Accelerations will always be finite, that alone is sufficient to rule out the wrong scenario.

3. Nov 18, 2016

### FallenApple

Right, if the ball starts rolling to the right with pure roll at that instant then that implies an infinite impulse was given to jump the Vcm to the one needed. Which is impossible

4. Nov 18, 2016

### jbriggs444

Experiment can be entertaining. Take a "Hula Hoop" (a length of plastic tubing perhaps 2 cm in diameter bent and joined end-to-end to form a circular hoop perhaps 1 meter in diameter). Hold it underhand and throw it horizontally forward at a gentle pace. Just prior to the point of release use a flicking or snapping motion of the wrist to impart a goodly amount of backspin.

Watch as the hoop travels away from you, stops and ultimately returns to your hand, staying upright throughout.