Help settling a physics argument

[Deseoso]

We were playing a game at New Years where you hold a wine glass from the bottom and you put it over a ping-pong ball on a table. Then by moving your hand in a circular motion you can cause the ball to roll around the inside of the glass. You can then lift the glass and transfer the ball to another glass and the ball doesn't fall out of the glass so long as you keep up the circular motion.

My understanding is that this works because the middle of the glass is wider than the rim of the glass and thus, while held upside down and moved in a circular motion the ball will move to the widest part of the glass.

THE ARGUMENT - The next day I saw my nephew trying the same trick with a perfectly straight tube but no matter how fast he spun it, the ball kept falling out.

My father believes that my nephew is simply incapable of spinning the tube fast enough to keep the ball rolling around one continuous line inside the tube.

I believe that the speed does not matter because the tube is perfectly straight meaning that gravity will always pull the ball off the line, towards the ground. Who is correct?

Edited January 1, 2020 by Deseoso

[Strange]

1 hour ago, Deseoso said:

I believe that the speed does not matter because the tube is perfectly straight meaning that gravity will always pull the ball off the line, towards the ground. Who is correct?

I would say you are basically correct, in this case.

But there are a some other factors that can come into play. For example, friction. You might have seen that trick (wall of death?) where a motorcyclist drives around the inside of a vertical circular wall. The friction of the tyres (generated by the outward pressure caused by the rotation) stops them slipping down. So if your nephew coated the inside of the glass with, say, honey, he might be able to keep the ball in the glass.

Also, I think the motorbike has to continually steer upwards slightly to counter the force of gravity. So if you nephew could learn to angle the tube as it is spun to provide a slight upward force, then that might work.

The diameter of the tube will make a difference too.

[swansont]

There needs to be an upward force which is exerted by the slope of the glass. In the motorcycle trick, the bike is angled upward a small amount to ensure there is this force.

Honey would be problematic, since it’s viscous and you might not be able to get the ball to move fast enough to provide a sufficient centripetal force. And I don’t see how it would give a net upward force.

[Strange]

1 hour ago, swansont said:

Honey would be problematic, since it’s viscous and you might not be able to get the ball to move fast enough to provide a sufficient centripetal force. And I don’t see how it would give a net upward force.

By itself it wouldn't but it would slow the rate of descent, perhaps making it easier to counterbalance the force of gravity. But then the stickiness would act against the upward force, too. So, yes, it is probably a daft idea!

[swansont]

1 hour ago, Strange said:

By itself it wouldn't but it would slow the rate of descent, perhaps making it easier to counterbalance the force of gravity. But then the stickiness would act against the upward force, too. So, yes, it is probably a daft idea!

If you tilted the glass at the right rate you could do this with a straight glass, but you would have to do it at the ball’s cycling frequency, so the ball always sees an upward force from the tilt. (It’s a little like how a Paul trap works for ions, by rotating the electric field)