Inclined plane, ball filled with honey.

[malinowo]

For my classes I want to prepare this experiment http://fizyczny.net/viewtopic.php?t=28422 But of course I need to collect some data and explain why is it so. I thought about gravity and torque and that ball inside works as lever arm. Do you have any ideas?

 

[my_wan]

This is actually quiet easy to explain. It would be easier with some diagrams I am not going to bother with but should not be difficult to understand.

To see the construction here is the video:

Just picture the ball on an inclined plane (not too steep). If, even without the honey, you put the steel ball above the plumb line of the contact point of the outside ball on the plane it will lift upwards. If the steel ball was attached to the inside of the larger ball it simply would not role down the plane at all. The honey merely slows the steel ball down enough that whenever the ball rolls it move the steel ball to the top side, which then takes longer to catch up because of the honey slowing it down. Once the steel ball moves below the center point it rolls again, which puts the steel ball back at the top side.

 

[chrisbaird]

The "trick" is that humans are so used to seeing balls that have a uniform density, that we think that all balls should behave like them. You can think of a round cylinder rolling as the infinite limit of a polygonal cylinder that keeps falling over. Take a cube for instance. Will a cube block roll down an incline? Only if the incline is steep enough so that the block's center of mass extends beyond the contact footprint of the block on the plane. The honey inside the ball has a way of sticking the center of mass behind the contact point so that it does not fall over. In reality, this is no more magic than sticking a block on a not-so-steep incline and saying "look, it doesn't roll".

Incidentally, you can play around with this concept in a much easier way. Get a round water bottle and fill it only half way. Place it on its side and let it roll. It will wobble as it rolls for the same reason. The water sloshes back and forth, changing the center of mass.