A very strange ping pong ball.  



Use a felt marker to color 1/2 of a ping pong ball.   Now drill a 1/8 inch hole in the ball as shown.   Place it under water and fill it 1/2 full with water.    Place the ball with the colored side down in a freezer and allow to freeze.    

When frozen the ball will have a strange ability.   When placed on a flat surface it will sit with the colored side down but when it is spun it will flip and will spin with the colored side up.    While spinning it is stable only with a the colored side up.  


figure 1

                                            Figure 1

Although I cannot give a good explanation for this effect I can say that it is due to the ball being out of balance.    Furthermore to be in balance  the ball must be a sphere with constant density.   If the ball was 100% full of frozen water it would balance fine and it would not flip over.    This means that it's moment of inertia while rotating about X would equal it's moment of inertia about Y and about Z.  

 This is clearly not true with a weighted hemisphere.



It is also possible to cut apart a ping pong ball, half fill one side with epoxy and then re-assemble it with super glue for a permanent spinning ball that will flip when spun.  


Return to Previous Page