094 – Celtic Rattleback

Reversing the direction of rotation  

Introduction to:  

angular momentum, conservation of angular momentum, torque.  

Material:  

Celtic rattleback made of glass, available as a ready-made product 

Instructions:  

• Place the rattleback on a flat surface and make it rotate rapidly.   
• Perform the experiment again, this time turn it in the other direction (e.g. if the first run it rotated clockwise, now make it turn anti-clockwise).  

Observation:  

• When spinning in one direction, the glass body retains its sense of rotation.   
• When spinning in the other direction, the rotation slows down and comes to a standstill.   
• A strong wobble occurs.   
• Subsequently, even a rotation in the opposite direction occurs!  

Explanation:   

• During the wobbling, a transfer of force to the base occurs. In reaction to this, the wobble stone experiences a torque which changes its angular momentum. Conservation of angular momentum applies only to an object on which there are no acting forces.   
• The total energy remains the same all the time: Initial rotational energy is transformed into kinetic and potential energy during wobbling, and then back into rotational energy.