Why does a Tumbling Kelly toy return to its original position? (The oval shape is made out of plastic and lead at the bottom).

Question

anonymous · Answer

Topic: Moments (Turning Forces)

apoorvk · Answer

the center of mass is so so low and close to the ground, that every time you punch it and sinks a bit lower, the surrounding air-filled plastic which gets squeezed, delivers a restoring torque which is sufficient (due to the CoM being so low, hence the radius of gyration thingy is greater) to bring it back to its vertical position.

anonymous · Answer

Thank you very much! :D

apoorvk · Answer

and ofcourse the CoM so low because of the heavy bottom!

anonymous · Answer

Mm, alright! Follow up question, what are the properties of plastic and the lead at the bottom that makes suitable materials for a Tumbling Kelly toy? :D

apoorvk · Answer

well, lead is very dense.. so as i mentioned above, helps make the bottom very heavy, that makes the centre of mass shift towards the base. as for the plastic, you may have guessed it helps to provide the restoring torque due to its 'elasticity' when the toy is at an angle with the ground.
try visualising the motion of the toy in slow motion, and am sure you 'll get a proper feel of what's happening in there.

anonymous · Answer

I actually took a Tumbling Kelly and checked it out. :P Thank you very much!