There are three discs made up of Iron, Aluminium and Acrylic respectively. Rotate the discs one after the other(Iron first, Aluminium second and Acrylic last). Observe which disc stops first.

What did you observe? 
The Acrylic disc stops first and the Iron disc stops at last.

The science behind it :
The energy stored in any object is proportional to its mass. Hence iron has more energy and stops at last.