What do you mean by mass element $dm$ ? 

How can general body be obtained ?

What is pure translational motion ?

A solid sphere rotates about a vertical axis on frictionless bearing. A massless cord passes around the equator of sphere, then passes through over a solid cylinder and then is connected to block of mass $M$ as  shown in figure. If the system is released from rest then the speed acquired by block after it has fallen through distance $h$ is

What is rotational motion and  axis ?  

What is rigid body?