Hi, I got a doubt while studying rotational mechanics. It is said in my book that when a sphere is purely rolling on a surface, total mechanical energy is conserved even though friction is acting. Isnt friction an external force? How can energy be conserved then?? Another doubt I got is the following: I understand that it is the torque due to friction that helps a body roll. Hence, friction actualy helps motion.. Then why does the rolling body ever come to stop?? Please help me get this cleared. thanks for your time.