Question

A body is moving along a circular path of radius r what will be he distance travelled and displacement of the body when it completes half a revolution

Answer 1

if it is moving in a circular path then it is moving only in in the boundary of a circle not in the interior 
the boundary of the circle is know as circumfrance 

to calulate complete circufrance we have formula 
circumfrance = 2π R 
where R is radius 

to calulate half distance divide 2π r by 2 
so distance in half will be 
2π R / 2 = πR meters 

displacement is the shortest distance from one point to another 
it it completes one half then displacement will be the diameter of the circle = 2 R meters 
since it is a vector you have to give its dirtection also like in N direction or S direction depending upon the initial and final positions