Four particles, each of mass m, are placed at the corners of a square of side a as shown in the figure. The position vector of the centre of mass is
a2(^i+^j)
The (x, y) co-ordinates of the masses at O, A, B and C respectively are (x1=0,y1=0),(x2=a,y2=0),(x3=a,y3=a) and (x4=0,y4=a).
The (x,y) co-ordinates of the centre of mass are given by
xCM=m×0+m×a+m×a+m×0m+m+m+m=a2
yCM=m×0+m×0+m×a+m×am+m+m+m=a2
Hence, the position vector of the centre of mass is a2(^i+^j), which is choice (b).