Two particles of masses 1kg and 3kg have position vectors 2^i+3^j+4^k and −2^i+3^j−4^k respectively. The centre of mass has a position vector:
−^i+3^j−2^k
Position vector of centre of mass
rcm=m1r1+m2r2m−1+m2r1=2^i+3^j+4^k,m1=1kgr2=−2^i+3^j−4^k,m2=3kgrcm=1(2^i+3^j+4^k)+3(−2^i+3^j−4^k)1+3=4^i+12^j−8^k4=−i+3j−2^k