A man (mass $=50 \mathrm{~kg}$ ) and his son (mass $=20 \mathrm{~kg}$ ) are standing on a frictionless surface facing each other. The man pushes his son so that he starts moving at a speed of $0.70 \mathrm{~ms}^{-1}$ with respect to the man. The speed of the man with respect to the surface is :
