A radioactive nucleus initially at rest decays by emitting an electron and neutron at right angles to one another. The momentum of the electron is 3.2 ×10-23 kgm/s and the momentum of neutron is 6.4 x 10-23 kgm/sec. The direction of the recoiling nucleus with that of the electron motion is:

Pe = 3.2 × 10-23 kgm/s, Pn = 6.4×10-23 kgm/sPR = Pe2+Pn2 = (3.2)2+(6.4)2×10-23 kgm/stan θ =PnPe  = 6.4×10-233.2×10-23 = 2According to law of conservation of momentum, the residual nucleus must move in a direction just opposite to that of PR→, as shown infigure. Hence, direction of the recoiling nucleus with that of the electron motion ∅ = π-θ = π-tan-1(2)..