A nuclide at rest emits an α-particle. In this process:

As Fext = 0, hence Ps→ = constantor Pα→+PN→ = constantBecause nuclide was at rest before emission of α particle, hence according to law of conservation of momentum, Pα→+PN→ = 0or mαvα→+ mNvN→ = 0 or vα →  = -mNmαvN→Thus, α-particle and the nucleus move in opposite directions. Further, as mN >> mα, hence vα>>vN