Q. A nucleus of mass number $A$ originally at rest, emits alpha particle with speed $v$, the recoil speed of the daughter nucleus is:
J & K CETJ & K CET 2000
Solution:
From law of conservation of momentum,
we have momentum before collision $=$ momentum after collision
The daughter nucleus has mass $A-4$
$\therefore m_{\alpha} v_{\alpha}+m_{N}\, v_{N}=0$
$\Rightarrow 4 v+(A-4) v_{N}=0$
$v_{N}=\frac{-4 v}{A-4}$
