The nuclear forces are

$A$ nucleus is at rest in the laboratory frame of reference. Show that if it disintegrates into two smaller nuclei,the products must move in opposite directions.

Consider the following statements ($X$ and $Y$ stand for two different elements):
$(I)$ ${ }_{32}^{65} X$ and ${ }_{33}^{65} Y$ are isotopes.
$(II)$ ${ }_{42}^{86} X$ and ${ }_{42}^{85} Y$ are isotopes.
$(III)$ ${ }_{85}^{174} X$ and ${ }_{88}^{177} Y$ have the same number of neutrons.
$(IV)$ ${ }_{92}^{235} X$ and ${ }_{94}^{235} Y$ are isobars.
Which of the above statements are correct?

Weak nuclear force always operates between

In a helium nucleus,there are

$A$ nucleus (of nuclear density $\rho$) disintegrates into two daughter nuclei with masses in the ratio $8 : 27$. The density of the smaller nucleus is