($A$) The amplitude of oscillation in the first case changes by a factor of $\sqrt{\frac{M}{m+M}}$, whereas in the second case it remains unchanged
($B$) The final time period of oscillation in both the cases is same
($C$) The total energy decreases in both the cases
($D$) The instantaneous speed at $x_0$ of the combined masses decreases in both the cases
From Conservation of momentum,
$MV _1=( M + m ) V _2$
$\frac{ MV _1}{ M + m }= V _2$
$\sqrt{\frac{k}{M+m}} A_2=\frac{M}{M+m} \sqrt{\frac{k}{M}} A_1$
$A_2=\sqrt{\frac{M}{M+m}} A_1$
In case $II$,
$A_2=A_1$
$T =2 \pi \sqrt{\frac{\overline{M+m}}{k}}$ in both the cases.
Total energy decreases in first case where as remain same in $2$ nd case. Instantaneous speed at $x_0$ decreases in both case.
Answer is $A , B$ and $D$ .
