- A${gdS}\left(x_{2}+x_{1}\right)^{2}$
- B$\frac{3}{4} g d S\left(x_{2}-x_{1}\right)^{2}$
- ✓$\frac{1}{4} g d S\left(x_{2}-x_{1}\right)^{2}$
- D${gdS}\left(x_{2}^{2}+x_{1}^{2}\right)$
$U _{ f }=\left(\rho Sx _{ f }\right) g \cdot \frac{ x _{ f }}{2} \times 2$
By volume conservation
$Sx _{1}+ Sx _{2}= S \left(2 x _{ f }\right)$
$x_{f}=\frac{x_{1}+x_{2}}{2}$
$\Delta U =\rho \operatorname{Sg}\left[\left(\frac{ x _{1}^{2}}{2}+\frac{ x _{2}^{2}}{2}\right)- x _{ f }^{2}\right]$
$=\rho \operatorname{Sg}\left[\frac{ x _{1}^{2}}{2}+\frac{ x _{2}^{2}}{2}-\left(\frac{ x _{1}+ x _{2}}{2}\right)^{2}\right]$
$=\frac{\rho Sg }{2}\left[\frac{ x _{1}^{2}}{2}+\frac{ x _{2}^{2}}{2}- x _{1} x _{2}\right]$
$=\frac{\rho Sg }{4}\left( x _{1}- x _{2}\right)^{2}$
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$(A)$ $\beta=0$ when $a= g / \sqrt{2}$
$(B)$ $\beta>0$ when $a= g / \sqrt{2}$
$(C)$ $\beta=\frac{\sqrt{2}-1}{\sqrt{2}}$ when $a= g / 2$
$(D)$ $\beta=\frac{1}{\sqrt{2}}$ when $a= g / 2$

