Question 2.13: A cube of side a floats with one of its axes vertical in a l...

A cube of side a floats with one of its axes vertical in a liquid of specific gravity SLS_{L}. If the specific gravity of the cube material is ScS_{c}, find the values of SL/ScS_{L} / S_{c} for the metacentric height to be zero.

2.13
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Let the cube float with h as the submerged depth as shown in Fig. 2.36. For equilibrium of the cube,

Weight = Buoyant force

a3Sc×103×9.81=ha2×SL×103×9.81a^{3} S_{c} \times 10^{3} \times 9.81=h a^{2} \times S_{L} \times 10^{3} \times 9.81

or, h=a(Sc/SL)=a/xh=a\left(S_{c} / S_{L}\right)=a / x

where SL/Sc=xS_{L} / S_{c}=x

The distance between the centre of buoyancy B and centre of gravity G becomes

BG=a2h2=a2(11x)B G=\frac{a}{2}-\frac{h}{2}=\frac{a}{2}\left(1-\frac{1}{x}\right)

Let M be the metacentre, then

BM=I=a(a312)a2h=a412a2(ax)=ax12B M=\frac{I}{\forall}=\frac{a\left(\frac{a^{3}}{12}\right)}{a^{2} h}=\frac{a^{4}}{12 a^{2}\left(\frac{a}{x}\right)}=\frac{a x}{12}

The metacentric height MG=BMBG=ax12a2(11x)M G=B M-B G=\frac{a x}{12}-\frac{a}{2}\left(1-\frac{1}{x}\right)

According to the given condition

MG=ax12a2(11x)=0M G=\frac{a x}{12}-\frac{a}{2}\left(1-\frac{1}{x}\right)=0

or, x26x+6=0x^{2}-6 x+6=0

which gives x=6±122=4.732,1.268x=\frac{6 \pm \sqrt{12}}{2}=4.732,1.268

Hence SL/Sc=4.732 or 1.268S_{L} / S_{c}=4.732 \quad \text { or } \quad 1.268

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