Question

A glass plate of length $10\text{cm}$, breadth $4\text{cm}$ and thickness $0.4\text{cm}$ weighs $20\text{g}$ in air. Half of the plate is immersed in water in such a way that its breadth is perpendicular to water. Calculate force due to surface tension and buoyant force of plate. Surface tension of water is $70\text{dyne/cm}$.

• A. $1.5\text{gf},8\text{gf}$
• B. $7\text{gf},8\text{gf}$
• C. $1.5\text{gf},3\text{gf}$
• D. $1.5\text{gf},6\text{gf}$

Answer 1

The correct option is A $1.5\text{gf},8\text{gf}$

Given, $l=20cm;b=4cm;t=0.4cm;$ weight$=20g$
Volume immersed$=\frac{\text{total volume}}{2}$
$T=70\text{dyne/cm}$

surface tension, $F_s=T\times 2(l+t)$
$F_s=\frac{T\times 2\left(10.4\right)}{980}$
$F_s=\frac{70\times 2\left(10.4\right)}{980}=1.5gf$

Now, buoyant force
$F_b=\frac{V_s}{2}\times {\rho }_L\times g$
$=\frac{l\times b\times t}{2}\times 1\times g=8gf$