Question

# Surface tension of liquid is $70 dynes/cm$. Its value in SI units is:A. $70N/m$B. $7 \times 10^{-2}N/m$C. $7 \times 10^{2}N/m$D. $7\times 10^{3}N/m$

Hint: Surface tension is the tendency of liquids to occupy minimum surface area. This occurs at the liquid- air interface. It can be expressed in different forms like $\gamma=\dfrac{dyn}{cm}=\dfrac{erg}{cm^{2}}=\dfrac{10^{-3}N}{m}=\dfrac{10^{-3}J}{m^{2}}$.Here we should convert the CGS units of surface tension to SI units.
Surface tension has dimensions of force per unit length or energy per unit area. Surface tension is denoted by $\gamma$ with SI units $N/m$ and CGS units of $dyn/cm$.
We know that $1dyne=10^{-5}N$ and $1cm=10^{-2}m$
Given that $S=70\dfrac{dynes}{cm}=\dfrac{70\times 10^{-5}N}{10^{-2}m}=70\times 10^{-3}N/m =7\times 10^{-2}N/m$