Q: A certain block weight 15 N in air. But it weight only 12 N when completely immersed in water. When immersed completely in another liquid, it weight 13 N. Calculate the relative density of (i) the block and (ii) the liquid.

Sol:(i) $\large Relative\; density\; of\; body = \frac{W_{air}}{W_{air}-W_{water}}$

Where W_{air} = 15 N (weight of the body in air)

And W_{water} = 12 N (weight of the body in water)

$\large RD_{block} = \frac{15}{15-12} = 5 $

(ii) $\large RD_L = \frac{Loss \;of \;weight \;in \;liquid}{Loss \;of \;weight\; in \;water} $

$\large = \frac{15-13}{15-12} = \frac{2}{3}$