Question

a solid body weight 2.1N in air. it's relative Density is 2.1.what will the body weights in water and in a liquid of relative density 1.2.

Answer 1

weight in air = 2.1 N

relative density wrt water = density of solid/ density of water = 2.1 

weight inside water = 2.1 N * [1 - 1/ 2.1 ]
                               = 2.1 * 1.1/2.1 N.
                              = 1.1 N

Relative density in that fluid = 2.1 / 1.2 = 7/4

Weight inside the fluid = 2.1 N * [1 - 1/(7/4) ] 
                                    = 2.1 * 3/7  N
                                    = 0.9 N

Answer 2

Explanation:

$given \: weight \: of \: body \\ in \: air \: = 2.10 \: n \\ r.d \: of \: body \: = 8.4 \: \\ weight \: of \: body \: in \: air = \\ r.d = \frac{w1}{w1 - w2} \\ or \: 8.4(2.1 - w2) = 2.1 \\ or \: w2 = \: \: \frac{2.1 \times 7.4}{8.4} = 1.85 \\ \\ ii.)upthrust \: due \: to \: water \\ = w1 - w2 = 2.10 - 1.85 \\ 0.25 \\ upthrust \: due \: to \: liquid \\ upthrust \: due \: to \: liquid \times r.d \: of \: liquid \\ 0.25 \times 1.2 = 0.30n \\ weight \: of \: body \: in \: liquid = weight \: of \: body \: in \: a \: air - upthrust \: due \: to \: liquid \\ 2.10 - 0.30 = 1.8n \\ \\ answer \: of \: 1st \: part \: is \: 1.85n \\ answer \: of \: 2nd \: part \: is1.8n$

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