Question

A body of volume 100 cm3 weighs 5kgf in air. it is completely immersed

Answer 1

Density of iron ρ = 7.8 x 103 kg/m3

Volume of iron piece V = 100 cm3 = 100 x 10−6 m3

Mass of the iron piece M = Vρ = 100 x 10−6 m3 x 7.8 x 103 kg/m3

                                            = 0.78 kg

(i) Weight of the iron piece in air = Mg = 0.78 kg x 9.8 m/s2

                                                   = 7.644 kg. m /s2 = 7.644 N

(ii) Upthrust = Volume of the solid x density of water x g

                 = Vρwg

                 = 100 x 10−6 m3 x 1000 kg/m3 x 9.8 m/s2

                 = 0.98 N

(iii) Apparent weight of iron piece in water = Weight in air − upthrust

                                                           =  7.644 N − 0.98 N

                                                           =  6.664 N