When a body is partially or completely immersed in a fluid, the fluid exerts an upward force on the body equal to the weight of the fluid displaced by the liquid.
Given m = 4 kg,
p = 4000 kg m^-3,
a = 1000 kg m^-3,
g = 10 m s^-2,
Wa = ?
Weight of ball in air = 4 x 10 = 40 N
Loss in weight = Density of water x (Mass of ball / Density of ball) x g
Or Loss in weight = 1000 x (4/4000) x 10 = 10 N
Apparent weight of ball in water=40 -10=30 N.