physics
posted by lisa .
A box with a volume V = 0.03 m^3 lies at the bottom of a lake whose water has a density of 103 kg/m^3.
(a) How much force is required to lift the box, if the mass of the box is 100 kg?
(b) How much force is required to lift the box, if the mass of the box is 70 kg?

Check your density of water again.
103 kg/m^3 is much too low. 
ah thanks, it's 10^3 kg/m^3

(a) The buoyancy force on the box is
V*(water density)*g = 294 Newtons
The box's mass is M g = 980 N
Force required to lift = 686 N
(b) same approach, but the box weighs 686 N
Force required to lift = 686  294 
thank you very much!