(1) As can be seen from Figure A, if the walking code number is 2g and the weight is 20g+20g=40g, the mass of cucumber is m = 40g+2g = 42g.

(2) According to the measuring cylinder shown in Figure B, the dividing value of the measuring cylinder is 4 ml; The total volume of cucumber and water is 84 ml.

The volume of cucumber V=84ml-40ml=44ml=44cm3,

The density of cucumber ρ = MV = 42G44cm3 ≈ 0.95g/cm3 = 0.95x103kg/m3;

(3) (1) The purpose of inserting nails into cucumbers is to change gravity;

(2) When the buoyancy of the object is less than the gravity of the object, the resultant force of the object is downward and the object sinks, then the conditions for the object to sink are: F float < g；;

(3) Make the substance with higher density than water into a hollow object, and put the object into water, so that the volume of boiled water discharged by the object increases, so that the buoyancy of the object is greater than the gravity of the object, and the object can float on the water surface.

(4) The gravity of cucumber is g = mg = 0.042kg×10n/kg = 0.42n,

If the cucumber floats in Figure C, then F floats = g = 0.42 n;

In Figure C, the buoyancy f ′ of cucumber when it is completely pressed into water = ρ water gV row =1.0×103kg/m3×10n/kg× 4.4×10-5m3 = 0.44n,

Then the force f = 0.44 n-0.42 n = 0.02 n required for the cucumber in Figure C to be completely pressed into the water.

So the answer is: (1) 42; (2)950; (3)① Gravity; ②F float < g； ③ Objects can be made into hollow objects; (4)0.42; 0.44; 0.02.