The question provides two different weights for the object, one in air and one in water. The weight in air is 40 N and the weight in water is 36 N. The difference in weight is due to the buoyant force exerted by the water on the object. The buoyant force is equal to the weight of the water displaced by the object. Since the weight in water is less than the weight in air, it means that the buoyant force is 4 N. Using the formula for density, which is mass divided by volume, we can calculate the density of the object. The density is equal to the weight divided by the acceleration due to gravity times the volume. Rearranging the formula, we can solve for the volume, which is equal to the weight divided by the density times the acceleration due to gravity. Plugging in the given values, we find that the volume is 0.0004 m^3. Finally, dividing the mass of the object by the volume, we can find the density. The mass of the object is equal to the weight in air divided by the acceleration due to gravity, which is 4 kg. Dividing the mass by the volume, we find that the density is 10,000 kg/m^3.