How much does sea water weigh per cubic foot?

Answer

The weight of sea water really depends on a number of variables, including the temperature, the amount of salt (salinity) and whatever other foreign items may be present, and the depth, thus the pressure.

But to get to the basic answer, seawater, at the surface, on average weighs 1027 kg/m3, or just over 64.1 lbs per cubic foot.

Answer

It is more correct to refer to the weight of salt water in terms of density. Salt water has greater density than that of fresh water obviously due to the additional density of salt. Fresh water has a density of 1000 kg/cubic meter vs. an average density of 1027 kg/cubic meter for ocean salt water.

As for the balloon volume vs. depth question, it is a simple relationship of Boyle's Law P1*V1=P2*V2 assuming constant temperature. V2(@33ftH2O)=(99ftH2O)*(6 cubic inches)/(33ftH2O)=18 cubic inches.

Answer

I’m sorry, but the answer to the balloon question given by the previous respondent is incorrect. Here is the correct solution worked out step by step.

Yes, the change in volume of the balloon can be calculated by Boyle’s law. It is inversely proportional to the change in ABSOLUTE pressure (assume the balloon doesn’t exert any pressure on the gas contained in it, and the pressure inside the balloon must equal the pressure outside the balloon). The problem is that the previous respondent neglected atmospheric pressure that is also exerted on the surface of the water. Just think about this: if it were inversely proportional to depth alone, the pressure at the surface would be 0 and thus the volume would be INFINITY. We know this is not the case. Pressure at the surface is actually 14.7 pounds per square inch. To ensure consistency of units, I will convert this pressure to pounds per square foot. (144 sq. inches in a sq. foot, so pressure is 14.7 * 144 = 2,116.8 lb/sq. ft.)

The liquid component of pressure at a certain depth is given by the formula: Pressure = gamma * depth, where gamma is density. Assuming seawater with a density of 64.1 lb/sq.ft., at 99 feet, the TOTAL ABSOLUTE PRESSURE EXERTED ON THE BALOON is

99 * 64.1 + 2,116.8 = 8,462.7 lb/sq. ft. at 99 feet depth

and at 33 feet, the TOTAL ABSOLUTE PRESSURE EXERTED ON THE BALOON is

33 * 64.1 + 2,116.8 = 4,232.1 lb/sq. ft. at 33 feet depth.

Since the ABSOLUTE pressure has gone down by a factor of 0.5, the volume of the balloon will go up by a factor of two. So 6 cu. in. x 2 = 12 cu. in.

Thus, the volume of the balloon at 33 feet is 12 cubic inches.

Note: Pounds, plural, is never abbreviated "lbs." It comes from Latin "libra" and is always abbreviated "lb."

First answer by Delia Mignoli. Last edit by Joepoidog. Contributor trust: 722 [recommend contributor]. Question popularity: 236 [recommend question]