Fluid Specific Gravity - NS Q Bank

trs494

2+ Year Member
I came across this question in the Q Bank for NS and was confused.. Can't you use specific gravity for any fluid? Not just relative to water's density?

My thought process was that since specific gravity is a term relative to the fluid a specific gravity greater than one means the density of the object is greater than the fluid it is in.. and it will therefore sink (sink SG = density object/density fluid).

I will attach the photos of the question and explanation. Let me know if I am wrong in my logic!

bobeanie95

2+ Year Member
The specific gravity of a fluid is relative to the density of water, so that Specific gravity=density of fluid/density of water. In this case, the density of the fluid will be greater than that of water since the value is 1.05.

What happens to the object is determined by the opposing forces, the weight of the object and the buoyant force: mg=pgVo. Since the density of the fluid, "p" and "g" is a constant, the weight of the object "mg" is proportional to Vo, the volume of fluid displaced by the object. In other words, the greater the object's weight, the more fluid will be displaced and the greater it will be submerged. Since specific gravity only gives you the density of the fluid "p", there's not enough information to determine how submerged the object will be without being given its weight or density.

OP
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trs494

2+ Year Member
The specific gravity of a fluid is relative to the density of water, so that Specific gravity=density of fluid/density of water. In this case, the density of the fluid will be greater than that of water since the value is 1.05.

What happens to the object is determined by the opposing forces, the weight of the object and the buoyant force: mg=pgVo. Since the density of the fluid, "p" and "g" is a constant, the weight of the object "mg" is proportional to Vo, the volume of fluid displaced by the object. In other words, the greater the object's weight, the more fluid will be displaced and the greater it will be submerged. Since specific gravity only gives you the density of the fluid "p", there's not enough information to determine how submerged the object will be without being given its weight or density.
that makes sense! Thanks!

andelJ94

a la wikipedia "the ratio of the density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas."

I think for the MCAT, we are supposed to assume the standard is water, like we do with pH, concentrations, standard solvent etc... Chemically speaking you COULD use any liquid as the reference, but this is not science class, it's the mcat.