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Q36P

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Physics For Scientists & Engineers
Found in: Page 441

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Short Answer

A hydrometer is an instrument used to determine liquid density. A simple one is sketched in Figure P14.36. The bulb of a syringe is squeezed and released to let the atmosphere lift a sample of the liquid of interest into a tube containing a calibrated rod of known density. The rod, of length L and average density ρ0, floats partially immersed in the liquid of density ρ. A length h of the rod protrudes above the surface of the liquid. Show that the density of the liquid is given by role="math" localid="1663764227560" ρ=ρ0LLh.

It is proved that the density of the liquid is ρ=ρ0LLh.

See the step by step solution

Step by Step Solution

Given data:

The length of the rod is L.

The length of the rod not immersed in the liquid is h.

The average density of the rod is ρ0.

The density of the liquid is ρ.

Buoyancy and gravitational force:

The buoyant force from a liquid of density d when volume of the liquid displaced v is,

B=gdv ….. (1)

Here, g is the acceleration due to gravity.

The gravitational force on a body of mass m is,

F=mg ….. (2)

Show that the density of the liquid is ρ=ρ0LL−h:

Length of the rod immersed in the liquid is L-h.

Let A be the cross sectional area of the rod. Volume of the rod immersed in the liquid is AL-h.

This is also the volume of the liquid displaced. From equation (1), the buoyancy force on the rod directed upward is,

B=ρgA(Lh)

The total volume of the rod is AL and thus its mass is ρ0AL. Thus from equation (2), the gravitational force acting on the rod directed downward is,

F=ρ0ALg

At equilibrium, these two forces cancel each other. Thus

ρgA(Lh)=ρ0ALgρ=ρ0LLh

Hence it is proved.

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