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 Form 4 Physics: Floating and sinking online lessons

Meaning and formulae for calculating relative density

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Answer Text:
Relative density:
-We have established the relative density as the ratio of the density of a substance to the density of water. Since by the law of floatation an object
displaces a fluid equal to its own weight hence the following mathematical expressions can be established.
Relative density = density of substance / density of water = weight of substance /(weight of equal volume of water=mass of substance / mass of equal volume of water
Applying Archimedes principle, the relative density‘d’; d = weight of substance in air / upthrust in water or d =# W /u#
Since upthrust is given by (W2 -W1)where W2 – weight in air, W2– weight when submerged.
Hence d = W / u = #W /( W2)# – W1, the actual density, ρ of an object can be obtained as follows
ρ of an object = d × 1,000 #kgm^-3#.


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