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  4. A metal ball of density 7800 kg / m 3 is suspected to have a large number of cavities. It weighs 9.8 kg when weighed directly on a balance and 1.5 kg less when immersed in water. The fraction by volume of the cavities in the metal ball is approximately :

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Q. A metal ball of density 7800kg/m3 is suspected to have a large number of cavities. It weighs 9.8kg when weighed directly on a balance and 1.5kg less when immersed in water. The fraction by volume of the cavities in the metal ball is approximately :

Solution:

∴ Volume where metal is present
=9.87800=1.256×10−3
Buoyancy =vρg=1.5g
⇒v×1000=1.5
v=1.5×10−3
fraction of volume
=1.5×10−3−1.256×10−31.5×10−3×100=16%