Engineering
Physics
Liquid Pressure
Calculus
Question

Consider a solid sphere of radius R and mass density ρ(r)=ρ0(1−r2R2)0<r≤R. The minimum density of a liquid in which it will float is:

2ρ03

ρ03

2ρ05

ρ05

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Solution

ρ=ρ0(1−r2R2)             0<r≤R

mg = B

∫ρ(4πr2dr)=ρL43πR3;∫0Rρ0(1−r2R2)4πr2dr=ρL43πR3

∫0Rρ04π(r2−r2R2)dr=ρ04π(r33−r55R2)=ρL43πR3;25ρ0=ρL