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how to determine the centre of mass of a solid hemisphere
consider the hemisphere shown below the solution , let the mass of it be M & radius be R
then , mass density , ρ = M / {2/3 ¶ R ^3}
from the symmetry of object we can say the C.M. will lie on the vertical axis shown in the figure
take a disk of dl length at the l distance from O, its C.M. will be at its centre means at the l distance from O
its mass dm = ρ dv
= ρ ¶ r^2 dl
= ρ ¶ ( R^2 - l^2 ) dl
C.M. of mass of hemisphere =
= 1/M ∫ dm l
= 1/M ∫ ρ ¶ ( R^2 - l^2 ) dl *l
= ρ ¶ / M ∫ l ( R^2 - l^2 ) dl
= ρ ¶ / M [R^2 * l^2 /2 - l^4 /4 ]0 R
= ρ ¶ / M ( R^4) /4
= ρ ¶ (R^4) / { ρ *2/3 *4 * ¶ R ^3}
= 3 R/ 8 from O at the axis of symmetry
the solid hemisphere is a symmtrical body about the coordinate axes, i mean is symmtrical about X , Y axes. therefore its centre of mass lies on its centre.
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