M=∫ρdV
M=∫0Rrk4πr2dr
M=4πk∫0Rrdr
M=24πkR2=2πkR2

FG=R2GMm=mω02R
⇒R2G24πkR2=ω02R⇒ω0=R2πkG
∵T=ω02π=2πkG2πR=kG2πR
⇒T2∝R
JEE Main 2020 — Physics Mechanics
The mass density of a spherical galaxy varies as rK over a large distance r from its center. In that region, a small star is in a circular orbit of radius R. Then the period of revolution,T depends on R as:
Held on 2 Sept 2020 · Verified 6 Jul 2026.
T2∝R
T2∝R3
T2∝R31
T∝R
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