arthham

JEE Advanced 2022 · Gravitation

Two spherical stars AA and BB have densities ρA\rho_A and ρB\rho_B, respectively. AA and BB have the same radius, and their masses MAM_A and MBM_B are related by MB=2MAM_B = 2M_A. Due to an interaction process, star AA loses some of its mass, so that its radius is halved, while its spherical shape is retained, and its density remains ρA\rho_A. The entire mass of lost by AA is deposited as a thick spherical shell on BB with the density of the shell being ρA\rho_A. If vAv_A and vBv_B are the escape velocities from AA and BB after the interaction process, the ratio vBvA=10n151/3\frac{v_B}{v_A} = \sqrt{\frac{10n}{15^{1/3}}} the value of nn is ____.

Answer: 2.32.3

Watch this solved on the board The full walkthrough, written out a line at a time with narration explaining each step as it appears. Pause anywhere and ask why a line is there. Free to watch

Or read the worked solution below. Jump to the steps ↓

Written solution

Step 1: Determine the mass and radius of star AA after the interaction.

The radius of star AA is halved while its density ρA\rho_A remains constant, meaning its volume is reduced by a factor of 88.

MA′=ρA⋅43π(R2)3=MA8M_A' = \rho_A \cdot \frac{4}{3}\pi\left(\frac{R}{2}\right)^3 = \frac{M_A}{8}

Step 2: Calculate the mass lost by star AA.

Subtract the final mass of star AA from its initial mass.

ΔM=MA−MA′=78MA\Delta M = M_A - M_A' = \frac{7}{8}M_A

Step 3: Determine the new radius of star BB.

The lost mass is deposited as a shell of density ρA\rho_A on star BB, which initially has radius RR.

Vshell=ΔMρA=78⋅43πR3V_{\text{shell}} = \frac{\Delta M}{\rho_A} = \frac{7}{8}\cdot\frac{4}{3}\pi R^3 VB′=VB+Vshell=43πR3+78⋅43πR3=158⋅43πR3V_B' = V_B + V_{\text{shell}} = \frac{4}{3}\pi R^3 + \frac{7}{8}\cdot\frac{4}{3}\pi R^3 = \frac{15}{8}\cdot\frac{4}{3}\pi R^3 RB′=R(158)1/3R_B' = R\left(\frac{15}{8}\right)^{1/3}

Step 4: Determine the new mass of star BB.

Add the deposited mass to the initial mass of star BB, where MB=2MAM_B = 2M_A.

MB′=MB+ΔM=2MA+78MA=238MAM_B' = M_B + \Delta M = 2M_A + \frac{7}{8}M_A = \frac{23}{8}M_A

Step 5: Write the expressions for the escape velocities.

Use the formula for escape velocity v=2GMRv = \sqrt{\frac{2GM}{R}}.

vA=2G(MA8)R2=GMA2Rv_A = \sqrt{\frac{2G\left(\frac{M_A}{8}\right)}{\frac{R}{2}}} = \sqrt{\frac{G M_A}{2R}} vB=2G(238MA)R(158)1/3v_B = \sqrt{\frac{2G\left(\frac{23}{8}M_A\right)}{R\left(\frac{15}{8}\right)^{1/3}}}

Step 6: Calculate the ratio of the escape velocities.

Divide vBv_B by vAv_A and simplify the expression.

(vBvA)2=234⋅GMAR(158)1/3GMA2R\left(\frac{v_B}{v_A}\right)^2 = \frac{\frac{23}{4} \cdot \frac{G M_A}{R \left(\frac{15}{8}\right)^{1/3}}}{\frac{G M_A}{2R}} (vBvA)2=238⋅4⋅(815)1/3\left(\frac{v_B}{v_A}\right)^2 = \frac{23}{8} \cdot 4 \cdot \left(\frac{8}{15}\right)^{1/3} (vBvA)2=23151/3\left(\frac{v_B}{v_A}\right)^2 = \frac{23}{15^{1/3}} vBvA=23151/3\frac{v_B}{v_A} = \sqrt{\frac{23}{15^{1/3}}}

Step 7: Solve for nn.

Equate the calculated ratio to the given expression.

10n151/3=23151/3\sqrt{\frac{10n}{15^{1/3}}} = \sqrt{\frac{23}{15^{1/3}}} 10n=2310n = 23 n=2.3n = 2.3

Answer: n=2.3n = 2.3

More from Gravitation

All 14 Gravitation questions →

Gravitation: formulas and what the chapter covers →