Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Gravitation question

2022 · Shift 1 · Q37
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Advanced
  3. /Physics
  4. /Gravitation
  5. /2022 · Shift 1 · Q37

Gravitation question

2022 · Shift 1 · Q37

JEE AdvancedPhysicsGravitationNumerical+3 / −1
Two spherical stars AAA and BBB have densities ρA\rho_{A}ρA​ and ρB\rho_{B}ρB​, respectively. AAA and BBB have the same radius, and their masses MAM_{A}MA​ and MBM_{B}MB​ are related by MB=2MAM_{B}=2 M_{A}MB​=2MA​. Due to an interaction process, star AAA loses some of its mass, so that its radius is halved, while its spherical shape is retained, and its density remains ρA\rho_{A}ρA​. The entire mass lost by AAA is deposited as a thick spherical shell on BBB with the density of the shell being ρA\rho_{A}ρA​. If vAv_{A}vA​ and vBv_{B}vB​ are the escape velocities from AAA and BBB after the interaction process, the ratio vBvA=10n151/3\frac{v_{B}}{v_{A}}=\sqrt{\frac{10 n}{15^{1 / 3}}}vA​vB​​=151/310n​​. The value of nnn is ‾\underline{\hspace{2cm}}​ .
Numerical answer
View written solutionFree

Correct answer: 2.2TO2.4

Step-by-Step Solution

1. Initial State of Stars A and B

Let the initial radius of both stars be RRR. So, RA=RB=RR_A = R_B = RRA​=RB​=R. Let their densities be hoA ho_AhoA​ and hoB ho_BhoB​. The initial masses are given by: MA=ρA×43πR3M_A = \rho_A \times \frac{4}{3}\pi R^3MA​=ρA​×34​πR3 MB=ρB×43πR3M_B = \rho_B \times \frac{4}{3}\pi R^3MB​=ρB​×34​πR3

We are given the relation MB=2MAM_B = 2 M_AMB​=2MA​. This implies: ρB43πR3=2(ρA43πR3)  ⟹  ρB=2ρA\rho_B \frac{4}{3}\pi R^3 = 2 \left(\rho_A \frac{4}{3}\pi R^3\right) \implies \rho_B = 2\rho_AρB​34​πR3=2(ρA​34​πR3)⟹ρB​=2ρA​.

2. Final State of Star A

After the interaction, star A loses some mass. Its new radius is halved: RA′=R2R'_A = \frac{R}{2}RA′​=2R​. The density of star A remains ρA\rho_AρA​. Its new mass, MA′M'_AMA′​, is: MA′=ρA×43π(RA′)3=ρA43π(R2)3=ρA43πR38M'_A = \rho_A \times \frac{4}{3}\pi (R'_A)^3 = \rho_A \frac{4}{3}\pi \left(\frac{R}{2}\right)^3 = \rho_A \frac{4}{3}\pi \frac{R^3}{8}MA′​=ρA​×34​π(RA′​)3=ρA​34​π(2R​)3=ρA​34​π8R3​ MA′=18(ρA43πR3)=18MAM'_A = \frac{1}{8} \left(\rho_A \frac{4}{3}\pi R^3\right) = \frac{1}{8} M_AMA′​=81​(ρA​34​πR3)=81​MA​.

3. Mass Transferred

The mass lost by star A is: ΔMA=MA−MA′=MA−18MA=78MA\Delta M_A = M_A - M'_A = M_A - \frac{1}{8}M_A = \frac{7}{8}M_AΔMA​=MA​−MA′​=MA​−81​MA​=87​MA​. This mass is deposited as a shell on star B.

4. Final State of Star B

The mass of the shell added to star B is Mshell=ΔMA=78MAM_{shell} = \Delta M_A = \frac{7}{8}M_AMshell​=ΔMA​=87​MA​. The new total mass of star B, MB′M'_BMB′​, is: MB′=MB+Mshell=2MA+78MA=(2+78)MA=238MAM'_B = M_B + M_{shell} = 2M_A + \frac{7}{8}M_A = \left(2 + \frac{7}{8}\right)M_A = \frac{23}{8}M_AMB′​=MB​+Mshell​=2MA​+87​MA​=(2+87​)MA​=823​MA​.

The density of the shell is given as ρA\rho_AρA​. Let the new outer radius of star B be RB′R'_BRB′​. The volume of the shell, VshellV_{shell}Vshell​, is: Vshell=43π((RB′)3−R3)V_{shell} = \frac{4}{3}\pi ((R'_B)^3 - R^3)Vshell​=34​π((RB′​)3−R3). Also, Mshell=ρAVshellM_{shell} = \rho_A V_{shell}Mshell​=ρA​Vshell​. 78MA=ρA(43π((RB′)3−R3))\frac{7}{8}M_A = \rho_A \left(\frac{4}{3}\pi ((R'_B)^3 - R^3)\right)87​MA​=ρA​(34​π((RB′​)3−R3)). Substitute MA=ρA43πR3M_A = \rho_A \frac{4}{3}\pi R^3MA​=ρA​34​πR3: 78(ρA43πR3)=ρA(43π((RB′)3−R3))\frac{7}{8}\left(\rho_A \frac{4}{3}\pi R^3\right) = \rho_A \left(\frac{4}{3}\pi ((R'_B)^3 - R^3)\right)87​(ρA​34​πR3)=ρA​(34​π((RB′​)3−R3)). Canceling common terms (ρA43π)(\rho_A \frac{4}{3}\pi)(ρA​34​π): 78R3=(RB′)3−R3\frac{7}{8}R^3 = (R'_B)^3 - R^387​R3=(RB′​)3−R3 (RB′)3=R3+78R3=158R3(R'_B)^3 = R^3 + \frac{7}{8}R^3 = \frac{15}{8}R^3(RB′​)3=R3+87​R3=815​R3 RB′=(158)1/3R=151/32RR'_B = \left(\frac{15}{8}\right)^{1/3} R = \frac{15^{1/3}}{2} RRB′​=(815​)1/3R=2151/3​R.

5. Escape Velocities

The escape velocity from the surface of a spherical body of mass MMM and radius RRR is vesc=2GMRv_{esc} = \sqrt{\frac{2GM}{R}}vesc​=R2GM​​.

For star A after the process: vA=2GMA′RA′=2G(18MA)R2=14GMAR2=GMA2Rv_A = \sqrt{\frac{2GM'_A}{R'_A}} = \sqrt{\frac{2G(\frac{1}{8}M_A)}{\frac{R}{2}}} = \sqrt{\frac{\frac{1}{4}GM_A}{\frac{R}{2}}} = \sqrt{\frac{GM_A}{2R}}vA​=RA′​2GMA′​​​=2R​2G(81​MA​)​​=2R​41​GMA​​​=2RGMA​​​.

For star B after the process: vB=2GMB′RB′=2G(238MA)151/32R=234GMA151/32R=23⋅2⋅GMA4⋅151/3⋅R=23GMA2R⋅151/3v_B = \sqrt{\frac{2GM'_B}{R'_B}} = \sqrt{\frac{2G(\frac{23}{8}M_A)}{\frac{15^{1/3}}{2}R}} = \sqrt{\frac{\frac{23}{4}GM_A}{\frac{15^{1/3}}{2}R}} = \sqrt{\frac{23 \cdot 2 \cdot GM_A}{4 \cdot 15^{1/3} \cdot R}} = \sqrt{\frac{23GM_A}{2R \cdot 15^{1/3}}}vB​=RB′​2GMB′​​​=2151/3​R2G(823​MA​)​​=2151/3​R423​GMA​​​=4⋅151/3⋅R23⋅2⋅GMA​​​=2R⋅151/323GMA​​​.

6. Ratio of Escape Velocities

Now, we find the ratio vBvA\frac{v_B}{v_A}vA​vB​​: vBvA=23GMA2R⋅151/3GMA2R=23GMA2R⋅151/3GMA2R=23GMA2R⋅151/3×2RGMA\frac{v_B}{v_A} = \frac{\sqrt{\frac{23GM_A}{2R \cdot 15^{1/3}}}}{\sqrt{\frac{GM_A}{2R}}} = \sqrt{\frac{\frac{23GM_A}{2R \cdot 15^{1/3}}}{\frac{GM_A}{2R}}} = \sqrt{\frac{23GM_A}{2R \cdot 15^{1/3}} \times \frac{2R}{GM_A}}vA​vB​​=2RGMA​​​2R⋅151/323GMA​​​​=2RGMA​​2R⋅151/323GMA​​​​=2R⋅151/323GMA​​×GMA​2R​​. vBvA=23151/3\frac{v_B}{v_A} = \sqrt{\frac{23}{15^{1/3}}}vA​vB​​=151/323​​.

7. Finding the value of n

We are given the expression for the ratio: vBvA=10n151/3\frac{v_B}{v_A} = \sqrt{\frac{10n}{15^{1/3}}}vA​vB​​=151/310n​​.

Equating our derived expression with the given one: 23151/3=10n151/3\sqrt{\frac{23}{15^{1/3}}} = \sqrt{\frac{10n}{15^{1/3}}}151/323​​=151/310n​​.

Squaring both sides: 23151/3=10n151/3\frac{23}{15^{1/3}} = \frac{10n}{15^{1/3}}151/323​=151/310n​.

23=10n23 = 10n23=10n n=2310=2.3n = \frac{23}{10} = 2.3n=1023​=2.3.

The value of n is 2.3.

PreviousNext

More from Gravitation

  • The distance between two stars of masses 3MS and 6MS is 9R. Here R is the mean distance between the centers of the Earth and the Sun, and MS is the mass of the Sun. The two stars orbit around their common center of mass in circular orbits…2021 · Numerical
  • Consider a spherical gaseous cloud of mass density ρ(r) in free space where r is the radial distance from its center. The gaseous cloud is made of particles of equal mass m moving in circular orbits about the common center with the…2019 · MCQ
  • A planet of mass M, has two natural satellites with masses m1​ and m2​. The radii of their circular orbits are R1​ and R2​ respectively, Ignore the gravitational force between the satellites. Define v1​,L1​,K1​… Includes table2018 · MCQ
  • A rocket is launched normal to the surface of the Earth, away from the sun, along the line joining the Sun and the Earth. The Sun is 3×105 times heavier than the earth and is at a distance 2.5×104 times larger than…2017 · MCQ
  • A bullet is fired vertically upwards with velocity v from the surface of a spherical planet. When it reaches its maximum height, its acceleration due to the planet’s gravity is (41​)th of its value at the…2015 · Numerical
  • A large spherical mass M is fixed at one position and two identical masses m are kept on a line passing through the centre of M (see figure). The point masses are connected by a rigid massless rod of length l and this assembly is free to… Includes diagram2015 · Numerical
  • A planet of radius R = 101​×(radius of Earth) has the same mass density as Earth. Scientists dig a well of depth 5R​ on it and lower a wire of the same length and of linear mass density 10-3 kg m-1 into it. If…2014 · MCQ
  • Two bodies, each of mass M, are kept fixed with a separation 2L. A particle of mass m is projected from the midpoint of the line joining their centres, perpendicular to the line. The gravitational constant is G. The correct statement(s)…2013 · Multiple correct