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Gravitation question

2025 · Shift 2 · Q44
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Gravitation question

2025 · Shift 2 · Q44

JEE AdvancedPhysicsGravitationNumerical+4 / −1
A geostationary satellite above the equator is orbiting around the earth at a fixed distance r1r_1r1​ from the center of the earth. A second satellite is orbiting in the equatorial plane in the opposite direction to the earth's rotation, at a distance r2r_2r2​ from the center of the earth, such that r1=1.21r2r_1=1.21 r_2r1​=1.21r2​. The time period of the second satellite as measured from the geostationary satellite is 24p\frac{24}{p}p24​ hours. The value of ppp is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 2.3TO2.4

Step-by-step Solution:

  1. Understand the properties of the geostationary satellite (S1). A geostationary satellite orbits the Earth in the equatorial plane in the same direction as the Earth's rotation. Its time period is equal to the Earth's rotational period, which is 24 hours.

    • Time period of S1, T1=24T_1 = 24T1​=24 hours.
    • Orbital radius of S1 is r1r_1r1​.
  2. Understand the properties of the second satellite (S2). The second satellite orbits in the equatorial plane in the opposite direction to the Earth's rotation.

    • Let its time period be T2T_2T2​.
    • Its orbital radius is r2r_2r2​.
    • We are given the relation between the radii: r1=1.21r2r_1 = 1.21 r_2r1​=1.21r2​.
  3. Apply Kepler's Third Law of Planetary Motion. Kepler's third law states that the square of the orbital period of a planet (or satellite) is directly proportional to the cube of the semi-major axis of its orbit. For circular orbits, this means T2∝r3T^2 \propto r^3T2∝r3. Applying this law to both satellites: (T2T1)2=(r2r1)3\left( \frac{T_2}{T_1} \right)^2 = \left( \frac{r_2}{r_1} \right)^3(T1​T2​​)2=(r1​r2​​)3

  4. Calculate the time period of the second satellite (T2). Substitute the given relation r1=1.21r2r_1 = 1.21 r_2r1​=1.21r2​ into the equation from Step 3: (T2T1)2=(r21.21r2)3=(11.21)3\left( \frac{T_2}{T_1} \right)^2 = \left( \frac{r_2}{1.21 r_2} \right)^3 = \left( \frac{1}{1.21} \right)^3(T1​T2​​)2=(1.21r2​r2​​)3=(1.211​)3 Taking the square root of both sides: T2T1=(11.21)3/2=(11.12)3/2=1(1.1)3=11.331\frac{T_2}{T_1} = \left( \frac{1}{1.21} \right)^{3/2} = \left( \frac{1}{1.1^2} \right)^{3/2} = \frac{1}{(1.1)^3} = \frac{1}{1.331}T1​T2​​=(1.211​)3/2=(1.121​)3/2=(1.1)31​=1.3311​ Since T1=24T_1 = 24T1​=24 hours, the time period of the second satellite is: T2=T11.331=241.331 hoursT_2 = \frac{T_1}{1.331} = \frac{24}{1.331} \text{ hours}T2​=1.331T1​​=1.33124​ hours

  5. Determine the relative angular velocity. The angular velocity of a satellite is given by ω=2πT\omega = \frac{2\pi}{T}ω=T2π​.

    • Angular velocity of S1: ω1=2πT1\omega_1 = \frac{2\pi}{T_1}ω1​=T1​2π​.
    • Angular velocity of S2: ω2=2πT2\omega_2 = \frac{2\pi}{T_2}ω2​=T2​2π​.

    The satellites are orbiting in opposite directions. To find the time period of S2 as measured from S1, we need their relative angular velocity. Since they move in opposite directions, their angular velocities add up. ωrel=ω1+ω2\omega_{rel} = \omega_1 + \omega_2ωrel​=ω1​+ω2​

  6. Calculate the relative time period (T_rel). The relative time period is the time taken for the relative angle between the satellites to change by 2π2\pi2π. It is given by: Trel=2πωrel=2πω1+ω2=2π2πT1+2πT2T_{rel} = \frac{2\pi}{\omega_{rel}} = \frac{2\pi}{\omega_1 + \omega_2} = \frac{2\pi}{\frac{2\pi}{T_1} + \frac{2\pi}{T_2}}Trel​=ωrel​2π​=ω1​+ω2​2π​=T1​2π​+T2​2π​2π​ Trel=11T1+1T2=T1T2T1+T2T_{rel} = \frac{1}{\frac{1}{T_1} + \frac{1}{T_2}} = \frac{T_1 T_2}{T_1 + T_2}Trel​=T1​1​+T2​1​1​=T1​+T2​T1​T2​​

  7. Substitute the values to find T_rel. Now, substitute T1=24T_1 = 24T1​=24 hours and T2=241.331T_2 = \frac{24}{1.331}T2​=1.33124​ hours into the expression for TrelT_{rel}Trel​: Trel=24×241.33124+241.331T_{rel} = \frac{24 \times \frac{24}{1.331}}{24 + \frac{24}{1.331}}Trel​=24+1.33124​24×1.33124​​ Factor out 24 from the denominator: Trel=24×241.33124(1+11.331)=241.3311+11.331T_{rel} = \frac{24 \times \frac{24}{1.331}}{24 \left(1 + \frac{1}{1.331}\right)} = \frac{\frac{24}{1.331}}{1 + \frac{1}{1.331}}Trel​=24(1+1.3311​)24×1.33124​​=1+1.3311​1.33124​​ Trel=241.3311.331+11.331=241.331+1=242.331 hoursT_{rel} = \frac{\frac{24}{1.331}}{\frac{1.331 + 1}{1.331}} = \frac{24}{1.331 + 1} = \frac{24}{2.331} \text{ hours}Trel​=1.3311.331+1​1.33124​​=1.331+124​=2.33124​ hours

  8. Determine the value of p. The problem states that the time period of the second satellite as measured from the geostationary satellite is 24p\frac{24}{p}p24​ hours. Comparing our result with the given expression: 242.331=24p\frac{24}{2.331} = \frac{24}{p}2.33124​=p24​ Therefore, p=2.331p = 2.331p=2.331.

  9. Final Answer. The value of ppp is 2.331. This value is within the specified range of 2.3 to 2.4.

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