arthham

JEE Advanced 2025 · Gravitation

A geostationary satellite above the equator is orbiting around the earth at a fixed distance r1r_1 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_2 from the center of the earth, such that r1=1.21 r2r_1 = 1.21\,r_2. The time period of the second satellite as measured from the geostationary satellite is 24p\frac{24}{p} hours. The value of pp is _____.

Answer: 2.332.33

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: Identify the given parameters.

The geostationary satellite has a time period T1=24 hT_1 = 24\text{ h} and orbital radius r1r_1, while the second satellite has orbital radius r2r_2.

r1=1.21 r2r_1 = 1.21\,r_2

Step 2: Calculate the orbital period of the second satellite.

Apply Kepler's Third Law to relate the orbital periods and radii.

T2=T1(r2r1)3/2T_2 = T_1\left(\frac{r_2}{r_1}\right)^{3/2} T2=24(11.21)3/2T_2 = 24\left(\frac{1}{1.21}\right)^{3/2} T2=241.331≈18.03 hT_2 = \frac{24}{1.331} \approx 18.03\text{ h}

Step 3: Formulate the relative angular speed.

Since the satellites orbit in opposite directions, their relative angular speed is the sum of their individual angular speeds.

ωrel=ω1+ω2\omega_{\text{rel}} = \omega_1 + \omega_2 ωrel=2πT1+2πT2\omega_{\text{rel}} = \frac{2\pi}{T_1} + \frac{2\pi}{T_2}

Step 4: Calculate the relative time period.

The relative time period TrelT_{\text{rel}} is the time period of the second satellite as measured from the geostationary satellite.

Trel=2πωrel=T1T2T1+T2T_{\text{rel}} = \frac{2\pi}{\omega_{\text{rel}}} = \frac{T_1 T_2}{T_1 + T_2} Trel=24×18.0324+18.03≈10.30 hT_{\text{rel}} = \frac{24 \times 18.03}{24 + 18.03} \approx 10.30\text{ h}

Step 5: Solve for the parameter pp.

Equate the calculated relative period to the given expression 24p\frac{24}{p}.

24p=10.30\frac{24}{p} = 10.30 p=2410.30≈2.33p = \frac{24}{10.30} \approx 2.33

Alternatively, expressing pp analytically:

p=1+(r1r2)3/2p = 1 + \left(\frac{r_1}{r_2}\right)^{3/2} p=1+1.213/2=2.331p = 1 + 1.21^{3/2} = 2.331

Answer: 2.332.33 (or 2.3312.331)

More from Gravitation

All 14 Gravitation questions →

Gravitation: formulas and what the chapter covers →