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Gravitation question

2021 · 20 Jul · Shift 2 · Q48
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  5. /2021 · 20 Jul · Shift 2 · Q48

Gravitation question

2021 · 20 Jul · Shift 2 · Q48

JEE MainPhysicsGravitationMCQ+4 / −1
A satellite is launched into a circular orbit of radius R around earth, while a second satellite is launched into a circular orbit of radius 1.02 R. The percentage difference in the time periods of the two satellites is :
  1. A
    1.5
  2. B
    2.0
  3. C
    0.7
  4. D
    3.0
View written solutionFree

Correct answer: D

  1. Use Kepler’s relation for circular orbits

For a satellite in circular orbit,

T=2πr3GMT = 2\pi \sqrt{\frac{r^3}{GM}}T=2πGMr3​​

So,

T∝r3/2T \propto r^{3/2}T∝r3/2

  1. Let the first satellite have

r1=R,T1∝R3/2r_1 = R, \qquad T_1 \propto R^{3/2}r1​=R,T1​∝R3/2

For the second satellite,

r2=1.02R,T2∝(1.02R)3/2r_2 = 1.02R, \qquad T_2 \propto (1.02R)^{3/2}r2​=1.02R,T2​∝(1.02R)3/2

Thus,

T2T1=(1.02)3/2\frac{T_2}{T_1} = (1.02)^{3/2}T1​T2​​=(1.02)3/2

  1. Find the fractional change

Percentage difference in time period:

T2−T1T1×100=[(1.02)3/2−1]×100\frac{T_2 - T_1}{T_1} \times 100 = \left[(1.02)^{3/2} - 1\right] \times 100T1​T2​−T1​​×100=[(1.02)3/2−1]×100

Now, using binomial approximation for small change:

(1+x)3/2≈1+32x(1+x)^{3/2} \approx 1 + \frac{3}{2}x(1+x)3/2≈1+23​x

with x=0.02x=0.02x=0.02,

(1.02)3/2≈1+32(0.02)=1+0.03=1.03(1.02)^{3/2} \approx 1 + \frac{3}{2}(0.02) = 1 + 0.03 = 1.03(1.02)3/2≈1+23​(0.02)=1+0.03=1.03

Therefore,

% difference≈(1.03−1)×100=3%\%\text{ difference} \approx (1.03 - 1)\times 100 = 3\%% difference≈(1.03−1)×100=3%

  1. Check with direct calculation

(1.02)3/2=1.0301(approximately)(1.02)^{3/2} = 1.0301\text{(approximately)}(1.02)3/2=1.0301(approximately)

So actual percentage difference is about

3.01%3.01\%3.01%

which matches 3.0%3.0\%3.0%.

  1. Evaluate options
  • A: 1.51.51.5 ❌
  • B: 2.02.02.0 ❌
  • C: 0.70.70.7 ❌
  • D: 3.03.03.0 ✅

So the correct option is D.

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