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Gravitation question

2022 · 27 Jul · Shift 2 · Q46
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  5. /2022 · 27 Jul · Shift 2 · Q46

Gravitation question

2022 · 27 Jul · Shift 2 · Q46

JEE MainPhysicsGravitationMCQ+4 / −1
A body of mass m\mathrm{m}m is projected with velocity λ ve\lambda \,v_{\mathrm{e}}λve​ in vertically upward direction from the surface of the earth into space. It is given that vev_{\mathrm{e}}ve​ is escape velocity and$$\lambda(R : radius of earth)
  1. A
    R1+λ2\frac{\mathrm{R}}{1+\lambda^{2}}1+λ2R​
  2. B
    R1−λ2\frac{R}{1-\lambda^{2}}1−λ2R​
  3. C
    R1−λ\frac{R}{1-\lambda}1−λR​
  4. D
    λ2R1−λ2\frac{\lambda^{2} \mathrm{R}}{1-\lambda^{2}}1−λ2λ2R​
View written solutionFree

Correct answer: B

  1. Interpretation of the question

A body is projected vertically upward from the surface of the Earth with speed u=λveu=\lambda v_eu=λve​ where vev_eve​ is the escape velocity from Earth's surface.

We need to find the maximum distance from the center of Earth reached by the body.


  1. Use conservation of mechanical energy

Let Earth's radius be RRR and Earth's mass be MMM.

Initial energy at the surface: Ei=12m(λve)2−GMmRE_i=\frac12 m(\lambda v_e)^2-\frac{GMm}{R}Ei​=21​m(λve​)2−RGMm​

At the highest point, velocity becomes zero. If the maximum distance from Earth's center is rrr, then Ef=−GMmrE_f=-\frac{GMm}{r}Ef​=−rGMm​

So, 12mλ2ve2−GMmR=−GMmr\frac12 m\lambda^2 v_e^2-\frac{GMm}{R}=-\frac{GMm}{r}21​mλ2ve2​−RGMm​=−rGMm​


  1. Use escape velocity relation

We know ve=2GMRv_e=\sqrt{\frac{2GM}{R}}ve​=R2GM​​ Rightarrow v_e^2=\frac{2GM}{R}$$

Substitute into the energy equation: 12mλ2(2GMR)−GMmR=−GMmr\frac12 m\lambda^2\left(\frac{2GM}{R}\right)-\frac{GMm}{R}=-\frac{GMm}{r}21​mλ2(R2GM​)−RGMm​=−rGMm​

This gives GMmλ2R−GMmR=−GMmr\frac{GMm\lambda^2}{R}-\frac{GMm}{R}=-\frac{GMm}{r}RGMmλ2​−RGMm​=−rGMm​

GMmR(λ2−1)=−GMmr\frac{GMm}{R}(\lambda^2-1)=-\frac{GMm}{r}RGMm​(λ2−1)=−rGMm​

Cancel GMmGMmGMm: λ2−1R=−1r\frac{\lambda^2-1}{R}=-\frac{1}{r}Rλ2−1​=−r1​

1−λ2R=1r\frac{1-\lambda^2}{R}=\frac{1}{r}R1−λ2​=r1​

Therefore, r=R1−λ2r=\frac{R}{1-\lambda^2}r=1−λ2R​


  1. Match with options

This corresponds to:

R1−λ2\boxed{\frac{R}{1-\lambda^2}}1−λ2R​​

So the correct option is B.


  1. Verification of physical condition

For the body to reach a finite maximum distance, we need λ<1\lambda<1λ<1 If λ=1\lambda=1λ=1, then r→∞r\to \inftyr→∞, which is escape. This is physically consistent.

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