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BITSAT 2018
Exam
BITSAT
Year
2018
Subject
Physics
Chapter
Gravitation

BITSAT 2018 · Physics · Gravitation

An asteroid of mass m is approaching earth, initially at a distance...

An asteroid of mass mm is approaching earth, initially at a distance 10Re10{R}_{e} with speed vi.{v}_{i}. It hite oarth with a speed {v}_{f}, \left({R}_{e}\right and{M}_{e}areradiusandmassofearthare radius and mass of earth),then, then

Options

  1. A{v}_f^{2} = {v}_i^{2} + \dfrac{2Gm}{Re} \left(1 +\dfrac{1}{10}\right)
  2. B{v}_f^{2} = {v}_i^{2} + \dfrac{2G{M}_{e}}{Re} \left(1 +\dfrac{1}{10}\right)
  3. C{v}_f^{2} = {v}_i^{2} + \dfrac{2G{M}_{e}}{R\theta} \left(1 -\dfrac{1}{10}\right)
  4. D{v}_f^{2} = {v}_i^{2} + \dfrac{2Gm}{Re} \left(1 -\dfrac{1}{10}\right)

Correct answer

C. {v}_f^{2} = {v}_i^{2} + \dfrac{2G{M}_{e}}{R\theta} \left(1 -\dfrac{1}{10}\right)

Step-by-step solution

Initial energy of the asteroid is
Ei=Ki+Ui=12mvi2−GMθm10Re{E}_{i} = {K}_{i} + {U}_{i} = \dfrac{1}{2}m{v}_i^{2} - \dfrac{G{M}_{θ}m}{10{R}_{e}}

Final energy of the asteroid,
Ef=12mvf2−GMθmRθ{E}_{f} = \dfrac{1}{2}m{v}_f^{2} - \dfrac{G{M}_{θ}m}{R\theta}

According to law of conservation of energy, Ei=Ef{E}_{i} = {E}_{f}

12mvi2−GMθm10Rθ=12mvf2−GMθmRθ\dfrac{1}{2}m{v}_i^{2} - \dfrac{G{M}_{θ}m}{10{R}_{θ}} = \dfrac{1}{2}m{v}_f^{2} - \dfrac{G{M}_{θ}m}{R\theta}

vf2−2GMθRθ=vi2−2GMθ10Rθ{v}_f^{2} - \dfrac{2G{M}_{θ}}{R\theta} = {v}_i^{2} - \dfrac{2G{M}_{θ}}{10{R}_{θ}}

⇒ {v}_f^{2} = {v}_i^{2} + \dfrac{2G{M}_{θ}}{R\theta} \left(1 -\dfrac{1}{10}\right)

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