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JIPMER 2010
Exam
JIPMER
Year
2010
Subject
Physics
Chapter
Gravitation

JIPMER 2010 · Physics · Gravitation

Two spherical bodies of masses M and 5 M and radii R and 2 R...

Two spherical bodies of masses MM and 5M5 M and radii RR and 2R2 R respectively are released in free space with initial separation between their centres equal to 12R12 R. If they attract each other due to gravitational force only,then the distance covered by the smaller body just before collision, is

Options

  1. A1.2R
  2. B2.5R
  3. C4.5R
  4. D7.5R

Correct answer

D. 7.5R

Step-by-step solution

Let at OO there will be a collision. If smaller sphere moves xx distance to reach at OO, then bigger sphere will move a distance of (9R−x)(9 R-x). F=GM×5M(12R−x)2asmall =FM=G×5M(12R−x)2abig=F5M=GM(12R−x)2x=12asmall t2=12G×5M(12R−x)2t2…(i)(9R−x)=12abig t2=12GM(12R−x)2t2…(ii)\begin{aligned} F & =\frac{G M \times 5 M}{(12 R-x)^2} \\ a_{\text{small }} & =\frac{F}{M}=\frac{G \times 5 M}{(12 R-x)^2} \\ a_{\mathrm{big}} & =\frac{F}{5 M}=\frac{G M}{(12 R-x)^2} \\ x & =\frac{1}{2} a_{\text{small }} t^2=\frac{1}{2} \frac{G \times 5 M}{(12 R-x)^2} t^2 \ldots(i)\\ (9 R-x) & =\frac{1}{2} a_{\text{big }} t^2=\frac{1}{2} \frac{G M}{(12 R-x)^2} t^2\ldots(ii) \end{aligned} Thus, dividing Eq. (i) by Eq. (ii), we get x9R−x=5\frac{x}{9 R-x}=5⇒x=45R−5x⇒6x=45R∴x=7.5R\begin{aligned} & \Rightarrow \quad x=45 R-5 x \\ & \Rightarrow \quad 6 x=45 R \\ & \therefore \quad x=7.5 R \\ & \end{aligned}$$
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