question_answer

Two particles, initially at rest move towards each other under the effect of gravitational force of attraction. At the instant when their relative velocity is 3v where, v is the velocity of the slower particle, then the speed of the centre of mass of two given particles is

A)  \[1v\]              

B)  \[2v\]

C)  \[3v\]              

D)  zero

Correct Answer: D

Solution :

 According to Newton law of universal gravitation, every point mass attracts every other point mass by a force directed along the line connecting the two. The gravitational force is an internal force. Since, the two particles are initially at rest, their centre of mass is also initially at rest under the effect of internal forces, so the centre of mass remains in the state of rest.