The centre of mass of a body is a point at which the entire mass of the body is supposed to be concentrated. The position vector `overset rarr(r )` of c.m of the system of tow particles of masses `m_(1)` and `m_(2)` with position vectors `overset rarr(r_(1))` and `overset rarr(r_(2))` is given by
`overset rarr(r ) = (m_(1)overset rarr(r_(1)) + m_(2)overset rarr(r_(2)))/(m_(1) + m_(2))`
For an isolated system, where no external force is acting, `overset rarr(v_(cm)) =` constant
Under no circumstances, the velocity of centre of mass of an isolated system can undergo a change
With the help of the comprehension given above, choose the most appropriate alternative for each of the following questions :
An electron and a proton move towards eachother with velocities `v_(1)` and `v_(2)` respectively. the velocity of their centre of mass is

Options

A. zero
B. `v_(1)`
C. `v_(2)`
D. `(v_(1) + v_(2))/(2)`

Answer

A

Explanation

As the force of attraction is mutual, the system is isolated system without external force. Therefore,
`v_(cm) = 0`