Question

Two particles $A$ and $B$ initially at rest, move towards each other under the mutual force of attraction. At the instant when the speed of $A$ is $v$ and the speed of $B$ is $2v,$ the speed of the centre of mass of the system is:-

• A. $3v$
• B. $v$
• C. $1.5v$
• D. $zero$

Answer 1

The correct option is D $zero$

Given,

velocity of one particle $A$ is $v$

velocity of another particle $B$ is $2v$

We know that

$Centerofmass=\frac{m_1{v}_1+m_2{v}_2}{m_1+m_2}$

Here there is no external force therefore,

$F_{ext}=0$

No change in momentum in center of mass hence, center of mass cannot be zero but velocity may be zero hence, velocity of center of mass will be zero.