A body of mass 1 kg collides head on elastically with a stationary body of mass 3kg. After collision, the smaller body reverses its direction of…

A body of mass $\displaystyle 1$ kg collides head on elastically with a stationary body of mass 3kg. After collision, the smaller body reverses its direction of motion and moves with a speed of $\displaystyle 2 \mathrm{~m} / \mathrm{s}$. The initial speed of the smaller body before collision is $\displaystyle \_\_\_\_$ $\displaystyle \mathrm{ms}^{-1}$.