A body of mass 1kg collides head on elastically with a stationary...

Let ${u}_{1}$ be the initial speed of the smaller body before collision.

Let $v$ be the final speed of the bigger body after collision.

Applying conservation of linear momentum,

$1 \times {u}_{1} + 0 = - 2 + 3v ⇒ {u}_{1} = - 2 + 3v . . . \left(1\right)$

We know, coefficient of restitution for elastic perfectly collision is $1$.

$⇒ 1 = \dfrac{v + 2}{{u}_{1}} ⇒ v + 2 = {u}_{1} . . . \left(2\right)$

Equating \left(1\right)and\left(2\right)$, we have</p><p>${u}_{1} = 4m{s}^{- 1}.