Question

A lump of clay of mass $10g$ travelling with a velocity of $10cm{s}^{-1}$ towards east collides head on with another lump of clay of mass $10g$ travelling with velocity of $20cm{s}^{-1}$ towards west. If the two lumps coalesce after collision, what is its velocity, if no external force acts on the system?

• A. $15cm{s}^{-1}$ towards west
• B. $15cm{s}^{-1}$ towards east
• C. $5cm{s}^{-1}$ towards west
• D. $5cm{s}^{-1}$ towards east

Answer 1

Answer: (C)

Let velocity towards east be positive, then velocity towards west is negative.
If $v$ is the velocity of two lumps which are coalesced after collision, then from law of conservation of linear momentum
$m_1{v}_1+m_2{v}_2=\left(m_1+m_2\right)v$
$10\times 20-10\times 10=20\times v$
$v=\frac{100}{20}=5cm{s}^{-1}$
Hence, $5cm{s}^{-1}$ towards west.