Question

A mass of $20kg$ moving with a speed of $10m/s$ collides with another stationary mass of $5kg$. As a
result of the collision, the two masses stick together. The kinetic energy of the composite mass will be:-

• A. $600Joule$
• B. $800Joule$
• C. $1000Joule$
• D. $1200Joule$

Answer 1

The correct option is B $800Joule$

As there is $no$ external non-conservative force on the system (both masses altogether) the $linear$ momentum should

be conserved.

Thus we should have $m_1{v}_1+m_2{v}_2=(m_1+m_2)V$ (inelastic collision)

$V$ is the common velocity after the collision.

putting values we get $20\left(10\right)+5\left(0\right)=(20+5)V$ or $V=8m/s$

so the new KE will be $K=\frac{1}{2}m{V}^2=0.5\times 25\times 64=800Joule$