Question

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

• A. 600 J
• B. 800 J
• C. 1000 J
• D. 1200 J

Answer 1

Answer: (B)

Given, $m_{1}=20\, kg,\, u_{1}=10\, m /s$,
Using law of conservation of momentum,
$m_{1} u_{1}+m_{2} u_{2}=\left(m_{1}+m_{2}\right) v 20 \times 10+5 \times 0 =(20+5) \times v$
$\therefore v=\frac{200}{25}=8\, m / s$
Kinetic energy of the composite mass
$=\frac{1}{2}\left(m_{1}+m_{2}\right) v^{2}$
$=\frac{1}{2}(20+5) \times(8)^{2}$
$=25 \times 32=800\, J$