Question

A mass of $20kg$ moving with a speed of $10m/s$ 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)

800 J

Answer 2

By conservation of momentum

$m_{1}u_{1} + m_{2}u_{2} = (m_{1} + m_{2})V$

Velocity of composite mass

$V = \frac{m_{1}u_{1} + m_{2}u_{2}}{m_{1} + m_{2}} = \frac{20 \times 10 + 5 \times 0}{20 + 5} = 8m ⥂ / ⥂ s$

∴ Kinetic energy of composite mass

$= \frac{1}{2}(m_{1} + m_{2})V^{2} = \frac{1}{2}(20 + 5) \times 8^{2} = 800J.$