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 both 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, $m_1=20kg,{\mu }_1=10m/s$ $m_2=5kg,{\mu }_2=0$ Using law of conservation of momentum, $m_1{u}_1+m_2{u}_2=(m_1+m_2)v$ $20\times 10+5\times 0=(20+5)\times v$ $\therefore$ $v=\frac{200}{25}=8m/s$ Kinetic energy of the composite mass $=\frac{1}{2}(m_1+m_2)v^2$ $=\frac{1}{2}(20+5)\times {(8)}^2=2.5\times 32=800J$