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}}=10\,\,m/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}=8\,\,m/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=800\,\,J $