A) Zero
B) 562.5 J
C) 360 J
D) 180 J
Correct Answer: C
Solution :
Key Idea : In absence of any external force, linear momentum of a system is conserved. Conservation of linear momentum gives \[mv={{m}_{1}}{{v}_{1}}+{{m}_{2}}{{v}_{2}}\] \[9\times 0=4\times 15+5\,{{v}_{2}}\] or \[{{v}_{2}}=-\frac{60}{5}=-12\,m/s\] Hence, kinetic energy of 5 kg mass \[{{K}_{2}}=\frac{1}{2}{{m}_{2}}{{v}_{2}}^{2}\] \[=\frac{1}{2}\times 5\times {{(-12)}^{2}}\] \[=\frac{1}{2}\times 5\times 144\] = 360 J Note: As velocity of 2nd mass is negative it implies that its velocity is in opposite direction of 1st mass.
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