question_answer

Two solid rubber balls A and B  having masses 200 and 400 gm respectively are moving in opposite directions with velocity of A equal to 0.3 m/s. After collision the two balls come to rest, then the velocity of B is [CPMT 1978, 86, 88]

A)          0.15 m/sec

B)                       1.5 m/sec

C)          ? 0.15 m/sec

D)                       None of the above

Correct Answer: C

Solution :

Initial linear momentum of system = \[{{m}_{A}}{{\vec{v}}_{A}}+{{m}_{B}}{{\vec{v}}_{B}}\]                                          = 0.2 × 0.3 + 0.4 × vB Finally both balls come to rest \ final linear momentum = 0 By the law of conservation of linear momenum        0.2 × 0.3 + 0.4 × vB = 0          \                                 \[{{v}_{B}}=-\frac{0.2\times 0.3}{0.4}=-0.15\ m/s\]