A block A of mass 2m is placed on other block B of mass 4m which is placed on a fixed table as shown in figure. A small object C of mass 6m moving horizontally along a line passing through the center

Question

A block A of mass 2m is placed on other block B of mass 4m which is placed on a fixed table as shown in figure. A small object C of mass 6m moving horizontally along a line passing through the center of block B and perpendicular to its face with a speed v collides elastically with block B. Friction is absent between ground and block B and surface between blocks is rough. Then :-

TestHub · Accepted Answer

The collision is instantaneous, so finite friction cannot move A during it: $v_{A} = 0$ - (A) false. Elastic hit, C (6m) on B (4m): $v_{B} = \frac{2 ( 6 m ) v}{6 m + 4 m} = \frac{6 v}{5}$ - (B) true.

$v_{c m} (A + B) = \frac{2 m ( 0 ) + 4 m \cdot \frac{6 v}{5}}{6 m} = \frac{4 v}{5} - (C) true.$

Friction's impulse during the infinitesimal collision is negligible, so (C+B) momentum is conserved - (D) true. Answer: (B), (C), (D)

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