Question

Three particles A, B and C of equal mass move with equal speed $V=5m{s}^{-1}$ along the medians of an equilateral triangle as shown in the figure. They collide at the centroid G of the triangle. After the collision, A comes to rest, B retraces its path with the speed $V=5m{s}^{-1}$. What is the speed of C?

Answer 1

Initial momentum of the system is zero because of symmetry as shown in fig.

by the principle of conservation of momentum in elastic collision final momemtum should be zero.

after the collision A comes to rest. therefore velocity of C should be equal and opposite to the velocity of B as shown in fig.

hence the speed of C is $5m{s}^{-}1$ .