a particle of mass 1kg is thrown vertically upward with a speed 100m/s . After 5sec it explodes into two parts . one part of mass 400g comes back with speed 25m/s ,what is the speed of the other part just after explosion?(g=10)

Let the velocity of the mass just before it explodes be v m/s.

Therefore, v = 100 - gt = 100 - 10(5) = 50 m/s.

Let the velocity of the 600 gm mass be u.

Applying law of conservation of momentum:

1000 x 50 = 600u - 400 x 25

Therefore, 600u = 50000 + 10000 = 60000

Therefore, u = 100 m/s in the upward direction

Hence, option (C) is the correct choice.