In my Physics textbook there is sample problem in which a firecracker placed inside a coconut of mass M, initially at rest on a frictionless floor, blows the coconut into three pieces (A, B and C) that slide across the floor. Mass of piece C is 0.30M and its final velocity is 5.0 m/s. We have to find speed of piece B, with mass 0.20M
The author made some conclusions before solving the problem:
The coconut and its pieces form a closed system
The explosion forces are internal to that system
No net external force acts on the system and Therefore the linear momentum of the system is conserved
My question: If Coconut and its pieces form a closed system then the explosion forces are external forces doing work on the system. How can linear momentum be conserved when an external force is doing work in that system? How can the sum of linear momentum of all its pieces be zero when an external force is doing work on the system?