Simplify the following text for a Grade 8 native English-speaking person.
Original: Momentum is a conserved quantity ( law of conservation of linear momentum ) , meaning that if a closed system is not affected by external forces , its total momentum can not change .
Simplified: Using vector addition to add the momentum of the bullet to the momentum of the gun ( equal in size but opposite in direction ) gives a total system momentum of zero . The momentum of the gun-bullet system has been conserved .