Simplify the following text for a Grade 10 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 . Momentum is sometimes referred to as linear momentum to distinguish it from the related subject of angular momentum .
Simplified: Momentum is a conserved object , meaning that the total initial momentum of a system must be equal to the total final momentum of a system .