Conservation Of Linear Momentum
Suppose that the net external force acting on a system of particles is zero (the system is isolated) and that no particles leave or enter the system (the system is closed).
P = constant
If no net external force acts on a system of particles, the total linear momentum P of the system cannot change
Vector equations and, as such, each is equivalent to three equations corresponding to the conservation of linear momentum in three mutually perpendicular directions as in, say the coordinate system. Depending on the forces acting on a system, linear momentum might be conserved in one or two directions but not in all directions.
Note that we focus on the external forces acting on a closed system. Although internal forces can change the linear momentum of portions of the system, they cannot change the total linear momentum of the entire system.
CHECKPOINT 4: An initially stationary device lying on a friction less floor explodes into two pieces, which then slide across the floor. One piece slides in the positive direction of an x axis. (a) What is the sum of the momenta of the two pieces after the explosion? (b} Can the second piece move at an angle to the x axis? (c) What is the direction of the momentum of the second piece?