In this chapter, we introduce the linear momentum of a particle and the law of conservation of linear momentum of a system of particles under certain conditions. We use this law and the conservation of energy to analyze translational motion when particles collide. For a system of isolated particles, or an extended object, we introduce the concept of center of mass to show that conservation of linear momentum applies under certain conditions, as it does for isolated particles. At the end of this chapter, we treat systems with variable mass. We first consider cases where the mass increases with time and then we consider cases where the mass decreases with time.