The linear momentum of a particle is conserved if :

The law of conservation of linear momentum states that the total momentum of a system remains constant if the net external force acting on it is zero [1]. This principle is derived from Newton's Second Law of Motion, which defines force as the rate of change of momentum; if the force is zero, the change in momentum is zero, implying momentum is conserved. While torque relates to the conservation of angular momentum, linear momentum specifically depends on the vector sum of external forces. In an isolated system where all internal forces cancel out due to Newton's Third Law, the absence of a resultant external force ensures that the initial and final momenta are equal. Therefore, for a particle or system to maintain constant linear momentum, the net force must be zero [1].

Sources

1. [1] https://www.khanacademy.org/science/ap-college-physics-1/xf557a762645cccc5:linear-momentum/xf557a762645cccc5:untitled-303/v/conservation-of-linear-momentum