Momentum is defined as mass times velocity. It is a vector quantity measured in kg*m/s. Impulse is the change in momentum caused by a force over time. Impulse is equal to force times time. In collisions, momentum is always conserved while kinetic energy may or may not be conserved depending on whether the collision is elastic or inelastic. Explosions also conserve momentum as the total momentum of the system before must equal the total momentum after.