In case (2), two masses with momentums ⁠ p 1 = m 1 v 1 ⁠ and ⁠ p 2 = m 2 v 2 ⁠ collide to produce a single particle of conserved mass ⁠ m = m 1 + m 2 ⁠ traveling at the center of mass velocity of the original system, v c m = ( m 1 v 1 + m 2 v 2 ) / ( m 1 + m 2 ) . The total momentum ⁠ p = p 1 + p 2 ⁠ is conserved.