# Momentum and Collisions - Mission MC4 Detailed Help Two Silly Putty balls of equal mass are moving in opposite directions at equal speeds. The balls collide head on and immediately stop. In this collision, ____. Click the button below to play an audio file.   Your browser does not support the audio element. Please download and view here. Each Putty ball's momentum can be calculated as the product of mass and velocity (see Formula Frenzy section). In this case, both balls have the same mass and the same speed. Thus, the momentum of each individual Putty ball is the same, except that the direction of the momentum is + (right) for one and - (left) for the other. For instance, one Putty ball may have +10 units of momentum and the other has -10 units of momentum. So before the collision, the total amount (sum) of momentum possessed by the system of two Putty balls is 0 units. The collision causes the Putty balls to immediately stop such that each ball has 0 units of momentum after the collision. And so the total amount of momentum possessed by the system after the collision is 0 units. The total system momentum is the same before the collision as it is after the collision. Total system momentum is clearly conserved. The momentum (p) of an object can be calculated from knowledge of its mass (m) and velocity (v) using the formula: p = m • v  