Big Idea! Momentum is always conserved (saved).
Consider a bat hitting a baseball. The law of conservation of momentum says that the total momentum of the bat and the ball before they collide will be exactly equal to the momentum of the two objects after the collision. We call these two objects a system. In other words, the total momentum considered to be conserved involves only these two objects. Both objects will change speed or direction once they collide but we know that their combined momentum before they collide will be exactly equal to their combined momentum after the collision.
Consider the example below.
Because the total of both car and truck after the collision must also be 30,000 kg m/s, a simple algebraic equation allows us to solve for the velocity of the truck after.
Check out Paul Hewitt describing this concept in a screencast here.
The Physicsclassroom.com provides an excellent description of the conservation of momentum here.
