CONSERVATION OF MOMENTUM
 

According to the law of conservation of momentum, the total momentum contained in two objects is the same after a collision as before. The characteristics of the collision do not matter as long as it is an ideal system. In an ideal system, there is no friction or other real-world imperfection, and the total system momentum never changes unless a new mass or force is introduced.
The law of conservation of momentum applies not only to systems having two objects or particles but also to systems having any number of objects or particles. However, the law holds only in a closed system, that is, a system in which the total mass remains constant, and no forces are introduced from the outside.
This is a good time to make an important announcement. From now on in this book, if specific units are not given for quantities, assume that the units are intended to be expressed in the International System (SI). Therefore, in the following examples, masses are in kilograms, velocity magnitudes are in meters per second, and momentums are in kilogrammeters per second. Get into the habit of making this assumption, whether the units end up being important in the discussion or not. Of course, if other units are specified, then use those. But beware when making calculations. Units always must agree throughout a calculation, or you run the risk of getting a nonsensical or inaccurate result.