Suppose the first ball comes in heading north. If it does not hit the target ball head on, the target ball flies off at some angle (say, to the northeast). It has an eastward component of momentum. To balance this, the first ball cannot stop, for if it did so, momentum would not be conserved in the east-west direction. It must head roughly northwest in such a way that its westward component of momentum cancels the eastward component of the target ball. The east-west momentum is then zero after the collision, which it must be because it was zero before the collision. See Figure 6.19 on page 102.