You need to understand the definition of angular momentum and the meaning of vector cross product. Angular momentum j is defined as J=r×p, where r is the potential vector of the object relative to the reference point O (the direction points to the object from the point O) and p is the linear momentum (also a vector) of the object. The cross product of two vectors is still a vector, the size of which is the area of a parallelogram composed of two vectors, and the direction adjustment follows the right-handed system. If you don't understand the concept of vector cross product, you can study it again. In addition, from an intuitive point of view, look at the two balls in the example. Do they rotate counterclockwise or clockwise after colliding with disks or rods? Example 1 Obviously, two balls have opposite effects on the disk. One ball makes the disk rotate counterclockwise and the other ball makes the disk rotate clockwise. Obviously, their angular momentum direction is opposite to O; But in example 2, both balls can promote the clockwise rotation of the rod, so it is obvious that the angular momentum direction of the two balls relative to O is the same.