I know that the CM frame is the frame in which the total momentum of the system adds up to zero.

For example, if I imagine two bodies **m1** and **m2**, with **m1 = m2**, one moving at the speed **v1 = v** and the other standing at **v2 = 0**, we can say that in the CM frame they are both moving at the speed of **|v\*| = |v/2|**.

How can I mentally visualize the position of the observer that sees both bodies moving at that speed? I think the observer should move at a certain speed too, but which speed?