Well, you might be surprised to know that a project manager has no formal authority whatsoever. In fact, in most organizations, the authority (or power, whatever you want to call it) still lies in the hands of the line/functional manager who often abuses it.
The project manager, unless working in a projectized organization does not own the resources and does not have any authority (power) over the resources. A lot of conflicts between the line manager and the project manager arise because of this situation (imagine trying to get people to do some work over whom you have no authority whatsoever). Smart project managers, in this situation, try to earn some informal authority over the resources.