Hi I have been dealing with a fluid mechanics pressure gradient problem and from a statistical view point I can see how it resolves itself but am puzzled as to how it can occur at the molecular scale from a conservation of linear momentum perspective if Momentum is a conserved quantity While the pressure gradient must resolve itself in the direction of the flow this requires a change of momentum vector within the flow without a resultant force. (without striking the container surface). I don't wish to fully explain the gradient I am looking at as it would take too long and is largely irrelevant to my question about Linear and Rotational momentum. The only way I can see this occurs is the use of Degrees of freedom to momentarily change linear to rotational momentum. then back again in the direction of the flow. The best analogy I can think of is a tennis ball on string (representing a molecular degree of freedom) is hit against in one direction it rotates around center mass and it hits another ball on the opposite side and imparts momentum in the reverse direction If we were to equate that to molecules (P representing momentum) Of course I acknowledge that the exact opposite is equally likely to occur so overall Momentum is Conserved but if you could examine on an individual event basis can rotational momentum be used to reverse a linear momentum vector without a resultant force. What am I not considering. Does the center mass go backwards ?