All of the elements of force, velocity have to be included as getting back to first prinicples, everything can be at least partially explained. Forget what you see, hear, feel and only consider what accurate sensors yield which can be put into a model describing the event and give quantifiable figures which really can't be argued like opinion can. Consider a group of billard balls in a circle with some type of force in the center. When all are excited, like a point source they travel in all directions with a similar velocity. Now consider the same billard balls arranged in a cylinder with finite depth and diameter and when the billard balls are excited, some go sideways while others press on forward along the path of least resistance. All have to encouter some type of reverse/negative force which seeds them to a steady state. The ones going to the sides are attentuated and reflected possibly colliding with another billard ball from the opposite direction forcing their velocity to zero. They may just be the entropy of the system but they have to be accounted for. From first principles, Force equals mass times acceleation(F=ma) and all systems strive to reach some sort of equlibrium(Thermodynamics) unless continually excited. Although I may have mixed wave and partical physics, either one should be able to expain the event. A combustion engine, a boiler, etc. are good examples and loosely are similar to what we have here with the wad be the piston which is being ejected. What we are looking for is how is the heat/expanding gas, whatever, converted into work.

Kind Regard,

Raimey
rse