Question

Throughout my time learning physics I have been imbued with the notion that forces cause accelerations, period. Accelerations don't cause forces, and they aren't merely correlated phenomena. By causality, I am content with the following definition: Connection between two events or states such that one produces or brings about the other; where one is the cause and the other its effect.

That is to say, an object experiences an acceleration because it is exposed to a net force; the force does not arise because of the acceleration. However, some philosophical thinking on Venturis has shaken my confidence in this idea. If the acceleration of the fluid through the constriction is caused by an unbalanced force, what causes the unbalanced force in the first place? Another way of asking the question is, how is the bounding geometry causally linked to the pressure distribution of the flow? My only answer as yet is that there's no other way to satisfy mass, momentum, and energy conservation simultaneously, but that seems decidedly unsatisfying. Is there any causality implied by Newton's 2nd Law?

Answer 1

Newton's first law states that a force is the cause of acceleration. In the case of fluid passing through a constriction, the acceleration is caused by an unbalanced force, which in turn is caused by the bounding geometry and pressure distribution of the flow.

Step-by-step explanation:

Newton's first law states that for any change in velocity to occur, there must be a net external force acting on an object.

This means that a force is the cause of acceleration, and not the other way around.

When a fluid passes through a constriction, the acceleration of the fluid is caused by an unbalanced force.

The unbalanced force arises due to the bounding geometry and the pressure distribution of the flow, which are causally linked.