When an object is immersed in a fluid, say or example water, then the force exerted by the fluid is always perpendicular to the surface. I'm unable to understand why is this so?
My books writes :- (NCERT Class 11 th Physics Part 2)
When an object is submerged in a fluid at rest, the fluid exerts a force on its surface. This force always normal to the object's surface. This is because if there were a component of force parallel to the surface, then the object will also exert force on the fluid parallel to it as a consequence of Newton's third law. This will cause the fluid to flow parallel to the surface. Since the fluid is at rest therefore it cannot happen. Hence, the force exerted by the fluid has to be perpendicular to the surface.
I have some counter-arguments for this explanation.
As you can see from the picture that if the fluid exerts a force F1 then the block would exert a reactionary force -F1 and this should cause the fluid to move upward in a beaker but why don't we observe it in real life?
F3 on block would compel the block to exert -F3 on the fluid which should cause the fluid to flow in the beaker.
I tried to satisfy myself that reactions of F3 and F4 would cancel each other but we also know that pressure i.e. force (if area is constant) varies with depth and therefore I couldn't account for the absence of this upward motion. For sideway motion it is satisfactory that reactions of F3 and F4 would cancel each other out (although I'm not sure over here too).
So, I request to please solve my misconception. Books like these tend to be logical but always ends up being illogical.
At last I want to make another request: Please give a plausible explanation why the fluid force is always normal to the surface?
My question is addressing Newton's third law in explaining the "normal" nature of the fluid pressure.