hobbitle's Eng Questions Thread

[hobbitle]

Hey peeps,

I'm going to have a crack at starting a thread to contain all of my questions that are from my eng-related subjects.  Only have one this semester but maybe this will continue into next year.  I basically have not done any physics and I'm discovering this has resulted in a big gap in my knowledge. 

So my first question is about fluid dynamics and the two flat plane experiment.

I understand and can draw the steady state profiles of the flat plane experiment (depending on which plane is moving) and also understand the derivation of Newton's Law of Viscosity:

tau_zx = F_x/A_z = -mu dv_x/dz

But there's this one related concept that I'm struggling to understand. F_x is the force applied by the particles below the designated plane on to the particles above the plane.  So basically I understand this to be a friction force and so then there will be an opposing force that is equal and opposite applied by the particles above the plane to the particles below the plane.  But then why/how is there any net movement at all?

[Hancock]

This is a classic Newton's 3rd Law problem. The reason that there is net movement is because there is an equal and opposite reaction force acting on different bodies. For example: if I hit a ball rightwards, there is a force imparted on the ball in the positive x-direction but there is an equal and opposite force on my hand in the left direction.

Although the vector sum of the forces is 0, the reaction force but N3 law doesn't act on the ball as well. Therefore there is net movement.