 So, the definition of shear stress or shear force starts by a description of clume friction. See, you've probably already come across clume friction as part of your introductory physics, but let's recap on what that is now. So imagine you had a block that was sitting on some sort of surface. We know that that block would subject the surface to a normal force due to its own weight. So the weight of the block would create a normal force acting on that surface. Let's call that N. If I subjected that block to a horizontal force, so if I tried to push it in this direction, initially when I subjected a small force so that it wouldn't move, and it would only start moving when the applied force overcame the frictional force. So we know that there's a frictional force created by the interaction of the surface in the block. We'll call that F, and we'll call the applied force A. So the block would only start moving when A was greater than F. So when A is greater than F, we know that the block would move. We also know that F is proportional to or equal to the applied the normal force multiplied by some coefficient of friction. So F equals the normal force multiplied by coefficient of friction mu. Okay and because of this this relationship we can put these two things together and we say that the block wouldn't move until A was greater than the applied the normal force multiplied by the coefficient of friction. So instead of looking at this as a cartoon of a block, let's simplify it into a force diagram. So we take away the block and the surface and we just represent the forces acting on a single point. So let's have a point here and we have our normal force. We have an applied force. We have a frictional force. And what's missing here is some sort of resistive force from the surface. So let's say we have a resistive force R. So we've taken that cartoon and we've represented it as a simple force diagram. Now what we can do next is we can combine these two resistive forces. I suppose the normal resistive force from the surface and the frictional force into a single force acting at some angle called that phi from the vertical. So we've combined these two forces into a single force. So that new resistive force is acting at phi from the vertical. And then this angle is given a special name. It's called the angle of friction and it's given the symbol phi. So tan phi must be equal to the frictional force over the normal force. So we can see here that the larger the angle of friction, the larger this angle is, the larger the frictional force that's generated proportional to the normal force. So we can see that larger angle of friction means that we've got a rougher or a more frictional surface. So we can see that this angle of friction has a relationship with the coefficient of friction where the coefficient friction f equals n times mu. If we write it just in terms of mu you have mu equals f over n. So we can see that tan phi is equal to mu. So it's quite convenient to think of surfaces or materials in terms of their angle of friction because you take out the or you resolve the relationship or you resolve two parameters, the relationship between the frictional force and the normal force into just one parameter. So just going back to this cartoon for a second, if we wanted to represent this in terms of stress rather than force, what I could do is take my normal force n and divide that by the contact area between the block and the horizontal surface. So if I took n and divided it by the contact area what I'd be left with is my stress but the same would be true if I did that for my frictional force and what I'd be left with is a shear stress. So what would happen do you think if I drilled a hole through this block? So I drilled a hole through the center of the block and I injected water into that hole so I filled some water into the hole. So the hole goes all the way through the block and comes out the other side so the water would then permeate through until it spread between the two surfaces. You can see that we'd have some level of water pressure between these two surfaces and that would then lower the amount of applied force that I needed to move the block. So you can see that the importance of effective stress when it comes to whether the block slides or not is really quite important. So my effective stress would be the, so my effective stress would be equal to this total stress minus whatever pore, water pressure was between the block and the surface. So that's why it's important to represent soils in terms of effective stress rather than total stress because it's the effective stress that governs where the soils fail.