 So the question now is, how do we write shear stress or shear force into the convention of dealing with stresses within soils? Well, if we return back to this shape, which represents an element of material, and it could be soil, but it could be any material, and we know that this material is static. So for this material to be stationary, it has to fulfill two conditions. The first is static equilibrium, and that means that the material isn't sort of moving around on the board here, but we're putting two forces into it. So what that means is that if we resolve those two forces into the same reference plane, and I recorded another video which shows how to rotate forces into different reference planes, but if you rotate A and B into the same reference plane, they should be equal and opposite. Okay, so let's rotate A and B into the vertical reference plane, and to do that we know that B has a vertical component, and that vertical component is equal to B cosine beta, and A has a vertical component, and that's equal to A cosine alpha. So for static equilibrium to be true, A cosine alpha must be equal to B cosine beta. But if this material is stationary, it must also satisfy rotational equilibrium, and that means that the material doesn't rotate about its center point. So to determine that we need to resolve A and B into the horizontal component. So if we resolve A into its horizontal component, it would be a shear force acting on this surface, and that would be equivalent to A sine alpha, and similarly B has a shear force acting at B sine beta. So we can get rid of the blue lines. So for rotational equilibrium to be true, if we take moments about the center point, and I've included another video which shows you how to take moments, if we take moments about the center point, for rotational equilibrium we have to balance the clockwise moments with the anticlockwise moments. And for rotational equilibrium, the moments that are acting in this direction are A sine alpha and B sine beta. So you can see that the moments caused by these two forces are acting in the same direction. So if we take the distance from the center to Bx, but we take A sine alpha and add it to B sine beta, both of those acting through a distance of x. So multiply both of those by x. Now those must be equal to, or there must be another force acting in an opposite direction if this material is in rotational equilibrium. So what forces are acting in this direction? Well, those must be shear forces acting on the two planes that don't have any normal force. So there must be two forces acting in this direction. And if we give those just a generic shear force, let's say I call that tau, then the moments acting in this direction would be equal to the moments acting in the other direction. So we can say that that would be equal to 2x times by tau, or 2 tau x. And in this case, what we can do is just cancel out the x's. So what we're left with here for rotational equilibrium is A sine alpha plus B sine beta equals 2 tau. So an important observation comes out of this, and that is that we can have shear force or shear stress acting on a plane that doesn't have any normal stress or normal force acting on it. So we're going to return to this again when we start thinking about calculating shear stress and normal stress in different reference planes and how we relate that on a Mohr's circle diagram. But it's really important now just to note down that we can get shear stress acting on planes that we maybe might not expect it to be on. And certainly planes where we don't get any normal force.