 Okay, so we have our fluid flow with this boundary layer that has a difference of velocity from the bottom to the higher up creating a shear, and that shear creates what we call the Bernoulli effect, which are these fluid forces that both push downstream and lift up on a grain. So now we have the question about when does a sedimentary grain actually move? So we have our surface, and we have a grain sitting on the surface here. We're going to break down the forces, so the gravity depends on the mass and the gravitational acceleration. The gravitational acceleration is constant for all grains, so the real variable among the grains is how much they weigh. And this of course depends on the volume of the grain and the density of the grain. Different minerals have different densities. Then we have our fluid forces that lift up and downstream, and the fluid forces can be divided into two different components. One is the downstream component here, so we have our downstream, and then we have a lift force from that pressure difference where we have pressure medium and pressure low, that pressure difference with some high pressure in here and low all the way down on the downstream side. We have this lift force. So the grain can roll if the downstream force is big enough, but it won't actually be lifted up off the bed unless the lift force is greater than the gravitational force. So basically we can say that the grain can roll due to the downstream force, and then the grain can get lifted if and only if the lift force is greater than the gravitational force. So the amount of lift that you get from this pressure difference has to create a force that's big enough to pick it up off. If that force that's created is big enough, if it's larger than the force due to gravity, it will lift off the bed, and it'll get pulled up into the flow and downstream along the combination of the force, the gravity force and the fluid force. So it's really key that there's a force on every grain where there's a pressure difference from the bottom to the top, but it will only lift if it has a big enough upward force. That's true for planes as well. The reason we need a runway to take off is that the plane has to be going fast enough that the Bernoulli effect on the top of the wing overcomes the gravitational force that's holding that plane to the ground. And so as the plane accelerates down the runway, that lift force gets bigger and bigger and bigger and then it eventually goes off the ground. The same thing is true, well it's a little different for grains because they're sitting there, but the stronger the flow is over the top of those grains and the bigger the difference the flow is at the top from the bottom, that bigger that lift force is going to be. So if we think about it that way, we basically can conclude that a steeper flow gradient, let's say change, steeper yield change creates more lift force. So a couple of videos ago I talked about what controls the characteristics of the boundary layer and we said that if there's a more turbulent flow, the boundary layer gets compressed and I'll draw that in here. So if your boundary layer is somewhere in here, the variation in the flow is greater close to the bed and for the same size grain that will create a larger lift force. Thanks for watching.