The amount of friction depends on the fluid viscosity and the velocity gradient ... When a fluid is stationary, its molecules are in a constant state of motion with a random velocity v. For a gas, v ...

Once disk picks up enough angular velocity to produce a no-slip condition at the contact ... the subsequent rolling without slipping sees the friction force going to zero, with the disk continuing to ...

points 1 and 2 lie on a streamline, the fluid has constant density, the flow is steady, and there is no friction. Although these restrictions sound severe, the Bernoulli equation is very useful, ...