Ch 18: Fluid MechanicsWorksheetSee all chapters
All Chapters
Ch 01: Intro to Physics; Units
Ch 02: 1D Motion / Kinematics
Ch 03: Vectors
Ch 04: 2D Motion (Projectile Motion)
Ch 05: Intro to Forces (Dynamics)
Ch 06: Friction, Inclines, Systems
Ch 07: Centripetal Forces & Gravitation
Ch 08: Work & Energy
Ch 09: Conservation of Energy
Ch 10: Momentum & Impulse
Ch 11: Rotational Kinematics
Ch 12: Rotational Inertia & Energy
Ch 13: Torque & Rotational Dynamics
Ch 14: Rotational Equilibrium
Ch 15: Angular Momentum
Ch 16: Periodic Motion
Ch 17: Waves & Sound
Ch 18: Fluid Mechanics
Ch 19: Heat and Temperature
Ch 20: Kinetic Theory of Ideal Gasses
Ch 21: The First Law of Thermodynamics
Ch 22: The Second Law of Thermodynamics
Ch 23: Electric Force & Field; Gauss' Law
Ch 24: Electric Potential
Ch 25: Capacitors & Dielectrics
Ch 26: Resistors & DC Circuits
Ch 27: Magnetic Fields and Forces
Ch 28: Sources of Magnetic Field
Ch 29: Induction and Inductance
Ch 30: Alternating Current
Ch 31: Electromagnetic Waves
Ch 32: Geometric Optics
Ch 33: Wave Optics
Ch 35: Special Relativity
Ch 36: Particle-Wave Duality
Ch 37: Atomic Structure
Ch 38: Nuclear Physics
Ch 39: Quantum Mechanics

Concept #1: Fluid Flow / Ideal vs Real Fluids


Hey guys. So, in this video we're going to start talking about fluids in motion or fluid flow and the first thing we're going to cover is the distinction between real fluids and ideal fluids, let's check it out. Alright, so the motion of fluids or the flow of fluids can get pretty complicated it's actually an area, that's still under active research in physics but we're going to simplify things by using a model called ideal fluid. So, the idea that fluids are pretty complicated but if we eliminate a ton of the complexities, the more complicated complexities of real fluids you end up with something that's called ideal fluids, they don't exist but they are a way to make things more manageable, cool? And there are some things you should know first you should know that these fluids will always be, ideal fluids will always be incompressible, that's a simplification, incompressible means constant density. Remember, density has to do with how tightly packed molecules are and in ideal fluids that's going to be constant, in real fluids molecules could get tighter or less dense, less tightly packed depending what's going on, okay? So, incompressible constant density, that simplifies things a bunch and the second thing you should know is that ideal fluids are always going to have what's called laminar flow, and laminar flow just means steady flow. So, if you look at water going to a pipe, if it's let's say see-through pipe or something like that you would see that there's just a constant stream of water that looks very clean and neat as opposed to real fluids that could have what's called turbulent flow, turbulent flow or turbulence, you could have turbulence if the liquid is moving or the fluid more generally is moving too fast, okay? So, imagine, if water is going way too fast that it goes through a little a little clot type thing here, some sort of constriction, then it could be that the water starts going all over the place and this is turbulence and this is generally bad news, lucky for you, you're probably not going to see any turbulence questions, you may just have to know this conceptually, cool? And the third distinction between them and I'm going to start over here is whether this motion is going to have viscosity, whether the fluid is going to have viscosity or not, okay? So, the defining characteristic, this is the most important of the three here, the defining characteristic of real fluids is that they have what's called viscous flow, viscous flow, in other words, the liquid has viscosity and viscosity has to do with the thickness, thickness of the fluid. So, for example, honey, right? If you get a cup full of honey and you turn it like this it's going to move very, very slowly it's because there's a lot of viscosity, ideal fluids have no viscosity at all they have no resistance, viscosity is essentially fluid friction, it's essentially fluid friction and it works very similar to air resistance or a kinetic friction and that it slows it down, okay? So, real fluids could have viscosity and they could have a viscous friction, ideal fluids are always going to have what's called non viscous, not very creative name, non viscous flow, in other words, no friction. So, that's the big difference, if you have an ideal fluid it's going to flow, it's going to flow smoothly with no friction, a real fluid could have turbulence and it has viscosity, okay? Now, lucky for you, most problems you see and maybe even all of the problems you see will be about ideal fluids, in fact a lot of professors don't even get into real fluids. So, if yours doesn't your life is simple, we're going to also assume ideal fluids unless something says explicitly that this is a real fluid or if they refer to viscosity which is fluid resistance, right? So, if they say that there's some sort of viscosity then that means there's resistance which mean it's a real, which now it means that you have a real fluid.

In most real fluid problems you're going to have viscosity because it's the defining characteristic but you're not going to have turbulence and you're also not going to have compression of the fluid, okay? So, then you're really not going to see this most of the time, you're not going to see this most of the time but you are going to see this quite a bit. So, you're going to have no turbulence and no compression of the fluid, okay? So, really it's going to come down to whether or not it has viscosity. So it is a quick intro so you can know some of the terminology and that's it, let's keep going.