In my training as a Certified Flight Instructor, I am required to teach Lift Theory.  While simple explanations have sufficed in the past, as I progress in my training and understanding, I want to be able to understand lift in greater detail. During my last lesson, I stumped my Instructor. I asked about the cause of the increased velocity above an airfoil and he could not explain it so I am trying to research it and come to some clear understanding.  I have a hunch described below. Please let me know in semi-layman terms if I am on the right track.  Known:    As Velocity increases, Pressure decreases and vice versa. (Bernoulli) The air moving across the top of an airfoil moves faster than the air below the airfoil. Some have said that this is because the air above has a farther distance to travel but this has been proven untrue. Higher pressures below the airfoil and lower pressure above the airfoil create a portion of the lifting force of the airfoil. Additional lift forces are created by air being directed down by angle of incidence and the shape of the wing (Newton). This occurs from the air running into the bottom of the wing and being forced downward.  Lift is also occurring because the air above is moving at a higher velocity than the air below, and when they meet at the trailing edge, the faster air pushes the slower air   downward. (Vortices) From the Cambridge University video (see References) showing time-lapse video, we see that the air above the airfoil moves faster such that it reaches the trailing edge sooner than air below the airfoil.    Unknown:   This proves that the air over the airfoil moves faster but what is the cause for that air to move faster?   My Hypothesis:   Because of the angle of incidence and angle of attack to the relative wind, a higher pressure is created upon impact at the separation. This upper airfoil high pressure is considered ‘high’ relative only to the air pressure at other areas of the upper airfoil such as the center of pressure or the trailing edge of the wing.  (It is not meant to be considered ‘higher’ pressure than the air   under the airfoil.  Also due to angle of incidence and angle of attack, an even lower pressure is created at the trailing edge of the wing.    Attached is a screen shot of the Cambridge video, with my hypothetical pressures jotted in. Because high pressure air always seeks lower pressure air, the air above the airfoil should get an extra push and accelerate to a velocity faster than that of the air under the airfoil.  I found (see attached) a diagram of Pressure Coefficient distribution across airfoil geometry.  This diagram is from an unrelated paper treating a separate subject of shapes of trailing edges. It however is the only graph I could find detailing what pressures occurred where on the airfoil.   References: 1  Cambridge University video   2  Morphing Trailing Edges with Shape Memory Alloy Rods. November 2002 URL:   1st Silvestro Barbarino   15.46 · Sikorsky Aircraft 2nd Wulf G Dettmer 3rd Michael Ian Friswell 45.16 · Swansea University
101
I understand the common explanation of lift, which describes the airflow over the top of the wing as moving faster than the air below the wing.  However, I don't quite understand why the air moves faster.  I've read some explanations citing a circulation effect among others, but haven't found a good, clear explanation for the exact cause.  Can someone help me out?
116295
Today Derek Muller posted a video about a  that can go downwind faster than that downwind itself. The vehicle is custom made for that case only: the case of going straight downwind. The vehicle has three wheels, I estimate about 5 meters from front wheel to back wheels, and and a two-bladed propellor is mounted about 5 meters up. The mechanical connection between the propellor and the wheels is such that when the vehicle is rolling forward the propellor is moving air from the front of the vehicle to the rear of the vehicle. The vehicle is as lightweight as possible for its size, so it's quite flimsy. This is definitely not a practical device, it is a proof-of-concept device.  In the video Derek himself indicated that he was not quite confident that he understood the physics of it. I anticipate that questions will start coming in on physics SE, so I present this case as a self-answer. How can this vehicle, when going straight downwind, go faster than the wind itself? Previous question about that vehicle:
113968
I have a bit of misconception about weight which I want to clarify.   The air pressure is explained as the weight of the air column above our head acting per unit area. But since air is not continuous how can the weight of all the air molecules (above our head) be acting on our head? I mean we would only feel the weight of the molecules near the surface of our head (if not then why not?) but how do we feel the weight of molecules so far away?  I have edited this question and asked  follow up question (so that it remains specific).
28559
I know that a hydraulic jump is formed when a zone of supercritical flow discharges into a zone of subcritical flow, but why exactly is a hydraulic jump formed?  From a mechanical point of view, are the only forces acting in the diagram weight and friction?   If someone could actually label the diagram with forces and explain them to me, I'd really appreciate it.
52761
I know (from Kinematics) that for an object moving linearly with an acceleration and without air resistance the following equations can be used to determine v(velocity) or x(position of the object) at any time:  $v=v_0+at$  $x=x_0+v_0t+\frac{1}{2}at^2$  Where $x_0$ is the position of the object at the start of accelerated movement, $v_0$ is the velocity at the start of the motion and a is the acceleration.  Now if I want to add air drag, say we take the formula for air drag as:  $D=0.5CA\rho v^2$  $C$ is a coefficent, $A$ is the reference Area, $\rho$ is the density and $v$ the velocity. Since $D$ is a force the air drag results in an acceleration, so in other words: The resulting acceleration is $a-D/m$, however $D$ depends on $v$, which depends on the acceleration. And that's where my problem is, I simply can't wrap my mind around that.  How would I go about getting the Equations for $x$ and $v$ including air drag (and don't forget there's acceleration too)?
122331
The rotor blades of a lift based wind turbine are shaped like airfoils, so the wind flowing around them creates a lift force which in turn moves them around. From a thermodynamic viewpoint and like answered in  kinetic energy is extracted from the wind during this.  But what goes on from a mechanical perspective? How exactly is the kinetic energy of air particles transferred onto the rotor blades via lift? None of the sites I read gave an elaboration on the how.
75796
Raindrops are always pictured like this, people imagine they have this shape when they fall, but is this true? Doesn't this shape create too much drag? What shape do they really have? It would also be nice to know how is the actual shape formed regarding the flow of air.
110897
I am given a simple river system flowing down a slope where the speed of the flow downstream under the force of gravity builds up until resistive forces $R$ balance the component of gravitational force acting downstream $F$.  $$R=av$$ $$F=bh$$  The speed will eventually adjust to when $av=bh \implies v=\frac{b}{a}h$. We further assume that rain add water and we let seepage into the ground remove water. This will increase the depth of water at any point in the river at the rate $r(h, x, t)$ and the mass conservation law is given by  $$h_t +(hv)_x = r$$  Using Burger's equation and substituting for $v$, I get  $$u_t +uu_x =f$$  where $u=2\frac{b}{a}h$ and $f=2\frac{b}{a}r$. My question is how one arrives at the proposed $u$ and $f$ expressions.  If $R=av^2$ instead and following the same procedure I arrived at  $$h_t +(\sqrt{\frac{b}{a}}hh)_x = r$$  Can I reduce this to,  $$u_t +uu_x =f$$  where $u=\frac{3}{2}\sqrt{\frac{b}{a}}h\;\text{and}\;f=\frac{3}{2}\sqrt{\frac{b}{a}}r \;?$
45177
I'm having trouble understanding how a  (not the  that is generated as a result but the material itself) work. The particles are aligned in a specific direction to give rise to force but I don't see how this alignment gives rise to "attraction" or force.
31482
So, in typing the title of this question I was recommended , which confirmed my guess that this effect "propagates" at the speed of sound (though I just had a feeling, I don't really understand why).  I also only have high-school level physics experience.  I feel like I don't know physics well enough to really know what to ask correctly, but I'll try to explain.  So, I rotate my arm, and, even though tendons are just pulling it at a certain location, the rest of it follows suit.  Why is that?  What is happening at an atomic/molecular scale that ends up "conveying forces" over a distance?  And, something else I don't understand- what is so special about the speed of sound that makes it this "fundamental unit of translation", or something?  Maybe the better question is "what processes at an atomic/molecular scale lead to the speed of sound being associated with all of these behaviors", which ties both questions together.
112974
It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I am a diviner, a psychic, a "sensibil" as we call it. The right explanation clearly needs some electrodynamics although it's "everyday electrodynamics" and theoretical physicists are not trained to quickly answer such questions although each of us has probably solved many exercises that depend on the same principles.    When I am biking under the power lines – which probably have a high voltage in them – I feel a clear tingling shocks near my buttocks and related parts of the body for a second or so when I am under a critical point of the power lines. It is a strong feeling, not a marginal one: it feels like a dozen of ants that are stinging me at the same moment. It seems almost clear that some currents are running through my skins at 50 Hz. I would like to know the estimate (and calculation or justification) of the voltage, currents etc. that are going through my skin and some comparison with the shock one gets when he touches the power outlet.  Now,    my bike that makes this effect particularly strong is a ; the speed is about 20 km/h and the velocity is perpendicular to the direction of the current in the power line; the seat has a hole in it and there is some metal – probably a conducting one – just a few centimeters away from the center of my buttocks. It's plausible that I am in touch with the metal – or near touch; my skin is kind of sweating during these events and the liquid isn't pure water so it's probably much more conductive than pure water; the temperature was 22 °C today, the humidity around 35%, clear skies, 10 km/h wind;  may be between 22 kV and 1 MV and at 50 Hz, the altitude is tens of meters but I don't really know exactly.   What kind of approximation for the electromagnetic waves are relevant? What is the strength? How high currents one needs?   Does one need some amplification from interference etc. (special places) to make the effect detectable? (I only remember experiencing this effect at two places around Pilsen; the most frequent place where I feel it is near Druztová, Greater Pilsen, Czechia.)   Is the motion of the wheels or even its frequency important? Is there some resonance?  Does the hole in the seat and the metal play any role? Just if you think that I am crazy, other people are experience the effect (although with different body parts), see e.g.  and .  seems to suggest that the metals and electromagnetic induction is essential for the effect but the presentation looks neither particularly comprehensive nor impartial enough.  An extra blog discussion on this topic is here:
78707
And will it cool off faster if you blow across the top of the cup or directly into the coffee?   Does it have to do with the fact that when you blow across the top of the cup the velocity of the air increases which causes an area of low pressure above the cup, resulting in steam from the coffee to be forced up and out of the cup? Or is that not it?
53653
A mass is attached to a rope, and put into a circular motion. If I pull the string from the center, the tangential speed of the mass will increase (by conservation of angular momentum).   I am applying a force only in the radial direction, so how can the tangential velocity increase if there is no tangential force?
108474
In the paper clip experiment, the surface tension of water prevents the clip from falling, thus we can assume it exerts a force of $mg$ (weight of clip) upwards.   However, if you try to pull the clip out of water, surface tension opposes this motion also. If for simplicity we assume that the clip is a rectangle of length $l$ and breadth $b$, and the surface tension is $T$, then the liquid exerts a downward force of: $$T×2(l+b)$$  What is going on here? Why is there an inconsistency in the direction along which the force acts?
63659
Is the  real? It is a well-known statement that a butterfly, by flapping her wings in a slightly different way, can cause a hurricane somewhere else in the world that wouldn't occur if the butterfly had moved her wings in a slightly different way. Now, this can be interpreted as a figure of speech, but I think it's actually meant to be true. I can't imagine though that this is true. I mean, the difference in energy (between the two slightly different wing flaps), which actually can be zero (the only difference being the motion of the air surrounding the close neighborhood of the two slightly different flapping pairs of wings), is simply too small to cause the hurricane 10 000 miles away. So how can this in heavens name be true? By asking I'm making the implicit and realistic assumption that in the atmosphere no potential energies can be released when a small difference in the air conditions occurs (like, for example, the release of energy contained in the water of a dam when the water has reached a critical level and a small perturbation can cause the dam to break, with catastrophic consequences).
71066
In the  the person cuts 6 grooves on a wooden pencil and mounts a paper propeller using a pin on the soft eraser side.  As he starts rubbing the groves vigorously, the paper propeller starts spinning clockwise.  The mysterious thing is this: torque is generated on the paper from just translational motion (the rubbing of the grooves)! Where does this torque come from?! My current hypothesis is that the pencil traces a small circle if you hold it with your hand while rubbing the grooves, but I've disproved this theory when I mounted a replicated device on a clamp and performed the rubbing - the rotations still happens! I've ran computer simulations on this and have yet to determine the origins of the torque, which means the torque generation lies in non-ideality. An ideal system will dissipate the vibrational energy perfectly, generating no torque.  How can sound waves in a wooden media generate any torque as it leaves through a pivoted rigid body (the paper propeller)?  Can anyone shed some insights to a theory of how this motor works, and provide quantative/numerical solutions to this problem?  A numerical solution that quantizes all relevant parameters would be extremely helpful to explain this mysterious phenomena!  P.S. IMPT: Please provide numerical solutions to back your theory up! Please do not just provide qualitative solutions!  My current sentiments: It has something to do with acoustics which may tell us the efficiency of energy transfer to propeller and an non-ideality which generates torque, however the latter mechanism of origin of the torque remains shrouded in mystery.
71284
Possible Duplicate:          Today we were in our Literature class talking about the Renaissance and the Enlightement and our teacher also said that scientific experiments were being conducted, and she gave as an example the experiment in which they dropped objects with different mass from a tower to see which object would land first. She then said that she herself didn't know the outcome and since I'm known in my class as the #1, they asked me and I said that they landed at the same time (to my shock many classmates even disagreed with me about this fact). The teacher asked me to explain why and we haven't had anything about inertia in our physics class, so I was forced to use my own self-thought knowledge of physics: I said gravity does pull harder on heavier objects, but heavier objects have more resistance to move, in other words, they have more inertia. The entire class except for the teacher disagreed with me even though they usually don't. One fairly annoying kid asked me to explain why a feather falls way slower than a bowling ball, and I explained why and I also made the claim that in a vacuum they would fall at the same rate. Was I right or my class? Of course I know my explanation is lacking, but it has truth to it, right?