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Advice of Chris: added some physical definitions
In the aeroplane frame of reference, the angle between the chord line and the incident flow is the '''angle of attack'''.
= Fundamental physical concepts =
=== Force and moment ===
'''Force''' is the action on a body by a body. If you jump, you fall back onto the ground, and it is the gravitational force that pulls you down. Gravity is the action by the Earth on you.
A force combined with a leverage gives a '''moment'''. When you use a wrench to tighten a nut, you exert a force, and, combined with the leverage of the wrench, creates a "twisting force" on the nut which tightens it. The moment is proportional to the leverage, so if you cannot tighten a nut sufficiently, the solution is to use a longer wrench.
=== Mechanical work and mechanical energy ===
'''Work''' is a transfer of energy. When you lift a stone, your lifting force does work on it, in which process energy goes from you to the stone (and as a result, you will feel tired). If, as a result of doing work, energy goes into A, we say '''positive work''' is done on A, conversely, if energy leaves B, we say '''negative work''' is done on B. In this case, your lifting force do positive work on the stone, and the weight of the stone does negative work on you.
Energy can take many forms, such as '''kinetic energy''' (energy associated with moving with a speed) and '''potential energy'''. In this aerodynamic discussion we consider '''gravitational potential''' and '''pressure potential'''.
Gravitational potential is the energy associated with being in a (physically) high place. When you lift the stone, the energy that you give the stone becomes its gravitational potential because it has moved to a higher place. Pressure potential is the energy associated with being in a high pressure: you will feel tired after blowing up a hundred balloons, which is because the air inside a balloon has pressure potential, which ultimately comes from you, if you do not blow, the balloon does not inflate itself.
Energy is '''conserved''', which means it cannot be created nor destroyed. It can only go from one body to another body, or go from one form into another form. If you drop the stone, its gravitational potential will become kinetic energy, so it will pick up speed. When the stone hits the ground and comes to a stop, we say its mechanical energy has been '''dissipated''', but it has really gone into heat: we just do not notice.
=== Viscosity ===
A fluid has '''viscosity''' if it is "sticky". When you spill ketchup or syrup, it does not come off your clothes voluntarily: it sticks onto it. A fluid which has viscosity is said to be '''viscid'''. If a fluid has zero viscosity, it is said to be '''inviscid'''. Air is viscid, although its viscosity is so small that we seldom notice.
=== Miscellaneous ===
Being '''normal''' is being orthogonal (at an angle of 90 degrees) to the tangent line of the curve drawn at the location of interest.
= What a wing actually does =
